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Erratic Misfire

Enderz0r

NAXJA Forum User
Location
Seattle
Hey, I have a '91 cherokee with a 4.0L. For the last couple of weeks it's been Misfiring pretty erraticly, sometimes it will run just fine, and other times it misses so much I can't beleive I'm driving it. It ususally Idles fine (but not always) and it seems to happen most frequently just as I'm starting to accelerate, it'll also hesitate at freeway speeds ....haven't seen a check engine light since this started happening. Sometimes when it's missing really bad, it seems like it's not shifting when it should be (but I could be wrong)

I've replaced the distributor cap/rotor and the plug wires (the plugs look fine). I replaced the lifters about 2 months ago. Six months ago (when i bought the truck) I replaced the fuel filter, air filter, oxygen sensor and throttle position sensor.

Thanks for any help,
Jordon
 
you might have a bad wire even though they are new.I put a new set of cheap wires on my a few years back and had the same intermitten prob.
At night time in a really dark spot I popped the hood and started moving plug wires around.I saw the number two wire shorting out against the block and said to myself "well there's your problem"
It drove me nuts for a month trying to figure out what was wrong because it would only do it once in a while....
 
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Were the TPS and O2 sensors new that you installed 6 months ago?

IF yes, check for a wire in the TPS or O2 circuit that has lost insulation, may be is grounding out as it moves around. Also check the battery cables and clamps, see that they are not loose or corroded.

Maybe a cracked exhaust manifold throwing off the O2 sensor. Has the gas mileage dropped since the problem started.

X2 on rechecking for a bad HV wire, even if it is new.

If all else fails, might be a CPS going bad, but hard to be sure before it fails.

Any other symptoms or problems?
 
I bought the wires at schucks and asked for the best ones they had it was a $30 set of BWD premium wires. are these cheap wires?

as I said before I just replaced the TPS a few months ago, and there's no codes saying it's bad.
 
yeah, the sensors were new, I'll check the insulation, battery looks fine tho.
I haven't really checked the gas mileage since it has started, but it's been doing it more and more, so it seems like it has to be getting worse mileage. Is CPS crankshaft position sensor? does it seem weird to anyone that i'm not getting a check engine light? .... and the plug wire isn't just new, it's been replaced since the problem started, but i'll check.


the only other problem I could think of that might be related would be that there's a rattling noise coming from the catalytic converter, could that be throwing off a sensor?

I'll list the other problems anyways... ( it's been doing these things since I bought the truck 6 months ago)

i can turn off the igniton and remove the key w/o putting it in park

it won't go in to 4wd, the shifter moves and everything, but it stays in 2wd, it's like i didn't move the lever.

when I turn the heater on High i'll hear something slam shut and it'll stop blowing into the cab, I'll hear it "slam shut" sometimes when i'm driving fast even when it's set pretty low. I have to come to a stop, turn the blower down and turn the heat off and on before it'll work again. and it'll only come out the defrost no matter where I set it....
 
Enderz0r said:
Is CPS crankshaft position sensor? YES. does it seem weird to anyone that i'm not getting a check engine light? NO. .... and the plug wire isn't just new, it's been replaced since the problem started, but i'll check.

the only other problem I could think of that might be related would be that there's a rattling noise coming from the catalytic converter, could that be throwing off a sensor? AH, HA!!! Bingo, Winner!!!!:eek: The catalyst material has come loose, broken, and is periodically falling into the right possition to choke off the exhaust!!!! You need a new cat converter.

I'll list the other problems anyways... ( it's been doing these things since I bought the truck 6 months ago)

i can turn off the igniton and remove the key w/o putting it in park

it won't go in to 4wd, the shifter moves and everything, but it stays in 2wd, it's like i didn't move the lever.

when I turn the heater on High i'll hear something slam shut ( AC damper door is being actuated by vacuum from the AC/heat controls) and it'll stop blowing into the cab, I'll hear it "slam shut" sometimes when i'm driving fast even when it's set pretty low. Sounds like a controller valve (actuator is its proper name) vacuum hose is loose and leaking, somehwer in the AC dash or near the evaporator valve under the hood, but not related your posted problem. I have to come to a stop, turn the blower down and turn the heat off and on before it'll work again. and it'll only come out the defrost no matter where I set it....Definately a loose or cut and leaking vacuum tubing, been there done that, but should not be part of your engine miss problem, unles the hose from the MAP to the throttle body is loose, but when that happens it barely accelerates and back fires when you gas it
 
alright, well, good to hear you sound so sure that is the problem.... .. do know where I can find a cheap one? and is there anything wrong with buying the cheapest catalytic converter I can find?
 
I got mine from an Ebay store (Magna something Flowmaster) for about $50. Had it welded in at the local muffler shop for about $20. My old one was already welded in. THe OEM has a flange mount that is worth keeping if you still have it (Makes manifold and tranny work more accesable as I recall), but the flanged Cat converter is more expensive, about $80 - $100 IIRC.

My $50 one works fine, passed emissions tests (254,000 miles) at 5% of the emission limits.

Just Google search "EBay catalytic converter Jeep" and the year.
 
was it missing before the tuneup?
a shorting spark plug, broken porcelain around the the center electrode or a bent center electrode can do this

Check under the distributer cap, youll likely find a burned contact if a spark plug is shorting
this might have happened when you put the plugs back in, its all to easy damage a used plug.
 
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92DripCherokee said:
good time for a compression test.

His problem is not at idle and it is random, not a repeatable problem. He has a rattle in the cat converter, i.e the honeycomb has come loose. When that happens it bounces around inside the housing and periodically blocks the exhaust as it bounces around, which kills the power, take off, and makes it stumble.
 
on the way to work this morning it was hesitating especially bad, like every 3 seconds.
I was talking to a friend at work about it and he said his cat was making a similar noise (but not effecting the way his truck ran) and then the noise just went away.... it gave me a crazy idea (i'm not sure if this makes sense, but it did in my head because I've never seen the inside of a cat) on my way home I tried to rev the jeep up as much as possible to try and blow out whatever was plugging it up. for the ten blocks I drove after I got off the freeway it didn't hesitate at all, but it's come and gone like that before, so I'm not convinced yet.

would it make any sense that reving it up could have fixed it?...atleast temporarily

92drip...what am I looking for if i put my hand over the exhaust? debris?
 
also, I haven't actually heard the cat making any noise for a few weeks, when i did notice the noise i was out side of the jeep and I had to listen around for where it was coming from.... i tried listening again when I got home today, but I didn't hear it anymore.... that was also right after it hadn't been hesitating.
 
I hope debris isnt coming out the tailpipe!
The exhaust should come out with smooth evenly spaced pulses.

With a shattered catalyst you would feel the exhaust coming out erratically,

It would not produce a miss, but it damn sure would give you and overheat problem. Ive seen cherry red exhaust manifolds from a melted down cat,
 
Here's a crazy idea:pull the cat and take a long rod and punch out the honeycomb....you won't be left guessing for 4days if it's your cat that is causing the problem.(shakes head)
 
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