This may be a bit of a long story. I've been trying to figure this out for about a year now. I bought the jeep last year, and near the fall this problem cropped up. The jeep would jerk for a split second, almost like the trans was switching gears when it shouldn't. This would only happen after driving about 30 or so minutes on a warmer, 75 degree+ day.

Well, winter hit and the problem seemed to go away. Not once when it was less than 60 degrees out did it have an issue. Ran great until this spring, where the problem came back even worse. I've replaced quite a few things, trying to figure out this issue. Some parts needed to be replaced anyway, some did not. I'd like to figure this out because I've grown attatched to this jeep, its a lot of fun.

Anyways on to the problem. It started with that jerking. Check engine light never came on, voltmeter stays above 14, the rpm guage would drop down a few hundred rpms and then come right back up. Sometimes the problem doesn't happen and sometimes it is hard to drive the jeep. Today I took it out and it started jerking immediately so I came right back home and it has a rough idle.

So far I've replaced the distributor, spark plugs, wires, exhaust manifold, crankshaft position sensor, and throttle position sensor. I've tried adjusting the kickdown cable but that didn't stop the issue.

Things i've replaced not related to this issue as far as i know: starter, battery, oil pressure switch, valve cover gasket, and basic maintenance.

I hope I can get some advice from everyone, so far I've enjoyed reading the threads on this site very much, especially the build threads!

 

My first guess would be the upstream o2 sensor.

Has that ever been replaced? If not, that'd be my first move. Plus, you may even see an MPG out of it

 

I'm not sure if the o2 sensor has been replaced. I wasn't away o2 sensors could cause severe issues like this. I just thought you'd get worse gas mileage when they start to go bad. I average 16.5mpg with 32x11.5 bfg at ko's so i thought that wasn't bad.

If there aren't any more suggestions i'll pick one up and try it.

 

When they go bad, you'll get exactly what your symptoms are actually (rough idle, some bucking/hesitation/bogging while in motion, etc). There are probably 5 new threads a day on rough idle. MANY times, it is caused by a bad upstream o2.

You should also pull your IAC and give it a good scubbing with some carb cleaner. A dirty or faulty IAC can sometimes contribute to a rough idle. So if you have some carb cleaner laying around and an old toothbrush, there's something you can try for free.

My money is on the o2 though. Check out rockauto.com as they have good prices for NTK o2 sensors

 

I forgot to say before that I already cleaned out the IAC so, I suppose I'll have to do the o2 sensor. Any tips on getting it out, as i'm sure it will be pretty much stuck.

 

Disconnect the vacuum hose on your fuel pressure regulator and (this is gonna sound weird) suck on it. If you can taste fuel replace it. I had similar symptoms on my 90. Hope this helps

 

I sucked on the vacuum line from the fuel pressure regulator and (thankfully) did not taste fuel.

02 sensor then I guess.

I found a couple other threads with similar issues where people suggested alternator, but mine seems to keep the system charged just fine. Is it possible to have an issue with the alternator but have it still charge a vehicle?

 

The alternator is not likely (but is a poss. 'cause it's tied into the PCM) but I would try to see if you can get any codes out of it before spending $$$$$$ throwing parts at it. IDK how to pull codes since in how I have an '89, but hopefully someone who has one can tell you how to make the check engine light flash it out. Wish you well.. BTW Cruiser 54 has a ground refreshment procedure that costs nothing but time and can fix all kinds of stuff.. Try it first..

 

Not a bad idea to refresh your grounds. I know you said you dont have a check engine light on but you may have stored codes that could put you in the right direction.

Still vote the o2 sensor though. As for getting it off, PB blaster and heat will help you out a lot

 

A friend of mine had a similar issue with his early 90's Dakota and after replacing everthing it turned out to be the cat converter punched it out and prob solved. If you replace the O2 and still have the issue try the cat

 

Erm.... if you're gonna take it off to punch it out, just slip a new cat on and be done with it.
They are available pretty cheap, new versions add very little back pressure, many localities require them for inspection, and the reduction in CO and other poisons helps us all out a bit.

A differential pressure test is the best way to check them. A small hole before and after the cat and a differential pressure gauge. Measures the back pressure directly.

But it's probably the O2 sensor.

 

You can replace the sensor and cat for around $70-80. A nice addition to your new manifold as well. I just replaced my o2 sensor and I had a similar split second jerking in warmer weather when I was barely on the throttle while accelerating. Happened once a week or two last year and it happened maybe twice this spring before I replaced the sensor. Not sure if I have had the issue yet, but regardless a new sensor cant hurt and will help in other areas if you are running the stock one still.

 

Well, I'm back with an update.

I replaced the o2 sensor with an NGK from rock auto. OEM plug so I didn't have to splice anything. The rough idle is gone, it starts up great and seems to run okay. Still jerking though, and a lot once it gets warm.

Any other suggestions? Its almost impossible to drive at this point, it jerks badly enough it feels as if something is going to break.

 

Im having a lot of the same problems you've mentioned. It really sucks because I've searched and dont know what else to do either. I've replaced my tps three times with new ones, and right when i got the jeep I put a new cat, exhaust pipe, and muffler on it. So i know its not my cat, well at least it shouldn't be, because its less than 6 months old.

I haven't gotten a new O2 sensor though, I think i will do that soon. Also, this intrigues me because mine happens a lot more often in warmer weather and almost never in cooler/winter weather.

What is this IAC sensor?

 

Forgive me if you've already mentioned it and I overlooked it....but what year is your Jeep?

If it's a Renix system, have you cleaned/refreshed all the grounds? Cleaned and re-seated all the relays?
Map sensor- good vacuum present and no problems with the line?
Checked fuel pressure once the engine warms up?
Checked the plugs/wires for arcing?
How about the coil? They have been known to act up once they get warm. If you can find one to swap in for test purposes, I'd try it.