Idle RPM's unsteady, too low, then too high

89' Jeep Cherokee, 4.0, 4x4

About 3 weeks ago, my Jeep began stalling and surging when I would come to a stop, or slow down and then re-accelerate out of curves. Idle park RPM= 400 Idle InGear RPM=220

Then about 2 weeks ago, I noticed the Idle in park was around 300 RPM's and in gear, 100-150 RPM's. This caused it to die when shifted from park to reverse/drive. If I held the gas enough to get the RPM's up to around 500 or so, it would stay alive. However, there was still that original stalling and surging.

Well, 3 days ago, I hopped into my Jeep and drove off, and noticed that the sputtering/stalling/surging had ceased to exist. Once I had stopped and put in it park, the RPM's went up to 1600. When back in gear, it was down to a little below 1000. This seems way too high, especially the InPark RPM's. This is where it is at right now.

Here's what I've done to the Jeep before and during....................

I used STP engine oil treatment about a month before all of this occured. This caused a bit of a change in oil pressure, like it would stay about 80 until engine warmed up, then it would be 40 @ idle, 60 @ 55mph, 80 @ 75mph...which was normal for my Jeep.

I also originally changed the fuel filter because of the surging, then plugs, wires, distributor cap, rotor, ignition coil(just cuz). All this was changed in the first week of experiencing these problems.

After the second week of these problems, I changed the air filter(wasn't really all that bad) and added some Lucas Oil Fuel System Cleaner to the 20 Gallons in the tank....2 days later, we arrived at the current problem.

anyone that can help, please let me know what you think!

Sounds like a vaccuum leak.

then why is it taking up to two weeks to show any signs of change, and if so, what vacuum?

That sounds a little like what mine is doing. I have a '88. I'll interested to find out what fixes this for you. Mine is actually dying after I start it(only if it's been sitting for more then a couple hours) If I keep my food on the gas for a little bit then I can put it in gear and take off and it runs ok...run pretty weak though, seems like it really only accelerates after it hits 35mph(sorry don't have a tac yet so I can't give the rpms).

Hope you get this resolved soon, be sure to post the results.

~Scott

yes, mine was dying too when i'd shift it into gear, but now the engine is racing too high for it to kill itself...about the idle air controller, where is it located in the engine compartment, how can I check it, and how much is a new one?

mrbill3 said:

It sounds like it is your idle air controller (Niehoff part #21771) is going bad. My 89 XJ was doing the high idle thing. I replaced the controler and went on an 800 mile trip and had no problems and still haven't had any.
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Where is this part # from...what store?

The controler is on the front of the throttle body right next to the throttle position sensor. This link: RENIX FSM will show you where it is. Go to page 70 of 100 (page 66 of the manual). I got the part at Kragen Auto Parts and it was $50.00. Good luck!!

Four things here - TPS, is it good and is it adjusted properly? IAC, is the area clean inside the TB (throttle body)? If your idle speed is changing the IAC is probably working but they do stick open sometimes and cause very high idle speed upon warm/HOT restart. 02, Renix will idle erratic with a bad and/or weak 02. Finally, vacuum leaks - intake manifold bolts, perhaps the MAP vac tube is ruptured?

Get the FSM download and study things. It's not that complicated but a few things need to be handled in the correct order with regard to TPS adjustment.

You might want to check your fuel pressure at the rail. About 30psi, and 40 psi with the vacuum line off the regulator.

A bad EGR valve or the control solenoid can cause an erratic and especially low idle. The control solenoid is on the drivers side fenderwell.

Sorry to drag up an old thread, but would disconnecting the vacuum to the EGR control solenoid(thus disconnecting the entire EGR system) cause these same problems? My control solenoid got stuck open, thus kept the EGR valve open at idle and kept stalling the motor, so I bought a junkyard one to pass emissions then disconnected it(as this one would still momentarily stick) and the Jeep runs great, but idle is a little erratic(I know my o2 sensor is bad also, will replace tomorrow).