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Unread 01-07-2005, 03:34 PM   #1
Tshark299
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1987 YJ Wrangler 
 
Join Date: Mar 2004
Location: Upstate NY
Posts: 2,341
4.2L Misfiring Under 1500 RPMs

So my 4.2 has run amazingly well for the year that I've had the Jeep so far until a few weeks ago and I can't figure out what's wrong.

The problem is that most times it just doesn't run smoothly anymore... when the engine is cold or I've just driven on the highway for more than 5 minutes, it idles like it's running on 5 cylinders (whole Jeep shakes). And then when I go to accelerate, it's rough until it gets above 1500 RPMs.

It's weird, in city stop-and-go traffic, it seems to run as smoothly as ever. But if I cruise at any speed, especially at say 40 MPH or more, for more than a couple minutes it will run real rough between idle and 1500 RPM afterwards, until I do more stop-and-go.

I messed with the carb mixture and regapped the plugs which seemed to help a little, but its still not gone. I don't know what else to do!!

It really seems like one cylinder won't fire sometimes, but I'm not sure why, or what to do about it. I really need this thing tip-top before I make the drive back to school in a couple weeks, so PLEASE HELP!!

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'87 YJ Laredo - 4.0L, AX-15, NP231, 4.10s: 3.5" BDS, 31" Muds

'95 XJ - Donated drive train to the YJ, Parted out the rest

'51 CJ-3A - Buick 225 V6: Painless Wiring Harness, Warn Overdrive & Hubs, 2" Lift, 31" Muds
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Unread 01-08-2005, 07:04 AM   #2
BamaJeeper
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Location: Alabama
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While it's idling pull eah plug wire and reinstall one at a time until (if) you one to one that makes no change (worse) in its idle. If you unplug one that has no change in idle it's either a bad plug or most likely that wire.
Do you have a timing light or a friend with one? Check the timing too. If the timing mark jumps around when you are checking with the light it means the timing belt is loose.
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Unread 01-09-2005, 09:22 AM   #3
Tshark299
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1987 YJ Wrangler 
 
Join Date: Mar 2004
Location: Upstate NY
Posts: 2,341
Yeah, my friend down the street has a timing light I always borrow from him. Thanks for the info, I'll check it out.

Also I should add for anyone else who might read this... the ignition coil, cap, rotor, and wires were all replaced in the late spring.
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'87 YJ Laredo - 4.0L, AX-15, NP231, 4.10s: 3.5" BDS, 31" Muds

'95 XJ - Donated drive train to the YJ, Parted out the rest

'51 CJ-3A - Buick 225 V6: Painless Wiring Harness, Warn Overdrive & Hubs, 2" Lift, 31" Muds
Tshark299 is offline   Reply With Quote
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