Electrical 1989 mustang 5.0 runs rich and stumbles in closed loop

mark32 · Aug 14, 2020

So I recently converted my 1989 4 cyl to a 5.0 using all the correct engine harnesses from a 1989 car. The ECU I'm running is the A3M out of a 93 car.

So the car starts right up no problem. After let's say 60 seconds of idling the car from what I think goes in to closed loop and once it does it runs like crap. It will pop, stumble, and die if I dont give it gas. After about maybe another 2 minutes of it trying to stay running in closed loop and running like crap it gives up and throws a check engine light and then goes in open loop and runs fine. I'm guessing it reverts to the pre configed fuel tables.

I ran a self test and got codes 21, 42, 34, 92.

I did convert a 93 o2 harness to work with the 89 and I'm pretty confident I did it right unless i got the banks backwards (left and right o2's). I did probe into the o2 wires at the ECU and I did get readings from the o2 sensors. I got a range from .02-.012

I know that code 21 is the coolant temp sensor and I did replace that and I haven't checked again to see if the 21 code went away but the car still runs bad.

I'm not sure what to try next. Any help is much appreciated.

jrichker · Aug 15, 2020

Have you dumped the engine running codes and done a cylinder balance test?

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

Revised 09-Sep-2017 Added reminder to write down the stored codes and engine running codes.

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.

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.

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.

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 dump the codes and then 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

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 www.midwayautosupply.com/Equus-Digital-Ford-Code-Reader/dp/B000EW0KHW Equus - Digital Ford Code Reader 3145.
It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.
Order it at Walmart for a better price and free shipping

Write down the codes that the computer outputs since they will give you information on problems that are stored in the computer's memory

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 1300-1500 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. With the test jumper in test position, start the engine and let it stabilize. It should flash a 10 and then a 4 and maybe an 11. If no 11, then there are other codes that will be dumped.

Write down the codes that the computer outputs since they will give you information that the computer found when it is running. These are often different from the stored codes.

One of the first tests it does is to open the EGR all the way, this will cause the engine to stumble and almost die. If the engine dies here then you have EGR problems.
To start the cylinder balance test, briefly floor the accelerator past 2500 RPM and let off the accelerator. The engine will stabilize at about 1300-1450 RPM and the cut off the fuel injectors one at a time. The engine speed will drop briefly and the computer will turn the fuel injector for the cylinder under test back on. Then it starts the process for the next cylinder. 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

See View: https://www.youtube.com/watch?v=HDXrkKS4jTE
for a visual tour through the process. There is no voice narration so you have to listen carefully for the engine sounds. I posted the link for the benefit of Stangnet members who had questions about how to do a cylinder balance test. I do not own that video and I am not the creator.

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.

mark32 · Aug 15, 2020

Hey so here's the codes I got

42 92 34 koer

34 81 82 84 85 koeo

I got 9 for the cylinder balance test.

tab and tad are not hooked up I threw those away. Purge Canister is also not hooked up. Egr has been deleted/smog stuff is all deleted.

General karthief · Aug 15, 2020

Hook the purge canister back up. It vents the fuel tank and helps with part throttle cruise.

mark32 · Aug 16, 2020

I hooked up the purge canister and I also went ahead and bought some new spark plugs. The car still runs like crap in closed loop. If I start the car up it still does the same thing it will buck, pop, stumble untill it gives up and goes in to open loop.

Keeps throwing the same 3 codes.

I have the EGR plugged in but vaccum removed. I wanted to delete this but not sure if i did it right.

34 - EGR Valve Position/Pressure Feedback EGR Voltage Above Closed Limit (O,R), PFE or EVP circuit has intermittently failed above the closed limit of 0.67 volts (CM)

42 - System Indicates Rich -passenger side(R), No o2 Sensor Switching Detected. always rich -passenger side(CM)

92 - o2 Sensor Circuit Indicates Rich -driver side(R), No o2 Sensor Switching Detected. always rich -driver side(CM), Shift Solenoid 2 Circuit Failure (O)

jrichker · Aug 16, 2020

The code 85 tells me that the purge canister solenoid valve is either missing or non-functional. The code 42/92 tells me that the tank vent system is sucking in raw fuel from somewhere. The code 85 in combination with codes 42/92 points to a tank vent connected directly to the intake manifold.

The only other possibility is the fuel pressure regulator diaphragm has a hole in and the engine is sucking raw fuel from there. Disconnect the vacuum line from the fuel pressure regulator. Check it for evidence of fuel present in the line by removing it and blowing air through it. If you find fuel, the fuel pressure regulator has failed.
Or the fuel pressure is way out of the 37-41 PSI range.

Code 85 CANP solenoid - The Carbon Canister solenoid is inoperative or missing.

Revised 11 –Jan_2015 to add warning about vacuum leaks due to deteriorated hose or missing caps on vacuum lines when the solenoid is removed.

