Chasing A Power Loss Issue

elarm1

10 Year Member
Dec 17, 2010
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N.Va
Got some questions regarding a HP loss feeling with my car. The car seems as though it has an on/off switch. My suspicions were confirmed when I took it to the local 8th mile for some test and tune and the fastest that I could muster was a 8.38. I was running those times without the V3! Cruising around I really have no problems but when trying to access boost (around 3500k), it feels like the power just isn't there. I am seeing boost on my boost gauge at around 11-12psi and I am almost certain my belt isn't slipping but it is as if there literally is an on/off switch at anything 3500k 4500k range. I really need to check my a/f ratio but I have no gauge so before I take it to my tuner, what is a few things that I can check? Plugs are gapped at .32 and I have not checked my fuel pressure since I got it back from my tuner. It ran like a bat out of hell when I got it tuned but what gives? (M)ay (S)uddenly (D)ie may be first on my hit list...
enginenov2015.jpg
 
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Got some questions regarding a HP loss feeling with my car. The car seems as though it has an on/off switch. My suspicions were confirmed when I took it to the local 8th mile for some test and tune and the fastest that I could muster was a 8.38. I was running those times without the V3! Cruising around I really have no problems but when trying to access boost (around 3500k), it feels like the power just isn't there. I am seeing boost on my boost gauge at around 11-12psi and I am almost certain my belt isn't slipping but it is as if there literally is an on/off switch at anything 3500k 4500k range. I really need to check my a/f ratio but I have no gauge so before I take it to my tuner, what is a few things that I can check? Plugs are gapped at .32 and I have not checked my fuel pressure since I got it back from my tuner. It ran like a bat out of hell when I got it tuned but what gives? (M)ay (S)uddenly (D)ie may be first on my hit list...
enginenov2015.jpg
As you said the msd but also maybe the fuel pressure is dropping off under load


Sent from my iPhone using my fingers while my auto correct makes me seem illiterate
 
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It may be intoxicated. I see an open container on the firewall. ;)

I agree, bypass the MSD and see how it goes. You probably don't need it anyway unless it's a 6btm and your're pulling timing with it.

Good Luck chasing the bug. I hate bugs!
 
Yeah guys, I'll try the MSD because it was the first on you guys list! I don't know maybe she is drunk! I was wondering if anybody would catch that. Its not a 6btm but do really think I do not need my MSD @Onefine88 ?
 
Yeah guys, I'll try the MSD because it was the first on you guys list! I don't know maybe she is drunk! I was wondering if anybody would catch that. Its not a 6btm but do really think I do not need my MSD @Onefine88 ?

This is a tough one to diagnose. I'm no expert but in my experience if it was leaning out up top it would be popping through the intake under WOT and/or pinging like a jack hammer.
But, I have seen MSD boxes fail but the engine still able to run. And, I've seen them fail leaving the engine unable to start.
But as for your question:
My car makes more than 500 to the tires without an ignition box and runs flawless. My tuner of many years "Who I trust Greatly" said he'll tell when I need one. I know alot of guys around my area not using MSD box's and making plenty of powa.
Run a good coil, gap the plugs accordingly and don't be concerned about an added potential MSD box failure. Ever notice that the Nascar boys run two?:eek:

"One less thing"

Good Luck
 
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Quit guessing and diagnose; many of the tools are easily available. Fuel pressure gauges can be borrowed from your local auto parts store.

Simple first step: do a cylinder balance test. The first part of a cylinder balance test dumps the codes and gives you a clue if there are any computer or sensor problems.

Cylinder balance test: use this to find dead or weak cylinders:

Revised 25 March 2012 to add necessity allowing the KOEO tests to finish before starting the engine and the need for a properly functioning IAB/IAC to run the cylinder balance test.

The computer has a cylinder balance test that helps locate cylinders with low power output. You’ll need to dump the codes out of the computer and make sure that you have the A/C off, clutch depressed to the floor and the transmission in neutral. Fail to do this and you can’t do the engine running dump codes test that allows you to do the cylinder balance test.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, have the clutch depressed to the floor, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.


