1998 GMC Jimmy 4.3 CPI w/155k
Went to leave work Friday and it was running horribly, dead miss on one cylinder. Scanner showed misfire on #3.
Pulled the plug and its clean, visually firing fine. (ran briefly with plug out and hooked up) Plugs/wires/cap/rotor have 50K all good quality stuff. Plugs are OE type Delco units good for 100k. (I won't push them that long).
Fuel pump 2 years ago, filter last year.
Drove slowly home on 5, about 17 miles.
Next morning it started and ran normally. Have put 200 miles on it without an issue. Scan tool shows everything perfect, like scary perfect for something with this many miles.
Current guess is a poppet valve in the spider got stuck then freed itself? I've had it the last 55K miles, always Top Tier gas and fuel filters every 20K. Currently digesting a tank of premium with a Seafoam chaser.
Who thinks I should do a MPI conversion? Maybe a different issue?
155k isn't all that much for the drivetrain but 50k is a long time for the cap and rotor on one of these. Also plugs and wires.
I wouldn't put too terribly much faith in misfire cylinder identification in this timeframe.
Lots of misfires in cylinder 3 makes me think failing fuel pressure regulator IF the plug was soaking wet. And IF the cylinder ID is accurate...
I'd want to see how big the sparks are when it's misfiring. Or look under the hood when it's dark (and the engine's misfiring) to see if I can see any sparks jumping from the plug wires anywhere.
The plug was bone dry when I pulled it. Had a normal light tan color.
Erf. How quickly can you get to the injector connector?
If you can get it to act up again, quickly check the injector resistances while the engine is hot. The pins are paired, and you don't really need to hustle to check that it's in "spec", one will probably be way different from the others.
If it's intermittent, it may be an injector winding opening up when hot, then making good contact internally again when it cools off. I've seen an injector test normally when cold and 95 ohms hot.
This is an odd failure mode for these because the windings stay fairly cool, but it fits the failure scenario.
No experience with a 4.3, but I've got 2 trucks with vortec 5.7's. So same thing +2
Any used one I pick up I just slam lower intake gaskets and a mpfi spider assembly in, unless its running good and obvious that has been done already. Just do it all at once, it'll take you a saturday but its not that bad.
Distributor gears wear on them, but that would be consistent. As said throw a new cap and rotor at it too, or at least inspect it. Vortecs seem to eat them for whatever reason, my theory is worn distributor and too much end play in the assembly.
In your shoes I'd just run it and if/when it happens again make a move. Did you happen to look at fuel trims?
A review of my maint notes shows cap / rotor / wires / plugs have 40k not 55.
Fuel trims are good.
Check the upper bushing on the distributor, just take the cap off and see if it has side play. Mine did and had random missfires. Also the spider could be leaking on that cylinder, which would pull the leaking fuel into the ports anyways. Get the 2001-up improved spider with the injectors at the ports, direct replacement. That made a big difference on my 99 4.3L. On a side note, all 1996 up are MPI with 6 injectors, not a single injector like the 95 and earlier ones.
I'd still get a visual on the cap/rotor. My '96 had a cap/rotor on it with about 30k, I pulled it while diagnosing a no start condition and I was shocked how terrible it looked for such low mileage. All AC Delco stuff too
