I want to fix my car that is emitting rich black smoke.

[LILCBRA]

So this is what you're looking to fix, correct?


View: https://youtube.com/shorts/I9sbMwCajng?si=fRvGYAypcOJ4UV5c



View: https://youtube.com/shorts/KxnDDvf7qPc?si=Ye-XEVhJI9UApeY5

 

[sands9330014]

So this is what you're looking to fix, correct?


View: https://youtube.com/shorts/I9sbMwCajng?si=fRvGYAypcOJ4UV5c



View: https://youtube.com/shorts/KxnDDvf7qPc?si=Ye-XEVhJI9UApeY5

Yes, this car.

 

[LILCBRA]

From what I can tell by what you've shared, you have the correct fuel injectors. I think I would start by confirming fuel pressure at the fuel rail. If memory serves, it should be around 39psi with the vacuum hose disconnected from the regulator. If that is correct, it could be a little more in depth with the engine, such as worn rings.

I've watched a couple more of the videos you've posted, it seems that the engine runs well otherwise? Maybe it has a surging idle? But I was flipping through and didn't watch a full video of anything. The spare tire on the fender had me intrigued....

For anyone else who is curious, I believe this is what our friend is working on.

Mitsuoka Made Cruella De Vil a Car Out of a Fox-Body Mustang

Japan holds a lot of weight all across the automotive world. The small island somehow gives the world its most reliable and sensible vehicles like Honda

www.motorbiscuit.com

 

[Noobz347]

I have this injector.

Those are the correct injectors.

Question: Do you have a facility near you that can clean and flow-test fuel injectors?

 

[sands9330014]

Thank you. I have replaced the ECU and the injectors are correct so I don't know what to fix. I know there is a small leak from the EGR but will that be a problem?

View: https://youtube.com/shorts/Z7aqJIjRObc?si=JEPgIkHk2rHAgcyX

 

[sands9330014]

Those are the correct injectors.

Question: Do you have a facility near you that can clean and flow-test fuel injectors?

Thank you. I was away from home for a while. I checked the injector discharge before. I didn't check the discharge volume. I don't have a place to test the flow rate.

 

[sands9330014]

From what I can tell by what you've shared, you have the correct fuel injectors. I think I would start by confirming fuel pressure at the fuel rail. If memory serves, it should be around 39psi with the vacuum hose disconnected from the regulator. If that is correct, it could be a little more in depth with the engine, such as worn rings.

I've watched a couple more of the videos you've posted, it seems that the engine runs well otherwise? Maybe it has a surging idle? But I was flipping through and didn't watch a full video of anything. The spare tire on the fender had me intrigued....

For anyone else who is curious, I believe this is what our friend is working on.

Mitsuoka Made Cruella De Vil a Car Out of a Fox-Body Mustang

Japan holds a lot of weight all across the automotive world. The small island somehow gives the world its most reliable and sensible vehicles like Honda

www.motorbiscuit.com

I checked the compression ratio. It's at 9psi on all cylinders. It's fine when the water temperature isn't high, so I don't think it's a mechanical problem with the engine.

 

[sands9330014]

I also checked the thermostat. I put it in hot water and saw it open fully at 188F. What else could be causing this? I got code 42, so I swapped the left and right o2 sensors with new ones and still got code 42. It's not a sensor problem, but I suspect the injectors. However, the injectors are also new. It's great before the water temperature warms up, so I'm not too suspicious of the injectors. I'm having problems with the closed loop.
There are no problems with compression, and the fuel pump has been replaced, as has the filter.
Please help.

 

[gkomo]

Potential corrosion or clog inside the water pathways in the block?

 

[Noobz347]

• Code 21: ECT (Engine Coolant Temperature) Sensor Out of Range
  The ECT sensor tells the computer how hot the engine is to adjust fuel delivery. If it’s reading incorrectly (too cold), the ECU might enrich the mixture excessively, leading to black smoke. Could be a bad sensor, wiring issue, or a thermostat stuck open keeping the engine too cool.

This is a problem that needs to be addressed. It [can] be caused by air left in the system.

• Code 26: MAF (Mass Air Flow) or VAF (Vane Air Flow) Sensor Out of Range
  For 1991, most 5.0L Mustangs use a MAF sensor (2.3L might differ). If it’s over- or under-reporting airflow, the ECU could dump too much fuel, causing a rich condition and black smoke. A dirty MAF, bad connection, or sensor failure is likely.

This is a problem that needs to be addressed. You need to figure out why this sensor is showing out of range.

• Code 42: HEGO (Heated Exhaust Gas Oxygen) Sensor Indicates System Rich (Right Side)
  This confirms the rich condition—too much fuel on the passenger-side bank. The O2 sensor is picking up unburned fuel in the exhaust, which aligns with your black smoke. Could be a symptom of the ECT or MAF issues, or the O2 sensor itself might be sluggish.

This is more likely to be associated with the MAF sensor code. Fix that one first.

• Code 92: HEGO Sensor Indicates System Rich (Left Side)
  Same deal, but for the driver-side bank. Both sides running rich points to a systemic issue (fuel delivery or air measurement) rather than a single injector or cylinder problem.

Same as above.

