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post #1 of (permalink) Old 04-12-2003, 05:45 AM
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Misfire problem with 88 YJ (Long/detailed)

I have been searching all over the net for a resolution to a misfire problem. Here's the problem:

When cold, the Jeep starts and idles for about 2 seconds then starts missing/misfiring at idle. If I try to drive, it doesn't want to take full throttle and starts missing and popping/banging out the intake, and won't pull over 2000 to 2500 rpm.

It also misses/misfires at idle after about 2+ miles of driving into the wind. It runs great however once it's warmed up, and pulls hard and smooth. Just throws a fit at idle unless I let it sit there for about 4-5 minutes.

If I start it cold, and I drive for 1-2 miles only, it misses BAD at idle and sounds like a clanking sync'ed with the misses, and that clanking is coming from under the jeep and not the engine. This totally goes away if I let it sit for 5 minutes then start it, then I only get the 2+miles in wind problem.

I am getting spark on all cylinders.
I have not done a compression check.

88 Jeep Wrangler 4.2L I-6
Howell TBI
8' Before Top Dead Center Timing
New plugs
New ignition module
New Coil
New Distributor Cap, rotor, and magnetic pickup


I started with Carter BBD Carb and removed the emissions and did a "Nutter" bypass. Ran rich, but fine.

Took a trip from Florida to Oklahoma, then when I got to Oklahoma I realized that I made the entire trip at about 10' AFTER Top dead Center timing. (The vacuum canister on the distributor was hitting the motor mount and turned the distributor to fix that problem, rewired and replaced plugs, and retimed to 8'.)
It ran fine from there to California.
It started giving me fits on starting, so I decided to get the TBI to resolve that problem (thinking it was the carb) and to pass emissions.
I replaced the ignition module, got a HD coil for it and replaced the plugs again and regapped to .040-inch, and got new distributor innards.

I still have this problem, and it is MUCH worse when it is below 40-50 degrees outside. If it is warm, then it almost goes away, but not fully, and I still get the "2+miles into the wind" problem, and it still gives me the problem if I drive only 1-2 miles (bad missing and popping/banging from intake.

I have heard that a catalytic converter meltdown could cause too much backpressure on the engine, and that might also be the clanking from under.
I also found something about the timing chain jumping which would affect idle.

Thanks for any help you could provide to save money on replacing good parts. [img]images/graemlins/laugh.gif[/img]
Tom
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post #2 of (permalink) Old 04-12-2003, 08:34 AM
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Re: Misfire problem with 88 YJ (Long/detailed)

1. Welcome to the forum.
2. Thanks for filling out your user profile.
3. Read the New Posters Message at the top of the forum.
4. Participate more often!
5. Two brains are always better'n one!
6. Have you pulled the ECM codes?
7. I'd be replacing the ICM a 2nd time...
8. This doesn't exhibit timing problems...
9. Have you had the back-pressure checked at a Muffler shop?
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post #3 of (permalink) Old 04-12-2003, 05:30 PM
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Re: Misfire problem with 88 YJ (Long/detailed)

I have not had the backpressure checked at the mufflershop yet, and the ECM hasn't given any codes. Well...only the code that says it's working properly. [img]images/graemlins/smile.gif[/img]

Tom
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post #4 of (permalink) Old 04-12-2003, 09:27 PM
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Re: Misfire problem with 88 YJ (Long/detailed)

The backfiring through the intake when cold sounds like it is running lean. I'm not familiar with the Howell TBI setup you are running. What does it use to enrich the fuel mixture when the engine is cold ? Most fuel injection systems use a coolant temp sensor that screws into the engine directly into a water passage. The coolant temp sensor may telling the ECU that the engine is warmer than the actual temperature causing a lean mixture until it warms up. That would be the first thing I would check. You would need to get the specs on the sensor from Howell and check it with an ohm meter. They ussualy have a given resistance at a specified temperature. You may have a bad sensor. I have also seen problems caused by using teflon tape on the sensor threads. This causes a poor connection to ground on the engine. The sensor must have a good ground to the engine to operate properly. I have also seen sensors that were damaged internally from being overtightened. I would rule out an exhaust problem as you say it runs strong once it is warmed up. If you had a restricted exhaust it would still run poorly even when it was warmed up.
post #5 of (permalink) Old 04-13-2003, 01:52 AM
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Re: Misfire problem with 88 YJ (Long/detailed)

ALready checked all teh Howell Sensors, and been through 2 ECM's and 2 oxygen sensors. When cold, you can hear the injectors firing much faster than when warmed up, and they go to normal operation once the engine hits about 140 degrees or so. My temp sensor runs at 160-190 during warm operations. This is when the engine seems to run better.

The engine can still be warm (120-170) and it will still give me the problem after I've started and driven about 2 miles. It usually takes me about 5+ miles of driving to get it running normally, but then it still give me problems when idling.

When its running strongly, I meant not misfiring (sorry for that) and it pulls good, not not the most impressive thing. It also seems to lag here and there while accellerating too.

Tom
post #6 of (permalink) Old 04-13-2003, 08:02 AM
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Re: Misfire problem with 88 YJ (Long/detailed)

1. Don't ignore the ICM.
2. Yes, you did change it, but...
3. It's responsible for timing...
4. It's responsible for initaion of Spark and the fule pulse.
5. The symptoms are consistant with a non EFI system,
6. That's got a bad ICM.
7. This part is famed for being bad out of the box...
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post #7 of (permalink) Old 04-13-2003, 08:49 AM
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Re: Misfire problem with 88 YJ (Long/detailed)

I would check to be sure your exhaust manifold bolts are torqued down and all your vac lines are connected and in good condition. Sounds like you've got a vac leak with a cold block that goes away when the metal gets hot and expands. I don't know where you live, but it's a common "spring thaw" problem after a northern winter.

Good luck,
Michael
post #8 of (permalink) Old 04-13-2003, 11:56 PM
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Re: Misfire problem with 88 YJ (Long/detailed)

This EFI system does not control the timing. I will call Howell about it again though, and see what they have in mind. Although this problem was happening before I upgraded to EFI (this problem is one major reason I did) i am not ruling out a bad ECM or chip.

As far as the gaskets go, I was planning on replacing both the intake/exhaust gasket and the head gasket anyway, so we'll see what happens there.

Tom
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