I recently traded for an 88 GT and it was running great. Until i tuned it up. (Plugs, wires, cap and rotor, oil change, and i put a cold air intake)nothing extravagant. Then the car started overloading itself with fuel. It usually happens at low cruizing speeds (3rd gear) and highway cruising speeds 75 and down (OD) since all this has happened i have been rezearching and had come up with faulty MAP sensor. I changed the MAP sensor but it did not fix the problem. Please if anyone has an idea im open to suggestion.
Engine 88 Gt Barn Find Sputtering Issues?
- Thread starter MeddleVertGT
- Start date
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Sponsors (?)
Barn car find checklist: first steps to get it back on the road.
First of all, carefully check the underhood area to make sure that rodents haven’t had a feast on the electrical wiring, air ducts and vacuum lines. Replace and repair any visible damage. Replace the battery if you haven’t already done so.
Next, drain ALL the engine fluids, transmission fluid, pump the fuel tank and fuel lines clear.
Get several cans of brake fluid, you will need it to flush the brakes. Loosen all 4 brake bleeders one at a time and flush the brake system by pumping the brake pedal. You will need to bleed the brakes when you finish flushing them. Brake fluid absorbs moisture if it sits for a long time and corrosive compounds start to brew themselves in the brake system.
Pumping out the old gas - do this before replacing the fuel filter if possible;
If the old pump still works, you can use it to pump the tank out.
1.) Separate the pressure line (the one with the Schrader valve on it) using the fuel line tools.
Look in the A/C repair section for the fuel line tools. They look like little plastic top hats. You will need the 1/2" & 5/8" ones. The hat shaped section goes on facing the large part of the coupling. Then you press hard on the brim until it forces the sleeve into the coupling and releases the spring. You may need someone to pull on the line while you press on the coupling.
OR
View: https://www.youtube.com/watch?v=vRTjYAxvaCs
Use a piece of garden hose to run from the pressure line to your bucket or gas can. Make sure it is as leak proof as you can make it. Fire and explosion are not part of the repair process...
2.) Jumper the fuel pump’s test point to ground.
Foxbody Diagnostic connector
Foxbody Diagnostic connector close up view[/b]
Turn the ignition switch to the Run position. the fuel pump will pump the tank almost dry unless the battery runs down first.
Some 5 gallon paint pails lined with garbage bags are good to hold the gas. The garbage bags provide a clean liner for the pails and keep the loose trash out of the gas. If you decide to use a siphon, a piece of 1/2" garden hose stuck down the filler neck will siphon all but a gallon or so of the gas.
The fuel filter is on the passenger side of the car on the body just over the rear axle housing. Pull the plastic clips out by grasping the tabs with a pair of needle nose pliers or a screwdriver.
]View: https://www.youtube.com/watch?v=i-JU54w7FL4]
At this point you have fresh engine fluids, fresh gasoline, new filters, new battery fresh brake fluid, and have replaced or repaired any rodent damage.
Then change ALL the filters (fuel, air & oil). Then put in new oil, new antifreeze, and 5 gallons or so of new gasoline. Be sure to replace the all the fan belt or belts. Reusing old belts are an invitation to break and leave you stranded or overheated.
Pull the spark plugs out and squirt some oil down inside the cylinders to lube them up. While the spark plugs are out, examine them for signs of damage or fouling. Write down which plug came from which cylinder and write down any negative things that you saw when you examined the spark plugs.
This next step is for those who have successfully removed and reinstalled a 5.0 Mustang distributor. Pre-oil the engine to insure that everything is lubed up good before the engine starts. Turn the engine up to where the distributor rotor points to #1 cylinder. Mark the distributor base with a scratch mark or something else that isn’t going to get rubbed off. Then remove the distributor and stick a ¼” socket on a12” extension on an electric drill. Place this rig on the oil pump shaft in the hole below the distributor. Turn the ignition switch to Run but don’t crank the engine. Run the drill counterclockwise while your assistant watches the oil pressure gauge.
For the less experienced DIY’er, here is a less complicated procedure.
While the spark plugs are out, crank the engine until you see the oil pressure gauge indicate pressure. If you don’t see any indication of oil pressure at the gauge after 30 seconds or so of cranking, you have some other problems. This is the time to stop and investigate them.
