1991 GMC Sonoma Logo

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Posted on Apr 06, 2009
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I have 91 sonoma 2.5 has fresh gas,good plugs,good cap and rotor new fuel filter starts but backfires and runs rough whats wrong?

  • 4 more comments 
  • blake_weathe Apr 06, 2009

    no firing order is good it runs on its own but leaks fuel from valve cover gasket now and i cleaned plugs fired truck up and ran it pulled plugs and they were so black with carbon what is wrong ?

  • blake_weathe Apr 06, 2009

    truck was low on fuel i topped it up ,i dont know where regulator is and o2 sensor may be broken would this cause problems , when i rev it up it catches and backfires still i have no money left it was at a garage and ran fine took it home and was gonna go out next night wouldnt run it runs now but horrible and sounds like **** but it will take all the fuel i can give it it was doing same thing as before im at wits end and need to figure this out asap? thanx for your time thundergod

  • blake_weathe Apr 06, 2009

    i havent had engine codes checked i have no way of doing that or getting there its throttle body and it was doing the same as before runs bad ,backfires and dies what gives?

  • Anonymous May 11, 2010

    I asked several questions...you did not answer all of them...I asked because I need to know, if I'm going to try to help you.

  • Anonymous May 11, 2010

    There is no fuel inside a valve cover...therefore it cannot leak fuel. (has to be coming from somewhere else if it's fuel.) What was the problem before you changed the items you listed...was it doing the same thing? Is it injected or carbureted? Have you had code check done? Most large parts stores will do this free. Ignore code for O2 sensor at this point...it definitely is seeing exhaust way out of range. You are either getting too much fuel or not enough air in there. (likely fuel) If injected have you checked pressure regulator? (also temp sensor)

  • Anonymous May 11, 2010

    Look on your intake manifold and see if firing order is stamped on it. Usually cyl #s are stamped on the ends of the runners near the head as well. sounds like you may have crossed a wire.

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Harvey N Tawatao

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  • GMC Master 3,854 Answers
  • Posted on Apr 06, 2009
Harvey N Tawatao
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Check the vacum line to your fuel pressure regulator. Keep it simple and start with that, dubble check your firing order again. Good luck.

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