Question about GMC Jimmy

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2005 Jimmy 4X4, 4.3L, P300 multiple misfire. Have had on scanner, misfires Cylinders 1 and 4. Have changed: both spark plugs and wires, coil and coil lead, distributor cap and fuel filter. Engine does not miss or show any signs of any problem until I exceed 55 MPH (93 KM/Hr)then within 1 mile engine light comes on and shows multiple misfires. Can drive hundreds of miles with engine light off until I exceed 55 MPH.

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  • Beretta01 Aug 01, 2009

    Have changed air filter and used injector cleaner in fuel as well, situation did not change
    .

  • Beretta01 Aug 01, 2009

    BTW vehicle has 27,000 km on it so should not be an intake problem.

  • Beretta01 Aug 01, 2009

    Engine light goes off by itself once speed reduced and computer goes through its next cycle check. Fuel pressure in rail shows about 45 - 50 psi usually. Will have to check it with scanner while running highway speeds in the next couple of days. Vehicle uses standard type distributor cap and rotor. Random multiple misfires code always shows cylinder 1 and 4 which should vary if the fuel screen or pressure irregular I would think. With scanner (MT2500) hooked up and engine running at 2,300 rpm in drive with brake on... shutting off injectors using scanner show injectors working properly as well.

  • Anonymous Mar 23, 2014

    misfire cylinder 1 and rough idle with engine light

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Try some injector cleaner in the fuel--there might be some water in the fuel also. Check the air cleaner.

Posted on Aug 01, 2009

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  • drallim Aug 01, 2009

    You need to monitor fuel pressure during driving above 55 MPH. If it drops, look for weak fuel pump or plugged sock filter inside the tank. Will this problem show up just running the engine at a good clip? Any failing sensors should show on the computer scan. Does the problem go away if you slow down below 55? Does the distributor use a magnetic trigger to fire the plugs?

  • drallim Aug 01, 2009

    You didn't say whether the engine, by itself, will display the faults if running at 55 MPH-RPM (no load). Is there any connection between the speedometer and the computer as in cruise control? Check for slop in the distributor shaft upper bearing. How did the problem show up--gradually over time, suddenly, always there, after some repair? If the problem only shows on the road, try disconnecting the speedometer at the transmission.

  • Psastocks Aug 26, 2010

    I have the same problem, issue started after repair and replacement of the engine. Old engine did not have the problem. Engine runs fine no sign of misfire, soon as I exceed 110km/h the engine light comes on, code is multiple misfires.

    The new engine and work is still under warranty, but I am wondering what the problem can be. The mechanic has changed the dist cap and the new engine only had 45000 kms

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