Diagnostics? Random misfires.

OK, I'm at my wits' end here. Please help with suggestions and ideas.

1996 2-door XJ, 4.0, 5-speed manual. (~4" lift but still running little tires)

Problem:
Random rough running for the last 6 weeks- sometimes minor hiccups or hesitation, but at other times MAJOR rough running (obviously misfiring. No backfiring, though, just a "bogged-down" feeling to the engine most times. When it's really bad though, the shifter will literally shudder in my hand. Will happen most often at steady load in lower RPM range. But it also happens under acceleration, and at idle, just not as often as it does at steady speed.

What I've already done:

-Pulled codes from the computer: 300 and 306. Random misfires, and misfire cylinder 6.

-Plugs, wires, and coil were installed new 4 months ago (Bosch plugs, MSD other) Checked them first anyways, as well as my rotor and cap. Rotor was a bit dodgy, so I replaced the cap and rotor. All plugs when pulled look normal -grayish - not black not white. Strong spark when wires are pulled off and held near ground.

-Code 300 again.

-Fuel filter: replaced. Had some gunk in it that was weird, so I used an additive to kill the bacteria in the tank. (This was evidently not the cause of my issue, though. see my post here for more info http://www.naxja.org/forum/showthread.php?t=66781 )

-Code 300 again... (getting sick of this yet?)

-Fuel rail & injectors cleaned at a garage (hi pressure thingamabob)

-Again, same code 300, also 306 again.

-Fuel pressure is at 50 PSI steady, which is good. Check valve on the pump seems to be OK, too (bleed off test).

-Checked all ignition system ground points by eye and wiggle. They are fine. Checked misc other grounds the same way, none found bad. (Did find a bad connector on my headlamp lead, though.) No effect.

-Took it back to the mechanic, left it for the day with him. He pulled code 300 and 306 again. He tested a number of things, including the injectors for duration(? - or something) and I think the TPS. He didn't find anything in his poking around with it, and OF COURSE it didn't run rough while he had it. (Intermittentcy will kill me, i think...)

Now, oddly, I have had a few days of running OK. Still not perfect running, but I have had no notable rough patches. But, sure enough - this morning I had another rough running condition come and go.

Please, have mercy, and help me out with this. I am getting really worried that I will harm the engine by driving it around, but I don't have another vehicle.

Thanks...

Had a similar problem with the wifes 98. Found a bad plug.Plug looked ok,but running on the scope it would show a miss-fire,the injector would not fire either leading one to look for a problem there.But it was a bad plug.
Wayne

Loose the Bosch plugs, they don't seem to work good in a 4.0L

One possibility might be the Map sensor, which on my 95 seemed to contribute to similar symptoms - missing under steady load, especially long hills and freeway ramps. However, the main culprit was worn-out, and those symptoms as well as those codes really do sound like an ignition problem, maybe a bad plug, a bad wire, or crossfire. I would do a very careful check of the wires for broken carbon cores, defects, etc., and maybe just throw in a new set of cheap autolite plugs.

I just noticed something else in your post: fuel pressure at 50? Unless they changed it in 96 that's way too high. It should be about 39 at the pump, regulated down to 31. Better check that again.

Wayne & langer1:
OK, plug swap coming up on Tuesday (hurricane Wilma will hit us tonight, so no wrenching on the XJ until Tues.) If it works, I owe you a beer for suggesting such an inexpensive fix. And if it is the plugs, what an odd thing - they always worked well in my 4-bangers, so I just bought them out of habit for the Jeep.

Matthew:
"Fuel pressure should be 339 +/- 34 kPa (49.2 psi +/- 5 psi) at idle." Straight out of the manual. (Its MPI, not TBI like alot of the earlier XJs). And I'll check wires by swapping locations. I wouldn't know how to actually troubleshoot a wire, they all look fine on the outside...
About the MAP, I'll price it out- I seem to remember that being an expensive part. I think I may rtyt these other (cheaper) ideas first, before spending a lot on the MAP solution.

Thank you both for the input, and I'll post again after the hurricane passes and I get under the hood again.

Um, well, hurricane Wilma apparently had different plans for me.

Haven't ben able to work on the Jeep, been busy cutting trees, hauling debris, etc. The whole area is trashed. Power came back on about three hours ago, though, at 110, but no 220 - lines disconnected at the pole...

Just keeping this thread alive, anyone else have ideas? The Jeep's doing it again, and bad...

