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Misfire <1500 rpm

A

arsxj1

NAXJA Forum User
Location
Tampa, FL
My 93 4.0L XJ has had a misfire for some time now. I started to notice that it only misses under 1500 rpm's. Once it reaches 1500 it instantly smooths out and runs fine. Again I have replaced the cap/rotor, plugs/wires, seafoam in brake booster line/in tank, checked all vacum lines and fixed any leaks and cleaned pcv.
 
Have you removed the distributor for any reason?
 
old_man said:
Have you removed the distributor for any reason?
Click to expand...
No I havent removed it, but I cant speak for the previous owner, he replaced the head gasket right before I bought it. I just bought it 2 months ago and it has done this since I bought it. I thought it needed a tune up but nothing has fixed the problem so far. And this problem comes and goes. Sometimes it runs fine for a few days.
 
Have you checked for codes?

"Key-on, Key-off, Key-on, Key-off, Key-on"

.....in less than 5 seconds and then count the flashes from the CEL and check back with us or look at 5-90's signature and click on the "tech" link and you will find the OBD1 codes list there.
 
A code will only show if the check engine light comes on correct? The light has never come on unless it was from me unplugging something then I turn it off and restart it and the light is gone.
 
I have unplugged both the map and cts while the jeep was running. That in turn turned the check engine light on. Could that be why?

I thought maybee internal at first but that doesnt explain why every once in a while it will run great with no misfire at all and sometimes are worse than others. If it were a compression issue it would be a constant problem correct?
 
arsxj1 said:
I have unplugged both the map and cts while the jeep was running. That in turn turned the check engine light on. Could that be why?
Click to expand...

Yep, that will do it, clear the codes and drive it then re-check.

arsxj1 said:
I thought maybee internal at first but that doesnt explain why every once in a while it will run great with no misfire at all and sometimes are worse than others. If it were a compression issue it would be a constant problem correct?
Click to expand...

One would think. May be some slack in the timing chain or worn distributor. I dont think either one of those would cause a bad miss though.
 
ehall said:
every jeep 4.0 engine I've ever heard has a jagged idle, with a slight miss every few rotations
Click to expand...
Yea but sometimes it will purr like a kitten... Not a single miss and run great. I dont understand it, I know its not going to be as smooth as say a toyota prius lol but c'mon!!! It gets so rough that if I let off the gas it chuggs forward or reverse whichever direction im going and the whole jeep shakes,
 
I'm thinking distributor indexing.
 
Sounds like a compression related misfire. Might have an oncoming valve problem- intermittent soon to be constant. There's really no mistaking a single cylinder misfire for anything else once you've seen it a few times. If it gets worse at light throttle it could well be ignition.

I'd be looking at watching the primary O2 voltage to see if it shows lean (close to zero volts) when it acts up, classic misfire.
 
old_man said:
I'm thinking distributor indexing.
Click to expand...

The dist. can be turned a few degrees, correct? And, this could put the cap in a better relationship to the rotor, if indeed it's off a little. Although, the problem sounds like a tune up, I've had a bad spark plug wire cause the intermittent misfire.
 
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