Here's how to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

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If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

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The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

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The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and drivability problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, clutch depressed to the floor and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10
4




Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus Digital Ford Code Reader (3145) Equus - Digital Ford Code Reader (3145It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.


Cylinder balance test

If you have idle or IAC/IAB problems and the engine will not idle on its own without mechanically adjusting the base idle speed above 625-750 RPM, this test will fail with random cylinders pointed out every time it runs. The IAC/IAB must be capable of controlling the engine speed to run in the 1400-1600 RPM range. Playing with the base idle speed by adjusting it upwards will not work, the computer has to be able to control the engine speed using the IAC/IAB.

Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Let it finish the Key On Engine Off (KOEO) code dump. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. Remember to keep the clutch pedal (5 speed) depressed to the floor during the test. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure


Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent/loan. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open with a plastic screwdriver handle between the throttle butterfly and the throttle housing. Crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 PSI. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

A battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Connect it up while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.

See the link to my site for details on how to build your own blow down type compression tester.
 
Finally got around checking the codes. Twin newborns have been suckling the life out of me but here are my codes.
Key on running
41
Key on not running
15
114
41
91
I haven't taken off the May Suddenly Die but what do you think my next testing step should be?
 
41/91 are lean codes(usually o2 sensors) if memory serves fwiw. My manual is out in the garage and I'm to lazy to go look lol. Bad ect sensor can also cause your issues. I'd test those 3 things and see if they're in range. Jrichker has a write up somewhere on here for the tests/ranges etc
 
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ECT changed to known good unit
Fuel pressure 40 psi
Smoke vacuum test
Stock ignition wired in and it runs worse. Breaking up at wot 5k. Check engine light wit same codes. Anything else should I check?
 
How about the TFI module...or the Mag pick up....also where is your MAF check for leaks around the connections....I had one come loose on my turbo car ran like crap once in boost normal driving it was fine...
 
Extensive testing done today. Cylinder balance test done and it saw a problem in number 8 cylinder, pulled the plug and found a cracked porcelain. Replace it and I no longer have a code 91. Got her up to temperature again and it saw a problem with number 4 and 7. Checked those plugs and saw no issues. I've been reading a lot about an orange wire that is a ground for the o2 sensor. From what I'm reading, it's supposed to be grounded on either the head or intake manifold. Mine is grounded through one of the bell housing bolts to the block. Really can't see that being an issue but maybe I'll move it. With my still existing code 41, it's bringing me to my passenger side o2. I will follow O2 Sensor Codes 41/42/91/92 Troubleshooting instructions and go from there. Any more advise?
@A5literMan it's on my list next and @MY 85 GT distributor is slaughterzone new and I'll double check my maf. I did clean it but saw no change
 
@jrichker that checklist is priceless! I'll get to the bottom of it, it's just a matter of time. Speaking of which, please give me a class on how oNE would check the resistance of my o2 sensors. I have a multimeter but I'm no electrician! Here's some of your old info pulled from another thread I will be using.
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 6-Oct-2014 to improve code 41 description due to clogged Thermactor air crossover tube

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.

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.

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 (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
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 theLow 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 wireon 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. 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.

Thanks in advance @jrichker
 
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@elarm1; the O2 sensor test path looks like the current version. Read the test path carefully and follow the instructions and you will be good. What you need to know to do the job is in there.


Fuel pump.
Wrong guess...
A fuel pump would affect both sides and give both 41 and 91 codes. The OP said he fixed the 91 when he replaced a spark plug with a cracked porcelain insulator. That leaves only the code 41 to deal with.
 
My my my! Well after 2 days of testing everything then finally going to the o2 sensors I found my problem. I had a low resistance from passenger side o2. I swapped it out for a known working unit and voila, no more cell and she runs like a bat outta hell! Sucks that I had to check everything twice but now I know everything works as it should. Once again @jrichker that checklist is priceless and gave me all the tools I needed to figure things out. I can now put it away for the winter knowing everything works. Thanks all again and happy motoring!
 
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