Thoughts On Block Seal Just To See If It Helps

from6to8

There's suction so I used that end O_O
15 Year Member
Sep 2, 2012
1,076
45
79
I have always had a low end miss hesitation lately I would say probably for a good bit of this yr and even after a complete few months of diagnosing, up to date maintenance and checks, trying other parts, ect. I do seem to have some coolant overtime, not anything fast, but have some coolant loss and can smell just a slight coolant, not very strong at all but I think its slight. I think its possible I have a slight intake or head gasket leak or maybe even a slight freeze plug leak and probably one in back of head that cant be seen ( I just had one on my pickup in same spot that was leaking and I couldnt see the leak).

I'm wondering if it's worth trying a can of K&W block seal in it to keep fix it temporarily as I have heard of people doing it and it lasting a few yrs or more. The main thing i want to know from you guys is the negative effects of using block seal as i think I have heard in the past about it clogging your radiator potentially or something and just want to know if there are any potential risks to using it. I don't want to worry about this little problem if potentially it will mean needing to spend more money as a result of it possibly messing up something else like a radiator or something. Also there is no coolant mixing with oil or anything. If there is a slight head gasket or intake gasket, that will cause a slight hesitation/miss possibly?

So thoughts guys??????????
 
  • Sponsors (?)


In a word

Don't

If you want to keep the car and ever want to work on that motor again. Snake oils in a can cannot repair a bad head gasket or any other leak. What it will do is gum up your motor and maybe band aid the problem for a few weeks.
 
ye
In a word

Don't

If you want to keep the car and ever want to work on that motor again. Snake oils in a can cannot repair a bad head gasket or any other leak. What it will do is gum up your motor and maybe band aid the problem for a few weeks.

yeah thats not a chance i want to take either lol. i will take it to advance one day and borrow their tester to determine exactly where its coming from
 
In a word

Don't

If you want to keep the car and ever want to work on that motor again. Snake oils in a can cannot repair a bad head gasket or any other leak. What it will do is gum up your motor and maybe band aid the problem for a few weeks.
Ok just checked the codes and here is what I got and putting them in the order in which I got them:

KOEO:


411- Cannot control rpm during KOER low RPM check (had to google as my chilton's manual didnt have that code)

511- EEC Processor ROM Test failed

189- system at Rich adaptive limit at part throttle, system rich (left)

556- Fuel pump primary circuit failure

KOER:


412 - Cannot control rpm during KOER high rpm check

116- ECT outside limits during KOEO Test

136 - HEGO shows system always lean left


On the Cylinder Balance test I did right after the KOER test, just left car running and pressed pedal to floor to about 4K and released to do that test and got these results after it showing fine last time i did the Balance Test.

It showed No. 1,3,4, and 5. Does that mean they arent firing correctly? Put on new NGK 02's last Dec 15 and less than 7K miles as in the HEGO code if hego is 02 related. I think I have read that sometimes things can be wrong that will cause other codes to show but not necesarily meaning everything it shows is actually wrong. I also never clear codes so could they be possibly or could some possibly be old codes?
 
Have you done any compression testing of the motor?
Have you done any compression testing of the motor?
nah i didnt. I'm going to unplug the chip and tidy up a few small things and see what the results are again and then the next step will be to do a compression test i guess.

When its time to rebuild im unsure if i want to do a 306 or 331. I wouldnt mind the more velocity for a better more heavy, hollow , deeper sounding engine. I'm not a racer and just want a nice setup so when and if i feel like street play at any minute I will have some decent firepower lol. What are your numbers with your 306 and are you satisfied? I've listened to a 386 and 406 on youtube doing the different flowmaster muffler sounds and man ole man i wouldnt mind doing one of those
 
nah i didnt. I'm going to unplug the chip and tidy up a few small things and see what the results are again and then the next step will be to do a compression test i guess.

