I searched around the forums a little, but can't find anything that matches this exactly.

I have a 1991 XJ (5-speed, 4.0) with almost 215k. In cool weather, it runs great (apart from intermittent engine knock and valve noise, and hard shifting). We finally started having some warm weather here in the mountains the past few weeks (highs in the 70s to 80s), and the Jeep is now idling a little rough. It's not regular, so it's not like a single cylinder is missing consistently. More like 1 miss or hesitation, then a second or two later, two misses back-to-back, then 1 a second later, etc. It isn't terrible -- just enough to be noticeable. The Jeep still has as much power as always. It's most obvious when listening right at the exhaust, but I may not have even noticed it, except that it also shows up when accelerating, but only when in the range of maybe 900 - 1200 RPM. There is a slight jerk at each hesitation that almost feels like a hitch in the transmission, but it corresponds to the engine noise so I think it's just a brief loss in power caused by the engine hesitating.

At higher RPMs, it smooths out, and I don't notice it. If I drive around town at 30 or 35 in 4th, for example, I will get it consistently. But, only on warmer days. When it's below 50, I really don't notice it. When I'm driving at highway speeds, I also don't notice it.

My first thought was that I might have gotten some bad gas, so I used a couple bottles of Heet (yellow, then red), and a half-can of SeaFoam in the tank. But, I ran that tank down to almost nothing, then filled up again, and I still have the problem. I am thinking I'll try new plugs, wires, cap, and rotor next, but wondering if anyone has run into this problem before. I am planning to buy a different vehicle (WJ, probably) in the next 6 months, so I don't want to spend any more money than is absolutely necessary to keep this thing running for that period. In other words, if this is an easy and cheap fix, I want to do it to make it run better. Otherwise, I will take my chances that it's nothing serious.

 

Nothing? Really?

 

You can start with new plugs.. just go with cheap, standard Autolites or Champions. There is a rebate on Autolites right now. I basically got mine for free from Rockauto. From there you can down the list starting with the cheapest components. That's the economical way. OR you can take it to a good shop and pay them to diagnose. Sure you have to pay them for the diagnosis, but they'll probably point you in the right direction which will save you money.

Also...my transmission had a kicking problem not too long ago.. drained and filled with Lucas transmission fix and max life fluid.. no more kicking. I know Lucas has worked for many folks. That may rule out the tranny for you. Hope this helps.

 

Still no resolution to my problem. In fact, it is getting worse (maybe because the weather is getting warmer).

I spent all of yesterday getting greasy. I tried a new MAP sensor, since the existing one was showing 3.6 v with engine off, key in ACC position (supposed to be between 4 and 5). Ran the same as before with the new sensor. The new one gave me the same voltage. The TPS is showing something like 4.2 with engine off, so I think that is OK. There has been some minor blowby into the airbox, but the CCV in the rear of the VC is clear and sucking air just fine. I found a leak in a vac hose, but that didn't fix the problem. Throttle body looks pretty clean. Some discoloration, but doesn't seem like enough to cause any airflow problems. Seems to work fine when it's cooler out, at least until the engine gets warmer.

I haven't done plugs, because this just seems like an air/fuel/sensor problem, given that it's temperature related.

I tested the resistance on the Coolant Temperature Sensor after the Jeep sat for a few hours after last running. It was showing 3.9 on my multimeter, which is not autoranging. Could that be the actual value, or is it supposed to be scaled? If I remember correctly, the resistance is supposed to be between 1000 and 7000 ohms (7000 cold, I think). The hesitation/miss was so bad I wasn't sure I was going to make it up the mountain over the weekend. HELP!

 

My jeep has been doing the exact same thing lately with the warmer temps here

 

Odd. Any leads?

 

I replaced map, tps, cps, and idle.... Only to have the fix come from the front o2 sensor. Good luck

 

No idea why it does it, I assumed maybe it was just normal


Mine has new plugs, wires, cap, rotor, injectors, and air filter. I have not checked compression yet though.

 

Rednek -- Were you getting a check engine light and code for O2 sensor? I'm not getting any codes at all.

 

O2 sensor would be my first guess, mostly because that's what I know the problem is with mine. But if you're not throwing any codes...still, worth going over with the multitester.

 

It was not throwing a code, I had a buddy at a shop hook up the good computer while driving and he said o2 was not reading. Its worth a try, get one and try it, if it does not work, just clean it and take it back. I took back every sensor I tried before that back with no questions to autozone/discount. Just make sure you get the upstream if you try the front one and downstream for the one by the cat. dealio or big30 were discount codes that worked at advanced for like $25 off.

 

So you're saying the O2 sensor (or any of the sensors) could be bad, and still not throw a code? There are two O2 sensors on a 91, correct? And I'm guessing it's the front one that could be an issue...?

 

OK. Where is the upstream one located?

 

It's on the front pipe shortley after the manifold well before the cat.

 

OK. When you say he "hooked it up to the good computer," you mean something that actually diagnoses problems real-time, and not just something that reads codes, right? And I assume it plugged into the port below the dash on the left side of the steering wheel? I read the codes (using the trick of cycling the key on and off 3 times and watching the blinking CEL), and there's nothing there, so I'm thinking the typical scan tool would tell me nothing. I wonder if a parts store has something that's more of an actual diagnostic tool...?