Put the old spark plugs back in if they look good; replace the ones that don’t. Have a spare set of new spark plugs handy for installation once you get the engine running. Why? The oil you squirted in the cylinders will lube things up, but it may also foul the spark plugs. Don’t foul the new plugs by putting them in cylinders that may oil foul the plugs.
Remove the distributor cap, and examine it and the rotor for signs of moisture and tracks. Lightly spray the inside of the distributor cap with WD40 to displace any moisture, and then wipe it up with a clean paper towel. Replace any cap or rotor that shows signs of damage, excessive corrosion or tracking that won’t wipe up with a shot of WD40.
Put the distributor cap back on, secure any loose wiring, vacuum lines, check the fluid levels, check belt tension and tighten any fasteners that you may have loosened. You are now ready to see if the engine will run.
Now that you have gotten everything prepared to drive the car, it is time to do some diagnostic work:
Dump codes sticky
Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
I highly suggest that you read it and follow the instructions to dump the codes. http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/
First of all, carefully check the underhood area to make sure that rodents haven’t had a feast on the electrical wiring, air ducts and vacuum lines. Replace and repair any visible damage. Replace the battery if you haven’t already done so.
Next, drain ALL the engine fluids, transmission fluid, pump the fuel tank and fuel lines clear.
Get several cans of brake fluid, you will need it to flush the brakes. Loosen all 4 brake bleeders one at a time and flush the brake system by pumping the brake pedal. You will need to bleed the brakes when you finish flushing them. Brake fluid absorbs moisture if it sits for a long time and corrosive compounds start to brew themselves in the brake system.
Pumping out the old gas - do this before replacing the fuel filter if possible;
If the old pump still works, you can use it to pump the tank out.
1.) Separate the pressure line (the one with the Schrader valve on it) using the fuel line tools.
Look in the A/C repair section for the fuel line tools. They look like little plastic top hats. You will need the 1/2" & 5/8" ones. The hat shaped section goes on facing the large part of the coupling. Then you press hard on the brim until it forces the sleeve into the coupling and releases the spring. You may need someone to pull on the line while you press on the coupling.
OR
View: https://www.youtube.com/watch?v=vRTjYAxvaCs
Use a piece of garden hose to run from the pressure line to your bucket or gas can. Make sure it is as leak proof as you can make it. Fire and explosion are not part of the repair process...
2.) Jumper the fuel pump’s test point to ground.
Foxbody Diagnostic connector
Foxbody Diagnostic connector close up view[/b]
Turn the ignition switch to the Run position. the fuel pump will pump the tank almost dry unless the battery runs down first.
Some 5 gallon paint pails lined with garbage bags are good to hold the gas. The garbage bags provide a clean liner for the pails and keep the loose trash out of the gas. If you decide to use a siphon, a piece of 1/2" garden hose stuck down the filler neck will siphon all but a gallon or so of the gas.
The fuel filter is on the passenger side of the car on the body just over the rear axle housing. Pull the plastic clips out by grasping the tabs with a pair of needle nose pliers or a screwdriver.
]View: https://www.youtube.com/watch?v=i-JU54w7FL4]
At this point you have fresh engine fluids, fresh gasoline, new filters, new battery fresh brake fluid, and have replaced or repaired any rodent damage.
Then change ALL the filters (fuel, air & oil). Then put in new oil, new antifreeze, and 5 gallons or so of new gasoline. Be sure to replace the all the fan belt or belts. Reusing old belts are an invitation to break and leave you stranded or overheated.
Pull the spark plugs out and squirt some oil down inside the cylinders to lube them up. While the spark plugs are out, examine them for signs of damage or fouling. Write down which plug came from which cylinder and write down any negative things that you saw when you examined the spark plugs.
This next step is for those who have successfully removed and reinstalled a 5.0 Mustang distributor. Pre-oil the engine to insure that everything is lubed up good before the engine starts. Turn the engine up to where the distributor rotor points to #1 cylinder. Mark the distributor base with a scratch mark or something else that isn’t going to get rubbed off. Then remove the distributor and stick a ¼” socket on a12” extension on an electric drill. Place this rig on the oil pump shaft in the hole below the distributor. Turn the ignition switch to Run but don’t crank the engine. Run the drill counterclockwise while your assistant watches the oil pressure gauge.