Check the connector on the number 6 injector. Make sure it is secure. Slowrider had this problem on his MJ. Crimped the connector a bit with some pliers so that it was a tight fit on the injector.

Nixt said:

Wayne & langer1:


Matthew:
"Fuel pressure should be 339 +/- 34 kPa (49.2 psi +/- 5 psi) at idle." Straight out of the manual. (Its MPI, not TBI like alot of the earlier XJs).

Click to expand...

I know they're MPI, as have been all 4.0's since their introduction in 87. I was surprised at the pressure because as far as I know all 4.0's from 87 through 95 share the same fuel pressure specs (214kpa or 31psi regulated, 269 or 39 unregulated).

So now I'm curious: does your 96 have a metal gas tank or a plastic one? I'm wondering if they changed the specs when they changed that.

About 2 years ago, my 95 Wrangler (4.0/Auto) did the same thing. Remove your distributor cap and rotor and check for any side to side movement in the distributor shaft. I could not find a bushing kit for this so I replaced the distributor with a remanufactured unit - problem solved. Hope this helps.

get rid of the bosch plugs and get some champions.

Matthew Currie said:

I know they're MPI, as have been all 4.0's since their introduction in 87. I was surprised at the pressure because as far as I know all 4.0's from 87 through 95 share the same fuel pressure specs (214kpa or 31psi regulated, 269 or 39 unregulated).

So now I'm curious: does your 96 have a metal gas tank or a plastic one? I'm wondering if they changed the specs when they changed that.

Click to expand...

Fuel pressure went up to 49psi on OBDII 4.0s (96 and later). They only have one line going to the fuel rail.

K

If you are approaching 100k miles, I'd consider replacing the upstream o2 sensor.

Dave

OK, back at it with the chase-down of the dread misfire...

All who reccomended plugs:
Replaced the Bosch plugs just now with Champions- test drive around the block - *still doing it*. (BTW - old plugs looked fine, a brownish gray - no black, no white...)

xjohnnyc:
Pulled all the injector connectors, sprayed 'em with electrical contact cleaner. They all seem to be making good contact, including #6.

fireman:
Ran out of daylight, but I'll check that distributor. I hadn't even thought of that... Could you describe how the 'play' felt when you jiggled it? There's bound to be a certain amount anyway, right?

Matthew:
corbinafly answered the fuel pressure question, and my tank is steel.

n5xl:
Do you mean the O2 sensor in the intake manifold or the first O2 sensor in the
exhaust manifold? (There are 2 O2 sensors on my exhaust, one in the man., and one post-cat) Oh, and I'm at 120K.

Keep the ideas coming, this is helping me even though the plug idea didn't work!

There is no o2 sensor in the intake.

Nixt said:

n5xl:
Do you mean the O2 sensor in the intake manifold or the first O2 sensor in the
exhaust manifold? (There are 2 O2 sensors on my exhaust, one in the man., and one post-cat) Oh, and I'm at 120K.

Click to expand...

He's talking about the O2 sensor in the exhaust manifold (pre cat). The PCM uses the signal from this sensor to adjust fuel injector pulse.

Oh yeah. Like Langer1 said, there is no O2 sensor in the intake manifold, but I have a feeling that you meant exhaust...

K

If it is still throwing codes, I would do the cheap troubleshooting first. I would swap #1 and #6 injectors. See if the code still says cylinder 6. If it does, then swap the injector drive connector between cylinder #6 & #5. Again see if it follows.

langer1 and corbinafly:
I mistakenly thought the 'intake manifold air temperature sensor' was also called an O2 sensor (hey, I'm new at all this sensor stuff...). Sorry for the confusion.

old_man:
Good idea. Will swap injectors before trying O2 sensor, to see if it is that injector. About the swap of the 'injector drive connector' - I'm afraid I don't know what that is... do you mean the clip that connects the injector, or something else?

Matthew (if you're still reading this thread):
You posted about a bad harness causing the same symptoms in an MJ in another thread. What are your thoughts (or anyone elses) about that for my issue?

And BTW -
A big thank you to everyone for working on this with me. I really appreciate it.

Will post again after trying the new stuff.

Just a qick update:

Since cleaning the injector connectors with the spray cleaner and re-seating the connectors well, I have not had another of the rough-run conditions happen.

I think it's resolved, so that's it for this thread.

Thanks a final time to all that helped!