unclecameron

I've got an intermittent miss on a newly rebuilt engine and just rebuilt transmission, here are the steps I've taken to try to find it and fix it, still not working: Replaced plugs, wires, cap/rotor, air cleaner (K&N), checked wires, bolts, timing, cleaned injectors, checked injector wiring, loosened fuel cap, checked compression (great on all cyl's), checked fuel lines on engine side. It will just intermittently miss, then run perfect. It ran perfect for about 10K after I rebuilt the motor, then the transmission went and I had it rebuilt, and it works fine. Next I'm on to dropping the tank (because I have to rewire the trailer lighting pigtail) and looking for fuel problems. For awhile it would throw 0302 and/or 0306 codes, now it just throws 0300 codes every once in awhile, and then stumbles, especially when cold, otherwise it runs amazingly strong, just can't figure this thing out, so I thought I'd report back if I get it figured out and report back as I replace more things. In the meantime, let me know if anyone has places to check that I haven't gotten to yet, I guess this thing might be new by the time I figure it out, hope not.

unclecameron

Okay, an update after swapping many more parts:

The tank fill and return lines were partly unseated, so I put them back where they should be and clamped them, no change.

I thought it may have something to do with the throttle position sensor, so I replaced that, it ran a little better, and the tranny stopped jumping in and out of overdrive, but the engine still had the same problem.

At this point I started looking back at the crank angle sensor, since the computer started setting codes about the crank angle sensor, or cam sensor, so I replace the crank angle sensor for the 3rd time, since a friend said it wasn't putting out a good square wave when he checked it with an oscilloscope. It ran better, but then totally died about 1/4 mile later on the side of the road and I limped it back to my friend's garage.

Then I swapped out the pickup sensor underneath the rotor, which was the other cause of that code. It ran much better and I tried to drive it home - made it 5 miles and died on the side of the road.

At this point I figure there are only two things left to replace: the coil and the computer. Another friend who tears apart Jeeps had a '98 GC w/4.0 that a guy just rolled, and it had both a computer and coil. I got them from him, and installed both, now it runs perfect, er, well, for the first 30 miles anyway. Whether or not this fixes it, I hope it helps some other poor schmuck who's trying to troubleshoot the same problem, this one is tough to diagnose, if I'd taken it to a dealer it would've cost thousands of $'s I'm sure, which would've made it pretty crappy in the cost-effective-vs-buying-something-else department. We'll see how it goes

unclecameron

After a couple thousand miles, it started bogging when you accelerate quickly, even if you pop it in neutral, but only after warmed up. It's like you're driving down a dirt road and hit a 2 foot mud bog suddenly, but without the bog. Just gutless all of a sudden, decreasing rpm, just seems to run out of power. If you accelerate steadily it doesn't do it, only when it's quick. It seems to be intermittent, but hopefully it's just a matter of the new computer adjusting to the "new" sensors in this motor, which would've had different values than the one it came out of. Sure hope it fixes things, otherwise the next step is probably the wiring harness...grrr.

unclecameron

FIXED IT! This was a tough one. Basically, the computer thought it was O2 sensor, or something engine related, but it was lying.

What was happening is there was debris in the tank and a partially plugged fuel filter, so when the vehicle resonated just right, the debris would float up and clog the filter enough to starve the engine and set a code, but as soon as you shut it off, the filter vacuum would stop, and the debris would float back down to the bottom of the tank. Sometimes it happened when you hit a bump, but not always.

I replaced the fuel pump/filter assembly and it works fine now. I think if I would've had a scanner that actually watched my ODBII stuff real time while I was driving and had the ability to freeze/record that data for a few seconds before/after the thing started acting up, I probably could've found it sooner. But all I have is a code scanner/eraser. So that's the next thing on my list when something like this happens again, because those scanners are getting cheap enough to afford for weekend mechanics instead of $thousands from Snap-On, so I might check that out.

This problem took me around a year to fix, and my GC has lots of new parts now, so it oughtta run forever Hope this helps someone else with the same problem, this one was tough!

Bustedback

Nine times out of ten a P0300 (random misfire) is set due to fuel starvation.

dave1123

I sincerely hope you cleaned the tank!

unclecameron

It actually set a variety of codes intermittently. Anyway, I hope this helps the next poor guy who has the problem. It would've been nice to know about the fuel starvation aspect many months ago, but at least this might help the next guy.