For the less experienced DIY’er, here is a less complicated procedure.
While the spark plugs are out, crank the engine until you see the oil pressure gauge indicate pressure. If you don’t see any indication of oil pressure at the gauge after 30 seconds or so of cranking, you have some other problems. This is the time to stop and investigate them.
Put the old spark plugs back in if they look good; replace the ones that don’t. Have a spare set of new spark plugs handy for installation once you get the engine running. Why? The oil you squirted in the cylinders will lube things up, but it may also foul the spark plugs. Don’t foul the new plugs by putting them in cylinders that may oil foul the plugs.
Remove the distributor cap, and examine it and the rotor for signs of moisture and tracks. Lightly spray the inside of the distributor cap with WD40 to displace any moisture, and then wipe it up with a clean paper towel. Replace any cap or rotor that shows signs of damage, excessive corrosion or tracking that won’t wipe up with a shot of WD40.
Put the distributor cap back on, secure any loose wiring, vacuum lines, check the fluid levels, check belt tension and tighten any fasteners that you may have loosened. You are now ready to see if the engine will run.
Now that you have gotten everything prepared to drive the car, it is time to do some diagnostic work:
Dump codes sticky
Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
I highly suggest that you read it and follow the instructions to dump the codes. http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/
Ive been through the car and nothing seems out of the ordinary (plugged in and ran new egr vaccum lines, previous owner must have had it disconnected) still i have the same result(no help with egrhooked up). It is still running way too rich. Backfiring and bucking in 3rd and od at cruising speeds. Would putting a cold air intake mess with the reading on the map sensor?
Just for the sake of dealing with one issue at a time.
Pull the cold air intake and install th factory set up, then slowly go through all the things you did in the tune up. If the problem is new since the tune, I’d look there.
And as stated above, pull both sets of codes.
Pull the cold air intake and install th factory set up, then slowly go through all the things you did in the tune up. If the problem is new since the tune, I’d look there.
And as stated above, pull both sets of codes.
Here ya go, the man himself answered it here.
https://www.stangnet.com/mustang-forums/threads/codes-41-91.851712/
https://www.stangnet.com/mustang-forums/threads/codes-41-91.851712/
MAP/BARO sensor operation and code 22
Revised 14-Nov-2014 to add wire colors for frequency & voltage testing and engine sensor wiring diagrams.
On a Speed Density car, the MAP/BARO sensor is connected to the intake manifold and acts to sense the manifold pressure. Lower vacuum inside the intake manifold when combined with more throttle opening measured by the TPS means more airflow through the engine. As airflow increases, fuel flow through the injectors needs to increase to keep the air/fuel ratio where it needs to be. When manifold vacuum increases, the engine is either decelerating or idling, and it needs to reduce the fuel flow through the injectors.
On a Mass Air car, the MAP/BARO sensor vents to open air and actually senses the barometric pressure due to changes in weather and altitude. Its purpose is to set a baseline for the computer to know the barometric pressure. As barometric pressure decreases, it leans out the fuel flow to compensate for less oxygen in the air. When the barometric pressure rises, it increases to add fuel since there is more oxygen in the air. The fuel requirements decrease as altitude increases, since the atmospheric pressure decreases.
Disconnecting the MAP or BARO sensor will set code 22.
Misconnecting the BARO sensor to vacuum on a Mass Air car will cause the computer to lean out the fuel mixture.
Code 22 or 126 MAP (vacuum) or BARO signal out of range. The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture by sensing changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it. There a very few DVM’s with a price tag under $40 that will measure frequency, but there are some out there.
Map sensor wiring:
black/white - ground
orange/white or +5 volts power
white/red signal out.
Measure the +5 volt supply using the orange/white and black/white wires
Measure the signal using the black/white and white/red wires.
The MAP/BARO sensor is mounted on the firewall behind the upper manifold on 86-93 Mustangs.
Baro or MAP test using a real frequency meter - run the test key on, engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout. And oscilloscope is very useful if you have access to one or know of someone who does. With an oscilloscope, you can see the waveform and amplitude.
