Jeep 4.0 - running rough at low RPMs

[HawkeyeScott]

Problem started yesterday, worse today. When driving slowly - like in a parking lot with my foot OFF the gas - my Cherokee begins to idle really rough. The RPMs dip really low. Then when I put my foot back on the gas and rev it slightly, it almost cuts out completely. But it doesn't and as soon as the RPMs pick up, it's fine. Until I let off the gas again and am coasting at low speeds.

Did it yesterday on a 3/8 tank of fuel. Filled up this afternoon and like I said, it's gotten worse. Called my mechanic and he said he'd need to see it to really take a good guess. Over the phone his first guess was carbon on the throttle body. Next guess was bad throttle sensor. The Jeep is not showing any codes.

Can't get to the mechanic till Monday so I thought I'd toss in some Seafoam and see if that would do anything.

I'd appreciate any ideas you might have on what I'm looking at here. Thanks.

 

[Texan4Life]

some guesses that come to mind:

crank or cam position sensor, throttle position sensor, throttle stabilizer

 

[Zaedock]

Originally Posted By: Texan4Life
some guesses that come to mind:

crank or cam position sensor, throttle position sensor, throttle stabilizer


That's what it sounds like to me.

Could be a fragged IAC as well.

 

[Gary Allan]

Clean the throttle body, first while running. Get the bypass idle air valve good and let the thing work a bit (opening and closing) trying to keep the thing running.

 

[Boomer]

Also llok for cracked vacuum hoses while you are under the hood.

 

[punisher]

Originally Posted By: Boomer
Also look for cracked vacuum hoses while you are under the hood.


If people followed that advice, we would have a lot quicker fixes as well as a lot less money spent throwing parts at a problem. Rule #1.....For idle concerns ALWAYS check for vacuum leaks (or leaks between MAF and throttle body) first.

 

[Gary Allan]

The 4.0 would account for a static vacuum leak as long as it was in range of the Idle Air Controller to compensate. My neighbor's Rubi had a big hose off the intake. You would never know it. Now when we put it back on, then the recovery from shifting and whatnot was erratic for a while.

I didn't see a year mentioned, but there was a TSB about a "seeking" issue on the 1998-99 model year ..maybe beyond.

 

[HawkeyeScott]

It's a 2001 Cherokee.

Thanks for the replies so far. This is a head scratcher.

 

[440Magnum]

I ran across an article months (years?) ago on a quirk that can happen with 2000 and later 4.0s with distributorless ignition. Basically, they have a cam position sensor that goes where the distributor would go on an earlier 4.0, and inside it there is a chopper and pickup. You have to correctly phase the chopper and pickup (using an allen wrench through a slot, IIRC) or else you get really odd driveability problems. But I don't think that would only show up at idle, I think it shows up at all speeds.

Since its an 01- you can pull codes and it'll probably tell you where it hurts. My bet is on the Idle Air Controller being gunked up. Like Gary said, on MAP systems the IAC can compensate for huge vacuum leaks, so I doubt that. But check anyway.

 

[HawkeyeScott]

UPDATE:

I just cleaned the throttle body, using CRC from NAPA. $5.69. And man, the TB was pretty loaded up. (And it should be. It has 134K and I have never done this before. This time yesterday I wasn't sure what the TB was or where it was.) I don't know if it will solve my problem, but it certainly didn't hurt anything to clean it.

 

[mechtech2]

Vacuum leaks in lines, their connections, and the intake are #1 for lousy idles.
Cleaning the TB is always a great idea - you will never regret it.
Your actual idle air motor may need cleaning, but be careful with strong solvents because of possible plastics that are used.

Was this really all of a sudden?

 

[HawkeyeScott]

Pretty sudden. It happened Thursday for the first time and then happened again Friday, when it was noticeably worse.

It has not happened today after the cleaning. Hopefully an overdue scrubbin' was all it needed.

 

[Greaser]

IDLE AIR CONTROL VALVE.

 

[yeti]

hawkeye -- at 134 k., take the IAC off and clean it anyway. it'll need it. be careful not to lose the o-ring, and spray cleaner inside the TB area where the IAC is housed, as well as the IAC.while waiting for the cleaner to do it's thing, have 1 beer. be gentle on the IAC as you wipe it dry.repeat process.

 

[mcrn]

I had a 96 ranger that did this and it was a cracked spark plug wire that was arching. You could see it under the hood idling in a dark area.

 

[artificialist]

Does a 2001 Jeep 4.0 use an EGR valve?

 

[Greaser]

Originally Posted By: artificialist
Does a 2001 Jeep 4.0 use an EGR valve?
NO

 

[HawkeyeScott]

Thanks guys. I'm going to make another thread with the latest update...in case someone else can use it later as a resource for a similar problem.

 

[artificialist]

Originally Posted By: Greaser
Originally Posted By: artificialist
Does a 2001 Jeep 4.0 use an EGR valve?
NO

Good.

One less item that could have caused this.