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Stumped on a misfire

autolaneautolane Member Posts: 1
edited August 2015 in GMC
Hello, I am at my wits end tearing my hair out over this. Been a mechanic for quite a few years now but i cant seem to figure this one out. Its my personal vehicle and time is limited to get it finished.
I have a 1999 GMC Jimmy 4.3
It has come a long ways since i first purchased it but theres something left i cant seem to figure out on my own. It didnt run when i got it, however with alot of work its going now.
So far i have replaced:
-Spark plugs and wires. (Plugs AC Delco / Wires i believe are house brand since they were out of the ACDelco brand wires)
- Cap and rotor (both AC Delco)
- Fuel pump
- PCV valve
- Intake Airtemp Sensor
- Catalytic Converter
- Water pump
- Cleaned the MAF very well and it works properly.
- Few other things...
I will note that when i got it, it had a blown head gasket. However i fixed that with two seperate rounds of Blue Devil Head Gasket sealer. One time to fix it, and the second time just to be sure.

Problem is that i cant get it registered because i cant get the check engine light to go off. only thing left that comes up varies. Sometimes its either a P0300 (multiple cyl mifsire), or a P0302 (Cyl #2 misfire). But never at the same time, its always one or the other.
The Check engine light stays on solid, but then when i get up to around 30 to 40 mph the light will start flashing. I am leaning towards it possibly being the Spider fuel injector malfunctioning. Because besides that the damn thing runs perfect!
Any help or input would be much appreciated thank you!


  • Options
    imidazol97imidazol97 Member Posts: 27,230
    edited August 2015
    Pickup module in the distributor? Or wobble/wear in the distributor rotor? Ignition coil?

    Is this an engine where the distributor shaft wears and the wobble allows too much gap
    for the ignition pickup module to sense the position of the rotating piece that triggers
    the spark?

    I have no experience with these other than listening a lot to car repair call in radio shows
    through the years and recalling some of the problems the experts pointed out to the callers.
    I believe it was Fords where the pickup modules deteriorated with age and heat.
    But I suggest asking around about the distributor and module for your Jimmy.

    Good luck.

    2014 Malibu 2LT, 2015 Cruze 2LT,

  • Options
    thecardoc3thecardoc3 Member Posts: 5,776
    Being able to change parts doesn't make one a mechanic, and believing in "head gasket sealer" helps even less.

    What testing have you done? Have you connected a full function scan tool and monitored live data during the misfire? If so, which cylinders are being reported as misfiring in the live misfire data? What are the long term and short term fuel trims at idle, cruise, and especially when the misfire occurs? Have you performed a compression test, and especially a running compression test on the cylinders where the misfire is reported at the engine speed where the misfires occur? (That BTW is best done with a pressure transducer and digital oscilloscope). You do still have to confirm the engines mechanical health before you spend much time looking elsewhere.

    Have you confirmed that you really do have good spark to the plugs with an HEI spark tester like the ST-125?

    Being suspicious of the injector assembly is fine, what exactly do you think could be going on with it? From there if what you suspect is in fact an issue, you have to ask yourself what other symptoms would that cause under varying operating conditions? Are those other symptoms occurring and helping confirm if there is an issue, or are they not occurring and making the suspicion less likely?

    How have you proven that the head gasket sealer actually worked? (very doubtful that it has)
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