Low RPM Stumble

2000, 4.0, AW4.
Intake, (lack of) exhaust.

Lets see, where do I begin....

Two years ago my 4.0 crapped out from unknown causes. Probably had something to do with the 0331 head, but I digress...

Had a local mechanic put in a donor engine from a rolled XJ. Also a 2000 with ~77k on the clock.

Ever since then my engine will "stumble" for lack of better word at lower rpms, especially at idle. What I mean by "stumble" is it seems like 1 of the cylinders fails to fire or perhaps fires too soon? I am not sure, but it makes a definite change in the exhaust tone and shakes the jeep slightly.

In my limited knowledge of automobiles, I ran two full tanks of gas with Lucas oil injector cleaner through it. No change.

Also, I dont think this should affect the engine but the second 02 sensor circuit is shorted, so it is unplugged.

At this point I am unsure of what else to do.

Any suggestions?

I had similar issues, Found that whoever did my last tune up did not believe in die-electric grease it seems. Wires were gone, cap and rotor were worn. And when I bought cap/rotor/wire I pulled the wire off my coil and ended up throwing a coil onto it as well, runs much better. Next on my list is that broken manifold of mine,

Explosive said:

I had similar issues, Found that whoever did my last tune up did not believe in die-electric grease it seems. Wires were gone, cap and rotor were worn. And when I bought cap/rotor/wire I pulled the wire off my coil and ended up throwing a coil onto it as well, runs much better. Next on my list is that broken manifold of mine,

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Hmmm this is a possibility, however, on the 2000-2001s they have coil packs instead of the standard ignition hardware. I suppose I could take apart all connections and make sure they are in good working order.

I imagine you are setting a check engine light for that post cat O2 sensor?

How fresh are your spark plugs?

You should put a code reader on the computer and see if there are any other stored codes that can help you in your current problem.

I'd resolve that O2 sensor issue as if you have one problem that is known and unresolved, then another one comes along, it makes figuring either of them out more difficult, as sometimes there are relationships between them.

If you decide to replace the O2 sensor, replace both of them while you are under there. Good investment, and keep in mind that oxygen sensors do have a finite lifespan.

Spark plugs are two years old/10k miles aprox. EDIT: Err wait, maybe I put those new plugs in the old engine... I should really keep records...

When I originally had this issue, I replaced both 02 sensors with bosch units.Installed right after engine swap.

Last time I checked the only stored code was the 02 sensor circuit problem. Ill check that again to make sure there are no other codes.

CherBear said:

Any suggestions?

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gas can and a match?

esriously though, with your post cat o2 error have you checked that the cat is not plugged/

87manche said:

gas can and a match?

esriously though, with your post cat o2 error have you checked that the cat is not plugged/

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I suppose it is a possibility, how ever I think it is unlikely for a couple of reasons:

1.) The cat only has 130k miles on it.

2.) It doesnt rattle or make noise.

3.) The problem completely goes away while the engine is under load.

4.) I would expect to have decreased power output, while I have observed none.

5.) The CEL code said " 2nd o2 sensor has higher than accepted values" Therefore the circuit has a short I would guess. So this doesnt seem to be affected by the cat.

Is there any easy way to check if your cat is plugged besides removing it and physically inspecting it?

pull your spark plugs and check the spark gaps
its all too easy to bend a ground terminal and short a plug, causing an erratic misfire.

The post cat o2 os there just to monitor the cat to make sure it actually works. It doesn't control fuel mixture. Ideally if you were to check it out with a meter you should see about a .5v signal going to the ecm. That means the A/F ratio is good coming out of the cat. I would suspect your getting a short from the 5v feed that powers the o2 sensor heater since it saying its over value as the max is usually 1.0v to the ECM.

As for the stumble I'd do new plugs and really check the coil rail and boots out. Look for carbon tracking on the boots and rail. It would look like someone took a pencil and drew lines to something that would make a ground.

You might try the tooth pick alignment of the synch sensor.

joe what is this? mine does the same when its running

X2, what is the toothpick trick....