When its time to rebuild im unsure if i want to do a 306 or 331. I wouldnt mind the more velocity for a better more heavy, hollow , deeper sounding engine. I'm not a racer and just want a nice setup so when and if i feel like street play at any minute I will have some decent firepower lol. What are your numbers with your 306 and are you satisfied? I've listened to a 386 and 406 on youtube doing the different flowmaster muffler sounds and man ole man i wouldnt mind doing one of those
Its a little firecracker! Enough to keep the young boys with civics and mild evos at bay for now but I have a trick or two up my sleeve come spring time!
 
Pull the spark plugs and compare the color between them. If coolant is getting into the combustion chamber, sometimes you can tell which cylinder(s) by difference in deposits on plugs. Ones that look significantly cleaner might be where the coolant is getting burned. The process has a steaming effect that cleans carbon off.

Try removing the lower radiator cap seal - the one that seals the rad from the overflow tank. Keep the upper cap seal in place. This trick will keep the coolant in the system by letting the expansion of coolant go into the overflow tank without letting the system build pressure and thus force a leak at a weak spot. You can then fill the overflow about half way (engine hot) and monitor the level in the overflow tank. When the coolant cools and contracts, it should pull some fluid from the overflow and back into the tank.

It's a bubba trick that has worked for me on several vehicles over the years. Just maintain the proper mixture of coolant/water and if your rad and fan are up to snuff, it should reduce or even eliminate leaking that happens under pressurized conditions only.
 
Pull the spark plugs and compare the color between them. If coolant is getting into the combustion chamber, sometimes you can tell which cylinder(s) by difference in deposits on plugs. Ones that look significantly cleaner might be where the coolant is getting burned. The process has a steaming effect that cleans carbon off.

Try removing the lower radiator cap seal - the one that seals the rad from the overflow tank. Keep the upper cap seal in place. This trick will keep the coolant in the system by letting the expansion of coolant go into the overflow tank without letting the system build pressure and thus force a leak at a weak spot. You can then fill the overflow about half way (engine hot) and monitor the level in the overflow tank. When the coolant cools and contracts, it should pull some fluid from the overflow and back into the tank.

It's a bubba trick that has worked for me on several vehicles over the years. Just maintain the proper mixture of coolant/water and if your rad and fan are up to snuff, it should reduce or even eliminate leaking that happens under pressurized conditions only.
I actually changed the plugs last week and some were a little tannish and i think 2 or 3 had a few small deposits of gunk but overall they all looked pretty good. I will try and post pics of them later.

still not sure what you mean by lower radiator cap seal. I know of only one radiator cap lol
 
The blue arrow points to the inner cap seal. Remove that one. The outer seal stays and keeps the coolant in the system but allows expanding and contracting coolant to readily flow into and out of the overflow bottle. Removing the inner seal prevents pressure build up and reduces leaks.

Insideofradiatorcap.jpg
 
The blue arrow points to the inner cap seal. Remove that one. The outer seal stays and keeps the coolant in the system but allows expanding and contracting coolant to readily flow into and out of the overflow bottle. Removing the inner seal prevents pressure build up and reduces leaks.

Insideofradiatorcap.jpg
 
ok will try that when i mess with it again. I have to recheck codes and stuff first and possibly get a compression test too
 
I just had issues on my 92 s10 as well where I replaced 2 freeze plugs behind the head, the fan clutch, upper radiator hose, and put in a can of bl0ck seal. The other day I noticed where some coolant had exited the overflow jug and it has no holes or anything in the jug so I'm wondering if the cap possilby could be bad and need replacing
 
I just had issues on my 92 s10 as well where I replaced 2 freeze plugs behind the head, the fan clutch, upper radiator hose, and put in a can of bl0ck seal. The other day I noticed where some coolant had exited the overflow jug and it has no holes or anything in the jug so I'm wondering if the cap possilby could be bad and need replacing

Very easy to test. Autozone rents for free a cap and coolant system checker. Pump it up to 16lbs and the cap should hold the pressure. If it doesn't , it's bad. Same for the system. It should hold the pressure for at least 15-20 minutes and not bleed down.
 
