Hesitation, stumble, backfire

Discussion in 'GM Powertrain' started by jackmup, Jul 5, 2011.

  1. jackmup

    jackmup New Member


    I have a 97 5.7 Vortec 3500 Express
    I have been working on this truck I just bought for a couple weeks

    It had been sitting for two years, It was billowing pure white smoke, smelt like raw fuel, missing- just like the previous owner! The ad said "run's great"

    They replaced most of the sensors and the pcm using a junkyard donor w/ 4.3
    It was fun figuring that out, some of the parts look identical; Maf etc..

    I've got the miss corrected; bad Autolite double platinum plugs, why would you buy those? and use a junkyard maf?

    The Raw fuel was the pcm, When I asked her if she flashed it she handed me the flasher relay! that's for real no joke!

    I have replaced or tested almost every sensor on the truck and repaired the wires they messed up. Just installed a new fuel pump over the weekend.

    It now starts fine, idles smooth but when cold it stumbles off idle unless I feather the accelerator easy. I let it run for 2 hours and it behaved much better. when it gets hot and I stomp on it it still sounds like it's loading up but doesn't backfire through the tb. It also smells a bit rich with a tiny smoke but still but I don't know how much crap is in the exhaust. That was the longest I had let it run for. It will do a brake stand and WOT sounds fine. It is only when I accelerate just off idle, more pronounced when it's cold

    Repaired already
    all code issues
    raw fuel smell
    smoke plumes
    cooling system

    Problems I need to resolve
    I have No codes BTW

    Stumbles, hesitates, backfires when cold (but not if i go easy)
    Seems to load up if I stomp on it when hot, but does not backfire
    A/C does not turn on and is charged; where is the orifice located?
    front end needs work; quite loose under there have new idler arms but the upper part on the frame could be bad as well.
    Brakes? it's broken all over.

    Replaced or tested
    PCM, wrong one
    EGR, pintle stuck
    Knock sensor
    MAF, wrong one
    MAP, seal torn
    Distributer, worn gear, broken screw in housing for cap
    New plugs NGK, Autolites don't work out of the box
    They had new wires on it so I left them
    ECT and waterpump
    Fuel pump and harness
    tested the poppets they all open and close fine
    replaced the fuel reg and gaskets while I was there
    sprayed the whole engine down with carb cleaner looking for leaks no changes

    Going to test the IATsensor today
    tested the IAC already that looks good
    I don't think it's the TPS it responds fine
    There is a friggin snake hiding in the engine compartment that won't leave.
    so some things don't get tested at the same time as you would expect.

    MY thoughts right now
    The cats could be partially plugged but I don't have a temp gun to check them...yet
    maybe it just needs to run for awhile, runs fine at higher rpm

    The spider assy is some how messed up, How do you test that?

    I do not own a tech2 either but was looking at another program.

    anyway that's the size of it. IMG_2614.jpg
  2. ws6tony

    ws6tony New Member

    as for checking the spider assembly, I know you can test each individual injector with an ohmmeter, can probably google the values, or check a haynes or whatever. But its not a definite test, mine all checked out fine doing that, but injector 2 still ended up being shot
  3. jackmup

    jackmup New Member

    How did you find out it was shot?

    It's like a rubiks cube. this truck starts and idles smooooth and runs great.
    Just has that stumble if I get on it.

    maybe I need to look at it with a techII
  4. jackmup

    jackmup New Member

    replaced the Cap and rotor and ign coil with AC DELCO and it's almost gone.

    It's horrible like every thing I replace makes it a little better, I think when the PCM went it fried all the sensors it could.

    Only thing left is the spider. Maybe in the fall.

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