Need Help Troubleshooting a 4.3L Vortec

Discussion in 'Trucks and Trailers' started by machine, Nov 13, 2000.

  1. machine

    machine LawnSite Member
    from OH
    Messages: 101

    I just got done putting a 4.3L engine back in a 96 GMC Jimmy 4x4. I am having a problem toubleshooting it. The engine fires but won't run. The fuel pumps runs, but the engine seems that it is not getting fuel. The fuel fliter is new and there no cut-off relay to reset like the fords have. So I don't know where to begin since there are some many damn censors. Any help would be helpfull in getting this truck to run.
  2. John DiMartino

    John DiMartino LawnSite Silver Member
    Messages: 2,555

    Did this vehicle run before you pulled the engine-is the engine that went in the same one,or out of a different truck?it sounds like the computer is not seeing the tach signal after it fires,so its not adding fuel.Just a wild guess,go over every connector and make sure they are all hooked up.Make sure the engine grounds are all hooked up,most post-engine swap problems are simply ground problems,pinched hoses,lines.If this doesnt work,you need to get a spark test,and get a noid light on the injector and check for fuel,If you dont have a clue-take it to a good shop-the 1 hr labor they charge to take a look at it might be worth it.The more info you give the shop-the better they will be able to get on the right track to finding it.
  3. machine

    machine LawnSite Member
    from OH
    Messages: 101

    John, the engine was not in running condition when I pulled it. It is the orginal engine which was involved in an accident when I bought it. The reason I had to pull the engine was because the oil pan was cracked, go figure. I did a compression check and all cylinders checked out.

    I'm concerned that impact caused by the wreck, may cause the computor to shut down certain sensors nessary for starting the engine.

    I will go over every connection today, but I'am puzzeled to where the sensor for the tack signal would be located.

    Thanks for your advise. If I don't get it started today, I will probably take it in.
  4. mike reeh

    mike reeh LawnSite Member
    Messages: 229

    well dont hold me to this but the fuel pump should be pressurizing the system whenever the key is "On"

    what do you mean the engine fires but wont run? Probably the easiest way to make sure its a fuel problem is buy a can of starting fluid and have someone crank the engine while you spray ether into the intake.. if its capable of running, it will run while you supply it with ether. That would seriously narrow it down.. If you can connect a fuel pressure gauge, that would be great.. if not, a noid light to make sure the injectors are pulsing would help immenseley and you can pick em up and many auto parts stores, especially specialty ones..

    good luck and let us know how it turns out!!

    mike reeh
  5. John DiMartino

    John DiMartino LawnSite Silver Member
    Messages: 2,555

    Machine-why did you pull the engine to put an oil pan on it?I havedone them by dropping out the aluminum center section(axle assy)it only takes about 45 min to get it all down,the the oil pan is cake to get to.I know you have the one that attaches to the trans to,I havent done one of them yet,maybe its in the way.If it was in an accident,the impact might have broke the inside of the mass-airflow sensor,try unhooking it,sometimes they will fire up and run in backup mode if you unhook the sensor,since the computer sees it as being so out if range it ignores it,and will allow it to run on the other sensors-just not as good as it should.
  6. machine

    machine LawnSite Member
    from OH
    Messages: 101

    John your right about dropping the front axle and transfer case, but the main reason I removed the engine was that the frame needed to be straigthten. This vehicle has been expensive learning expirence for me since I don't consider myself machanic, just someone who likes to toy around with these kind of things. This truck should of went straight in to the packer.
  7. John DiMartino

    John DiMartino LawnSite Silver Member
    Messages: 2,555

    I've fixed a few totals myself,they are not for the inexperienced.Good luck getting it running.
  8. machine

    machine LawnSite Member
    from OH
    Messages: 101

    Guys, thanks for your input so far, but I am still trying to toubleshoot it before I have to take it in. I shot the code P108 with the scanner and then tested the MAP (Manifold Absolute Pressure) Sensor. It was bad, so I replaced it and erased the code. No more codes came up, and the engine hasn't changed. I crank the engine it seems to fire reaching 500-800 RPM but will not start running, even with starting fluid.

    I thinking now that I have a bad computor, is there away to test it?

    Also I have the Delco II sercuity system for the radio. Is any way the system would not let you start the vehicle is the radio was messed with?
    The manaul reads that if the radio is taken it will render it inopperable.(assuming that is only the radio) The radio does work. One problem is that I don't have the password codes to arm or disarm the radio.

    Any More help would be Apreciated, Thanks.
  9. bandit_36

    bandit_36 LawnSite Member
    Messages: 49

    Check the crank sensor for cracks. It's the most common thing to happen in a crash. The radio being locked cannot keep the truck from running. E-mail me & I will tell you how to unlock the radio.
  10. Fierospeeder

    Fierospeeder LawnSite Member
    Messages: 51

    have you tried pressing the pedal a very little, and hold it. If it starts, your iac "idle air control" can be bad.
    just something to try

    also, if you have a timing light, try going to each spark plugwire and see if each plug wire is emitting a light from the timing light.

    If some of that are not emitting a light, its possible your ignition module is bad.

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