• Code 18: SPOUT (Spark Output) Circuit Open or Loss of TACH Input
  This one’s ignition-related. The SPOUT signal controls timing advance. If it’s open or intermittent, timing could be off, leading to incomplete combustion and contributing to the black smoke. Check the SPOUT connector (near the distributor) or TFI module wiring.

This code is also [no good] and needs to be addressed. You must find out whey you're losing tach signal.


Where did your ECU come from? How old is it? Has it ever been serviced?

I think @General karthief posted the [Surging Idle Checklist] above. You need to go through that checklist LINE-BY-LINE without skipping steps.

@jrichker was a pretty smart guy (and had the patience of a saint). That checklist has solved a lot of heartache.

 

[General karthief]

I agree. The 02 sensors being rich, is the 'punch line' the other codes stack up to tell the story, the investigation starts.
Another thing I have encountered with similar symptoms was a bad alternator, gauge read charging but not enough at idle.
Do the checklist, you may have more than one issue.

 

[sands9330014]

Potential corrosion or clog inside the water pathways in the block?

In other words, I don't think so. There may have been some air left.

 

[sands9330014]

• Code 21: ECT (Engine Coolant Temperature) Sensor Out of Range
  The ECT sensor tells the computer how hot the engine is to adjust fuel delivery. If it’s reading incorrectly (too cold), the ECU might enrich the mixture excessively, leading to black smoke. Could be a bad sensor, wiring issue, or a thermostat stuck open keeping the engine too cool.

This is a problem that needs to be addressed. It [can] be caused by air left in the system.

• Code 26: MAF (Mass Air Flow) or VAF (Vane Air Flow) Sensor Out of Range
  For 1991, most 5.0L Mustangs use a MAF sensor (2.3L might differ). If it’s over- or under-reporting airflow, the ECU could dump too much fuel, causing a rich condition and black smoke. A dirty MAF, bad connection, or sensor failure is likely.

This is a problem that needs to be addressed. You need to figure out why this sensor is showing out of range.

• Code 42: HEGO (Heated Exhaust Gas Oxygen) Sensor Indicates System Rich (Right Side)
  This confirms the rich condition—too much fuel on the passenger-side bank. The O2 sensor is picking up unburned fuel in the exhaust, which aligns with your black smoke. Could be a symptom of the ECT or MAF issues, or the O2 sensor itself might be sluggish.

This is more likely to be associated with the MAF sensor code. Fix that one first.

• Code 92: HEGO Sensor Indicates System Rich (Left Side)
  Same deal, but for the driver-side bank. Both sides running rich points to a systemic issue (fuel delivery or air measurement) rather than a single injector or cylinder problem.

Same as above.

• Code 18: SPOUT (Spark Output) Circuit Open or Loss of TACH Input
  This one’s ignition-related. The SPOUT signal controls timing advance. If it’s open or intermittent, timing could be off, leading to incomplete combustion and contributing to the black smoke. Check the SPOUT connector (near the distributor) or TFI module wiring.

This code is also [no good] and needs to be addressed. You must find out whey you're losing tach signal.


Where did your ECU come from? How old is it? Has it ever been serviced?

I think @General karthief posted the [Surging Idle Checklist] above. You need to go through that checklist LINE-BY-LINE without skipping steps.

@jrichker was a pretty smart guy (and had the patience of a saint). That checklist has solved a lot of heartache.

Thank you.
The ECT diagnosis was performed when the outside temperature was low, so I think that is the cause. The sensor has already been replaced.
I am using two types of MAF, but both have the same symptoms.
Code 42 came up, so I tried swapping the left and right o2 sensors, and 42 came back again, so I don't think the sensor is faulty.
I removed the spout and checked the timing, so I think the error code was left at that time. I purchased and installed this computer. I checked it without the air cleaner installed, but is that the cause?

 

[elarm1]

What is your timing set at?

 

[sands9330014]

What is your timing set at?

I tried ignition timing from 8 to 12 degrees.

 

[Mcmahst]

Did you say you replaced BOTH sensors, the ECT and the MAF?

 

[sands9330014]

Did you say you replaced BOTH sensors, the ECT and the MAF?

Thank you. Yes, ECT, ACT and MAF have been replaced.

 

[sands9330014]

Thank you. I think I did a cylinder test, as mentioned in the youtube video above. KOEO 11
KORE 21 26 42 92 18 ? It seems to be displayed. The cylinder balance test shows 8, 7, 6, 5, 3? I have replaced the injectors, but is there something wrong with the injectors? There is no black smoke when the engine is in open loop, but when the water temperature warms up and the engine enters closed loop, black smoke is emitted. The amount of gasoline used up is very fast.

I will follow the steps in @General karthief guide to fix this code. I have visually checked the operation of the thermostat, so I will wait for the gasket to arrive and reinstall it. My current thinking is that there was air in the coolant and it was not in perfect condition. I will bleed the air and try a little harder.

 

[General karthief]

I will follow the steps in @General karthief guide to fix this code.

If I referred to the 'surging idle checklist' that's not my work, the late jrichker put that together with help from other members.
Great minds think alike.........my mind is not one of them

 

[AeroCoupe]

Are the valves adjusted correctly?