Check vacuum lines for leaks and cracks. Check electrical wiring for loose connections, damaged wiring and insulation. Check solenoid valve operation by grounding the gray/yellow wire to the solenoid and blowing through it.
The computer provides the ground for the solenoid. The red wire to the solenoid is always energized any time the ignition switch is in the run position.

If you disconnected the carbon canister and failed to properly cap the vacuum line coming from under the upper intake manifold, you will have problems. You will also have problems if the remaining hose coming from under the upper intake manifold or caps for the vacuum line are sucking air.

Charcoal canister plumbing - one 3/8" tube from the bottom of the upper manifold to the rubber hose. Rubber hose connects to one side of the canister solenoid valve. Other side of the solenoid valve connects to one side of the canister. The other side of the canister connects to a rubber hose that connects to a line that goes all the way back to the gas tank. There is an electrical connector coming from the passenger side injector harness near #1 injector that plugs into the canister solenoid valve. It's purpose is to vent the gas tank. The solenoid valve opens at cruse to provide some extra fuel. The canister is normally mounted on the passenger side frame rail near the smog pump pulley.

Connecting the gas tank vent line directly to the intake manifold will result in fuel vapor being constantly sucked into the intake manifold. There is unmetered fuel that the computer cannot adjust for. The result is poor idle and poor fuel economy.

It does not weigh but a pound or so and helps richen up the cruse mixture. It draws no HP & keeps the car from smelling like gasoline in a closed garage. So with all these good things and no bad ones, why not hook it up & use it?

The purge valve solenoid connector is a dangling wire that is near the ECT sensor and oil filler on the passenger side rocker cover. The actual solenoid valve is down next to the carbon canister. There is about 12"-16" of wire that runs parallel to the canister vent hose that comes off the bottom side of the upper intake manifold. That hose connects one port of the solenoid valve; the other port connects to the carbon canister.

The purge valve solenoid should be available at your local auto parts store.

Purge valve solenoid:

The carbon canister is normally mounted on the passenger side frame rail near the smog pump pulley.
Carbon Canister:

Code 42 & 92 & 137 & 173 (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 passenger side sensor, as viewed from the driver's seat
Code 92 is the driver side sensor, as viewed from the driver's seat.

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:

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.

africansnowowl · Aug 16, 2020

If you figure out the solution to your problem, please post what the fix was. I’m dealing with the exact same problem and codes.

mark32 · Aug 23, 2020

I changed fuel reg made no difference. I also did vaccum smoke leak test no leaks found. I only have brake booster hooked up, fuel reg, PCV, carbon canister for vaccum.

Last time I did the o2 probing I swear I got a range from .2-.12. But this time when I probed pins 29 and 43 I got 14.46 volts on both O2's. What the heck is going on how are they showing 14 volts??

mark32 · Aug 24, 2020

I did change the ground location for the O2/injector harness, the ground was on the firewall before but was loose so I bolted it up to the back of the head. Not sure if this is causing the high volt reading. Should I buy new O2 sensors? I also cleaned the MAF today that didnt help either.

I also looked at the injectors and I found 4 injectors that had broken pintle caps. I replaced the injectors with 4 injectors from a 92 4 banger they looked the same but I'm just running them temporarily till i can fix the 19# orange injectors which i will be putting back in.

Currently the car runs good only at full throttle. During cruising it pops alot and stumbles. Pretty much i can't drive it unless I floor it lol.

jrichker · Aug 24, 2020

mark32 said:
I changed fuel reg made no difference. I also did vaccum smoke leak test no leaks found. I only have brake booster hooked up, fuel reg, PCV, carbon canister for vaccum.

Last time I did the o2 probing I swear I got a range from .2-.12. But this time when I probed pins 29 and 43 I got 14.46 volts on both O2's. What the heck is going on how are they showing 14 volts??

The high voltage was caused by the missing/defective orange O2 sensor ground. The O2 sensor heater voltage was trying to use the computer O2 sensor wire as a ground.

mark32 · Aug 24, 2020

When probing pins 29 and 43 I still get 12.60 volts on KOEO and 14.+ volts when running. I checked the resistance between the HEGO ground wire and pin 49 on the computer harness and it was a 0 resistance so that ground is working.

mark32 · May 10, 2021

This issue has been resolved. The issue was with the injectors. The injectors were leaking and not spraying correctly. Some of the injectors has broken pintle caps and the seals were worn and loose. I since rebuilt all the injectors with new caps,seals and filters, as well as cleaned them. Now the car runs great and does not run rich. The exhuast smells normal now too.

sands9330014 · Mar 22, 2025

black smoke help

Noobz347 · Mar 22, 2025

sands9330014 said:
black smoke help

Run the checklist that is [again] posted in this thread.

Electrical 1989 mustang 5.0 runs rich and stumbles in closed loop

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