Running Rough!

Discussion in 'GM Powertrain' started by cwarrenburgess, Apr 28, 2010.

  1. cwarrenburgess

    cwarrenburgess New Member

    Hello all! I need some help and/or advice regarding my truck because its driving me mad!

    I currently have a 1993 Chevy Suburban with about 161000 miles on its original 454 engine. It has always run smooth and strong until the past few months. In December of last year, the fuel pump went out and I replaced it, along with the fuel filter, plugs, plug wires, alternator, belt and battery. Then starting around February of this year the questionable behavior began to appear.

    The engine has begun to run extremely rough, especially when I am not accelerating or putting a load on it. If it is idling, you can hear an occassional miss, but it does not become obvious until I am at cruising speed, around 1200 to 2500 RPM, then the rough, choppy operation commences. If I drop down into drive from overdrive and bring the RPMs up to above 2500 or accelerate aggressively, the operation is as smooth as ever. It only runs rough when cruising. In addition, there is a very strong smell of fuel coming from the exhaust when it is idling, as if the fuel is not being burned, but passing through to the exhaust or too much is being sent to the engine to begin with.

    I thought it might be electrical, hence the new plugs, battery, wires, etc. That had no effect. I even tried a new MAP Sensor, but that did not help either. I have noticed one other thing. Before I replaced the fuel pump, whenever I would go to fill up on gas, there would be a vacuum on the gas cap when I removed it resulting in a hissing sound. Now, no matter if I have spent a full tank, or just topping off, there is no vacuum pressure whatsoever on the fuel cap. Its as if there might be a compromise in the integrity of the tank area, allowing the sealed fuel system to vent otherwise somehow. When I removed the old fuel pump, I also had to remove a lot of gummy material that seemed to assist in the sealing of the tank. When I installed the new pump, I did not replace the gummy material, relying on the provided o-ring seal to do the job.

    Could air somehow getting into the system be causing the problem? To me, it would seem to be having the same effect as leaving the gas cap off. Please, help with any input you could. I am trying to clear this up on the cheap, but unfortunately may have to fork over a lot of cash if its the heads, etc.

  2. Eddie Z71

    Eddie Z71 Epic Member 5+ Years 1000 Posts

    Hi, and welcome to the site!:glasses: Glad to have you here!
  3. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Welcome aboard. You might have a bit better luck posting your problem in the correct subforum..
  4. bigdaddy77084

    bigdaddy77084 Epic Member 5+ Years 500 Posts

    Did you check the cap & rotor..
  5. retired2001

    retired2001 Epic Member 5+ Years 5000 Posts

    Welcome to the GMTruckClub!
  6. cwarrenburgess

    cwarrenburgess New Member

    I inspected the cap and rotor and they appeared to be ok. There was a little scorching on the rotor but the cap seemed to be in near new condition. Could the scorching on the rotor be causing a missfire? Would that account for the heavy gas smell?
  7. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    Yes the scorching could be causing the misfire, a heavy fuel smell indicates everything isnt being burned in the cylinders or the Cat convertor.
    Have you checked codes and had a diagnostic run on the engine while its running?
  8. NE1469

    NE1469 New Member

    Miss at 1500 RPM under steady speed

    Mine is doing the same thing. Around 1500 RPM you really feel the miss but if you accelerate you don't. Parked it Friday morning and did not drive it until Sunday afternoon and it did not want to go when you give it gas. Was sputtering bad. Went back to the house and left it running for about 10 minutes and took off again. This time it ran fine other than the miss at 1500 rpm. Did not sputter anymore when I was accelerating. Seemed like when it warmed up it was fine other than the original miss. Had codes PO420 and another one that I think was P0421. Both were related to cat or O2 sensor. Do you think CAT?

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