If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer
Some basic checks you can make to be sure that the sensor is getting power & ground:
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.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 1.5 ohm.
The following power on check requires you to turn the ignition switch to the Run position.
Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or EGR sensors. Use the black/white wire for the ground for the DVM.
Diagrams courtesy of Tmoss & Stang&2birds
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.
Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif
TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif
Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif
93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif
T5 Cutaway showing T5 internal parts
http://www.veryuseful.com/mustang/tech/engine/images/5_Speed_Cutaway_Illustrated.jpg
Visual comparison of the Ford Fuel Injectors, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/Ford_Injector_Guide.jpg
CODE: 31 (KOEO) - EVP circuit below minimum voltage. Vref (5 volt reference voltage supplied by the computer) missing or broken wire or bad connection in circuit.
Revised 06-Aug-2016 to add clarification of the 10 pin connector possible problems
Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or MAP sensor located on the firewall near the center of the car. Use the black/white wire for the ground for the DVM.
With the sensor removed from the EGR and still connected, press the plunger and watch the voltage change on the brown/lt green wire. Pull the passenger side kick panel and measure the voltage at the computer. You will need to remove the plastic cover over the wires and probe them from the backside. A safety pin may prove very useful for this task. Use pin 27, EVR input (brown/lt green wire) and pin 46, signal ground (black/white wire) to measure the voltage. The orange/white wire is Vref and should always be 5 volts -/+ .25 volt. Be sure to measure Vref at the EGR sensor to rule out any broken wires or bad connections.
Measuring the voltage at the computer helps you spot broken wiring and intermittent connections. The 10 pin connectors are especially prone to connection problems, If the voltage checks at the EGR sensor are good but not at the computer, the 10 pin connector is suspect.
See the graphic for the 10 pin connector circuit layout.
Code 33 - Insufficient EGR flow detected.
Look for vacuum leaks, cracked vacuum lines, failed EGR vacuum regulator. Check to see if you have 10” of vacuum at the EGR vacuum connection coming from the intake manifold. Look for electrical signal at the vacuum regulator solenoid valves located on the rear of the passenger side wheel well. Using a test light across the electrical connector, it should flicker as the electrical signal changes. Remember that the computer does not source any power, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.
Check for resistance between the brown/lt green wire on the EGR sensor and pin 27 on the computer: you should have less than 1.5 ohm.
Backside view of the computer wiring connector:
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif
EGR test procedure courtesy of cjones
to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve or see the EGR test jig drawing below. Connect the test jig or to directly to manifold vacuum.
Do not connect the EGR test jig to the EVR (Electronic Vacuum Regulator).
apply 5in vacuum to the valve. Using the test jig, use your finger to vary the vacuum
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
if engine stumbled, connect EGR test jig to the hose coming off of the EGR Valve.
Use your finger to cap the open port on the vacuum tee.
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 2-5 in vacuum?
if not the EVR has failed
EGR test jig
The operation of the EGR vacuum regulator can be checked by using a test light applied across the wiring connector. Jumper the computer into self test mode and turn the key on but do not start the engine. You will hear all the actuators (including the EVR vacuum regulator) cycle. Watch for the light to flicker: that means the computer has signaled the EGR vacuum regulator successfully.
Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.
Revised 20-July-2017 to add note that the 94-95 uses a 4 wire O2 sensor.
Code 41 is the passenger side sensor, as viewed from the driver's seat.
Code 91 is the driver side sensor, as viewed from the driver's seat.
Code 172 is the passenger side sensor as viewed from the driver's seat.
Code 176 is the driver side sensor, as viewed from the driver's seat.
Code 43 is not side specific according to the Probst Ford Fuel injection book.
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.
Computer wiring harness connector, computer side.
Backside view of the computer wiring connector:
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 (L\RH O2 with a dark green/pink wire) and 43 (LH 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; note that the 94-95 uses a 4 wire O2 sensor.
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 29
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
Check the fuel pressure – the fuel pressure is 37-41 PSI with the vacuum disconnected and the engine idling. Fuel pressure out of range can cause the 41 & 91 codes together. It will not cause a single code, only both codes together.
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. The exception is that the 94-95 uses a 4 wire O2 sensor.
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. This puts an excess amount of air in the passenger side exhaust and can set the code 41. 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.
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