1990 V2500 Suburban - running way too rich

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by Boldt, Aug 28, 2009.

  1. Boldt

    Boldt New Member

    I have a great old 1990 V2500 that all of a sudden went really rich. I drove to work fine, parked, came out at the end of the day, and it immediately flooded when I tried to start it up.

    Trouble codes at that time were:

    15: Coolant temperature sensor, low temperature
    34: MAP sensor voltage low (faulty wiring, now working fine)

    and I believe:
    45: Oxygen sensor - rich exhaust

    Visual inspection of fuel injectors revealed a great deal of gasoline being consumed. Removing either fuel injector connector, the engine was able to start but obviously ran a little rough. It is apparent that the ECU is sending too much voltage to the injectors. Whether this is directly the ECU or if it's a failed sensor affecting the ECU remains to be seen.

    I assumed it's the MAP sensor because a low voltage is consistent with full-throttle operation, so it seems possible that's what caused the fuel injectors to open up.

    After replacing the MAP sensor and the wiring to it all the way to the ECM, I reset the ECM by pulling the battery for a while. I brushed the spark plugs, reconnected the battery, started it up, and ran it at about 1/2 to 3/4 throttle for a couple minutes to clean it out. I imagine considerable soot was in the engine after driving 3 miles home with a trailing cloud of black smoke. Probably not a good idea to operate it in that condition but I did what I had to do. The exhaust was good and black at first, but it cleared up after ~ 30 seconds or so.

    After releasing the throttle, the engine seemed to be idling normal and smoothly. Almost immediately when I attempted to drive, it shuttered, killed and was flooded out again. NO trouble codes this time.

    I theorize that there's something screwy with the ECM, maybe it needs to be replaced. It's sending too much voltage to the injectors. But ECMs expensive and I'm not convinced that's what's wrong. So at this point, it would help to narrow down:

    - Which sensors have the ability to affect fuel injection?
    - If these sensors are not to blame, what problems can cause these sensors to give an abnormal reading?


    I swapped in some spare injectors which behave identical.
    Last edited: Aug 28, 2009
  2. unplugged

    unplugged Epic Member 5+ Years 1000 Posts

    Don't know if this is your problem, but check the fuel pressure. Should be 9 -12psi I think. I had a case where the engine was running rich. Black smoke out the tailpipe, fouled plugs etc. Measured the fuel pressure on the feed side and it was 28psi. I moved my test point to the return line and 28psi. Hmmm..checked the return line and it was kinked from a 4x4 adventure. Replaced the line and presto problem solved.
  3. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    Probably the coolant temp sensor. If it tells the PCM the engine is cold, it will send more fuel. Another guy had one go bad, it was send a temp of like 100 degrees below zero.
  4. Boldt

    Boldt New Member

    Thanks for the replies. I tested the coolant sensor with an ohmmeter and indeed it is infinite resistance like you said, 2COR517. I took a 2.2 k-ohm resistor and inserted it directly into the plug to simulate a temperature sensor reading of about 86 degrees F (30 deg C). The engine still ran rich.

    This is at home so I don't have a way to test fuel pressure right now. That's a good idea, though. The vehicle was recently in an accident so I don't doubt something could be pinched. The rear driver side was hit and the truck proceeded to impact the next car in the parking lot on the opposite side. This was enough to pop the battery out of it's tray and dislodge the spare tire...

    Anyway thanks for the help. I'll go inspect the fuel line, but if anyone has any other ideas, I'm all ears.
  5. Boldt

    Boldt New Member

    UPDATE, more clues

    I just started it up and it ran rich and rough, so I accelerated slightly, and all of a sudden it started running normal, as if someone flipped a switch. The engine accelerated slightly on its own and I let go of the accelerator. It warmed up and idled like it should. The engine was running normally with no error codes reported. Because of a lack of error codes, maybe it's safe to say that it was not running in a compromise mode? Well... who knows

    I was able to move it about 15 feet to a different location so it was out of the way. Everything seemed normal, so I turned it off and tried starting it again after ~ 10 seconds and it was rich, spewing black smoke again. I shut it off and walked away.

    Also I don't see any problems with the fuel line but I can't test the actual fuel pressure right now.
  6. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    That smoke at startup could be due to a leaky injector. It could be leaking down the residual pressure by spraying fuel into the TB/intake which would all get burned off when the engine is started. Fuel pressure test should reveal the leaky injector.

    Autozone usually stocks the tester with the adapter you need. Try to get some spare O-rings for the fuel lines too.
  7. Boldt

    Boldt New Member

    I doubt that there is a problem with the injectors because I replaced the whole top half of the throttle body with a spare. This replaced both injectors and the regulator. This did not affect the behavior of the engine.

    This problem is intermittent because there have been a few times I thought it was fixed, only for the problem to return. One time I hit a bump in the road and it made the problem come back.

    I'm kind of stuck for a while before I can get to the auto parts store. So here are my 2 questions until I can get a tester:

    • If the pressure turns out to be too high, what could cause that? Can't be the regulator diaphragm because I replaced it. Is the only possibility a pinched return line?
    • If the fuel pressure turns out to be normal, what causes the ECM to send a bunch of gas to the engine?
      The only thing I can think of is the TPS which tests good. I'm thinking bad wiring to the TPS or just a bad ECM?
    Last edited: Aug 29, 2009
  8. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    Remember that just because you replaced a part, doesn't mean it's good. Especially used parts. Too high of a pressure could be from a defective regulator or pinched return line. Too low of a pressure could be defective regulator, clogged fuel filter, weak pump.

    The fact that a bump can make it go away seems like it would be electrical in nature. Possibly a bad sensor/connector, bad ground strap, etc.
  9. bucklti

    bucklti New Member

    Replace the 02 sensor.
  10. Boldt

    Boldt New Member

    Yes, it was the O2 sensor. She lives on once again.

    Thanks for your help you guys.

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