Very easy to test. Autozone rents for free a cap and coolant system checker. Pump it up to 16lbs and the cap should hold the pressure. If it doesn't , it's bad. Same for the system. It should hold the pressure for at least 15-20 minutes and not bleed down.
i think advance lets you use it no charge
 
Ok just checked the codes and here is what I got and putting them in the order in which I got them:

KOEO:


411- Cannot control rpm during KOER low RPM check (had to google as my chilton's manual didnt have that code)

511- EEC Processor ROM Test failed

189- system at Rich adaptive limit at part throttle, system rich (left)

556- Fuel pump primary circuit failure

KOER:


412 - Cannot control rpm during KOER high rpm check

116- ECT outside limits during KOEO Test

136 - HEGO shows system always lean left


On the Cylinder Balance test I did right after the KOER test, just left car running and pressed pedal to floor to about 4K and released to do that test and got these results after it showing fine last time i did the Balance Test.

It showed No. 1,3,4, and 5. Does that mean they arent firing correctly? Put on new NGK 02's last Dec 15 and less than 7K miles as in the HEGO code if hego is 02 related. I think I have read that sometimes things can be wrong that will cause other codes to show but not necesarily meaning everything it shows is actually wrong. I also never clear codes so could they be possibly or could some possibly be old codes?

The wandering cylinder balance test results are due to the 411 code. The computer must be able to hold a steady 1450-1600 RPM for the cylinder balance test to work correctly.

511- EEC Processor ROM Test failed same as code 15 -
Code 15 - No Keep Alive Memory power to PCM pin 1 or bad PCM (Memory Test Failure). The voltage to the Keep Alive Memory (KAM) is missing (wiring problem) or the KAM is bad. The KAM holds all of the settings that the computer "learns" as it operates and all the stored error codes that are generated as a result of something malfunctioning while the engine is running. Use a voltmeter to check the voltage to the pin 1 on the computer - you should always have 12 volts. No constant 12 volts = bad wiring. If you do always have the 12 volts, then the KAM is
bad and the computer is faulty.

If the computer has to "relearn" all the optimum settings every time it powers up, the initial 5-30 minutes of operation may exhibit surges, poor low speed performance, and rough idle.

Note that some aftermarket chips will cause code 15 to set. Remove the chip, clear the codes and retest.

Before replacing the computer, remove the battery ground cable for about 10 minutes. This will clear all the codes. Retest after several days of running. If the 15 code is gone, then don't worry about it. If it is still there, then you get to do some troubleshooting.

See the following website for some help from Tmoss (diagram designer) & Stang&2
Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

Diagram courtesy of Tmoss & Stang&2birds
94-95_5.0_EEC_Wiring_Diagram.gif


http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif


189- system at Rich adaptive limit at part throttle, system rich (left) Same as code 42/92 -
Code 42 & 92 (engine running) System rich - Fuel control or (memory) System was rich for 15 seconds or more (no HO2S switching) - Fuel control. Look for leaking injectors, fuel pressure too high, cylinder(s) not firing due to bad ignition.
Code 42 is the RH side sensor,
Code 92 is the LH side sensor.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.


Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.71316


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.



There is a fuse link for the O2 sensor heater power. According to Ranchero50, it is in the wiring near the passenger side hood hinge. Measuring the voltages will give a clue if it has shorted to the O2 sensor signal lead. The O2 sensor voltage should switch between .2-.9 volt at idle.

136 - HEGO shows system always lean left - same as 41/91 -
Code 41 or 91 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 29-Sep-2013 to add back in a clogged crossover tube as cause for code 41

Code 41 is a RH side sensor, as viewed from the driver's seat.
Code 91 is the LH side sensor, as viewed from the driver's seat.

