black sooty spark plugs and poor mpg

I am a new member and this is my first post. I need some help with my 4R.

I have a 1990 4R, with 3VZE engine, A/T and 4WD. My spark plugs and tailpipe are sooty black. Gas mileage is terrible at about 5mpg. Engine has all stock equipment.

This car passed CA smog in Feb. Around June, it started getting poor gas mileage.

I have installed new plugs. All 6 old plugs were dry, sooty black.

Compression check is good on all 6 cyl.

New air filter installed.

New MAF sensor installed. Ohm tests of the various connector pins says it’s OK.

New TPS installed. I carefully adjusted it per shop manual specs, using an ohm meter.

New gas pressure regulator installed.

Timing set correctly. 10 deg BTDC

Cold-start injector tests OK and does not leak. It was cleaned with fuel injector cleaner.

Cold-start injector switch seems OK, as it is not triggering except at startup.

No “check engine” light error messages.

New O2 sensor installed. This is also sooty black when removed for inspection.

EFI relay tests OK.

Fuses look OK.

Water temp sensor(back of engine block) tests OK at about 815 ohms

VSV(for FPU) tests OK. Airflow in correct directions and ohms test is good.

I sent the ECU off to be reprogrammed. They said it was bad, so I bought another used one. Problem persists. Maybe this ecu is also broken. Or maybe the problem is not the ECU. Part # 89661-35441

The engine acts like the ECU is holding the injectors open too long, causing a rich mixture. A sensor may be giving an bogus signal to the ECU.? Is it likely that all 6 injectors could be stuck “open”? They were rebuilt about 5 years ago.

At this point, I do not know what to do. Does anybody have a suggestion?

Thanks Guy

 

Welcome to the forum.

Sounds like a fuel issue to me but I'd still check for good spark with the new plugs at each cylinder if you haven't already.
You could check the voltage going to injectors. Then have them flow tested or replace them.

Was all this maintenance done at once? If not, did any issues pop up/ or get slightly better after specific maintenance was done?

 

When I suspected reduced mpg, a couple months ago, I pulled the plugs to run a comp check and ck plugs. At that time, all 6 old plugs were sooty black. Ran comp check and installed 6 new plugs and new air filter. Problem was not resolved, so I replaced the fuel pressure reg and then the MAF sensor. Problem persisted. Installed new TPS. Then sent ECU in for re programming. All this within the past 3 months. The car seems to run OK, but costs me about $1/mile to drive it.

Does anyone think all 6 injectors are bad? I can pull the intake plenum and check injectors. I have no issue with buying new injectors if it will solve the problem.

Does anyone think the ECU is the root-cause? Is there something I have overlooked?

 

One thing I forgot to mention was have you checked for vacuum leaks? A severe vacuum leak might make you run rich, which will eventually do damage.
Once you check out the injector voltage and flow check back in.

 

It seems like a vac leak, into the air plenum and undetected by the MAF or TPS sensors, would make the engine run lean.? Please correct me if I am wrong about this. I have checked and see no broken or damaged vac lines. The car seems to run OK, so I assume the injectors are all getting 12VDC.

The service manual says that black exhaust might be caused by a blocked fuel-return line or a stuck PCV valve. I checked the return line from the pressure regulator to the gas tank and it is clear. No blockage. I pulled the air plenum to get to the PCV valve. It looks like it might be blocked and the hose appears to be kinked. so I ordered a new one and the hose for it. Should arrive this week. While the plenum is off, I decided to pull some of the injectors and test them. They are not stuck in the open-position. All 6 injectors would need to be stuck "open" since all 6 spark plugs are sooty black. I tested the injectors under pressure and there is no leakage. They trigger properly with 12VDC and squirt fuel injector cleaner. At this point, I think the injectors are working properly and I will not replace them. I did rebuild these a few years ago.

If the PCV valve does not solve the problem, I will look more closely at the ECU. I looked at a couple videos of ECU's with bad capacitors.

 

Sounds like you're on the right track.

If you're able to fix it post it here. You may be able to help others once you find the cause.