Rough Idle/Difficulty Holding Idle

[JSimmons]

Hi all,

Got another mystery for everyone.

Okay, so I've got a 97 Wrangler with the 4.0 in it. All of a sudden this week, the rig has trouble holding idle and revving up to about 1750 RPMs. Above that, it smooths out and runs fine. A few times, it wouldn't even start-up. But, later on in the day when I would try again, it would start-up and have a rough idle.

Anyway, I'm going through the ignition system piece by piece. Starting with the cheapest stuff first. Spark plugs (didn't change anything) followed by the dizzy cap and rotor to be replaced tomorrow afternoon.

Does anyone else have any experiences similar to this w/ the 4.0? If so, how did you remedy it? Any well-known or common sources for this sort of problem? If the ignition pieces don't solve the problem I guess I'm going to starting sorting out the fuel system (pump and filter, etc.).

Thanks in advance.

[soup31314]

Well I get the rough idle and high rev on my 98 with a 2.5l and its due to the TPS. Is it putting out any codes??

[Ghost00TJ]

I would thimk maybe TPS. unless there is a Idle Air Control valve on these?
No check engine light?

[crazy4ink]

Check your TPS (Throttle Position Sensor). Also check and make sure that your MAP sensor has the little rubber hose elbow from it into the throttle body. It should be located on the back of the throttle body and held on with a single torx bolt. Do you cycle your ignition key to see what codes the Jeep will give you? Are you still running the same fuel as when the problem started? If so, you might have some bad gas. A bottle of Heet in the tank should clear up any water you might have in the fuel system. IIRC your fuel filter is going to be part of your sending unit/pump in the tank so you aren't going to be able to change it. (stupid engineer design on that one)

[usnavymitchells]

I had a Chevy Blazer that had a similar problem (almost to the "T") it turned out to be a clogged EGR. Only difference was I had a check engine light. This may sound stupid but try getting it on a highway get up to say 50-55 and then floor it. Kind of like the old carb days where you had to clean out the carbon build up. Had to do that to the Blazer once every couple of months, but it worked....

[JSimmons]

Hi all,

Haven't seen any codes flashing. Then again, this is happening at 0530 before PT...so maybe I haven't been paying that much attention to it.

I will check the TPS and the MAP sensor this weekend. The fuel filter/pump deal is a bad piece of engineering. I hope it ISN'T going to be one of those. That'll make another weekend of no fishing or golf.

I'll get it out on the highway and mash the skinny pedal and see what happens.

Thanks for all the ideas. Let y'all know how it turns out.

[JSimmons]

Yeah...so...I'm pretty much a dumbass.

Apparently it was just the coil wire and rotor that was messed up. After I replaced the spark plug wires, rotor and cap everything was good to go.

The rotor looked like it was totally fried/corroded. The coil mount on the dizzy cap was almost cooked black too. So, after I swapped everything out the rig ran fine.

There's a lot to be said for starting out with the cheapest fixes.

Hopefully, everything works out these next few days.

[crazy4ink]

well, glad to hear it was something really easy and inexpensive. Back to driving/idling normal now?

[JSimmons]

After a few days of driving around town and some light off-roading everything is still going just fine.

The plugs I took out did look like they were running a bit hot though. Hopefully, that is not indicative of some other hidden problem waiting to rear its ugly head.