Code 172 is the RH side sensor, as viewed from the driver's seat.
Code 176 is the LH side sensor, as viewed from the driver's seat.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.

Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.71316


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 43

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports, Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

Being you got strange results with two different O2 sensor codes that seem to be complete opposites of each other, check the Left Hand O2 sensor wiring and connectors.
 
Last edited:
The wandering cylinder balance test results are due to the 411 code. The computer must be able to hold a steady 1450-1600 RPM for the cylinder balance test to work correctly.

511- EEC Processor ROM Test failed same as code 15 -
Code 15 - No Keep Alive Memory power to PCM pin 1 or bad PCM (Memory Test Failure). The voltage to the Keep Alive Memory (KAM) is missing (wiring problem) or the KAM is bad. The KAM holds all of the settings that the computer "learns" as it operates and all the stored error codes that are generated as a result of something malfunctioning while the engine is running. Use a voltmeter to check the voltage to the pin 1 on the computer - you should always have 12 volts. No constant 12 volts = bad wiring. If you do always have the 12 volts, then the KAM is
bad and the computer is faulty.

If the computer has to "relearn" all the optimum settings every time it powers up, the initial 5-30 minutes of operation may exhibit surges, poor low speed performance, and rough idle.

Note that some aftermarket chips will cause code 15 to set. Remove the chip, clear the codes and retest.

Before replacing the computer, remove the battery ground cable for about 10 minutes. This will clear all the codes. Retest after several days of running. If the 15 code is gone, then don't worry about it. If it is still there, then you get to do some troubleshooting.

See the following website for some help from Tmoss (diagram designer) & Stang&2
Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

Diagram courtesy of Tmoss & Stang&2birds
94-95_5.0_EEC_Wiring_Diagram.gif


http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif


189- system at Rich adaptive limit at part throttle, system rich (left) Same as code 42/92 -
Code 42 & 92 (engine running) System rich - Fuel control or (memory) System was rich for 15 seconds or more (no HO2S switching) - Fuel control. Look for leaking injectors, fuel pressure too high, cylinder(s) not firing due to bad ignition.
Code 42 is the RH side sensor,
Code 92 is the LH side sensor.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.


Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.71316


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.



There is a fuse link for the O2 sensor heater power. According to Ranchero50, it is in the wiring near the passenger side hood hinge. Measuring the voltages will give a clue if it has shorted to the O2 sensor signal lead. The O2 sensor voltage should switch between .2-.9 volt at idle.

136 - HEGO shows system always lean left - same as 41/91 -
Code 41 or 91 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 29-Sep-2013 to add back in a clogged crossover tube as cause for code 41

Code 41 is a RH side sensor, as viewed from the driver's seat.
Code 91 is the LH side sensor, as viewed from the driver's seat.

Code 172 is the RH side sensor, as viewed from the driver's seat.
Code 176 is the LH side sensor, as viewed from the driver's seat.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.

Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.71316


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 43

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports, Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

Being you got strange results with two different O2 sensor codes that seem to be complete opposites of each other, check the Left Hand O2 sensor wiring and connectors.
 
Will unplugging the chip clear the codes? I just saw your repy Jrichker and havent read it all but i will shortly

.....................oops just saw that unpluging batt for 10 mins will

Also I just replaced the 02's last Dec about 7k miles ago
 
Last edited:
Will unplugging the chip clear the codes? I just saw your repy Jrichker and havent read it all but i will shortly
If you used a scanner to dump the codes, it will have a button or setting to clear them. Clearing the codes does not fix what caused them, it just erases them from memory so you have a clean sheet to log the codes.

Disconnecting the battery for 5 minutes does the same thing plus clears all of learned settings. If you change a sensor, or fix a sensor's wiring, I recommend disconnecting the battery. Reconnect the battery and drive at highway speeds for 15-30 minutes. The car may run a little rough until it "re-learns" the new sensor settings.