Idle giving me fits!!!!!

[The Lure Washer]

Ok so Im having fits over my 90 laredo! So To give you an example of whats happening If im sitting at a traffic light my vehicle has a tendency to fall on its face so to speak. The idle will dipp down to where its about to stall and then returns back to normal rpm. Also when im driving i can feel it surge like that as well. But it doesnt do it all the time. Ive already replace plugs, wires, cap, and rotor. I also cleaned out the throttle body and IAC. And the car ran like a champ for about 2 weeks. Then it started again, So I replaced the IAC and still it does it. What else could it be?

One other question and this will sound like a real noob question but where in the instrument cluster is my Check engine light. I have no idea where it is or if its even on because 3/4 of the bulbs in my dash were burned out.

Thanks in Advanced!!!!

~Matt

 

[4LowandGo]

For the idle... There are lots of things to check, but personally I'd start with checking all vaccuum lines. Those really cause idle problems if they aren't addressed.

 

[88XJSport]

There is no check engine light on any 84-90 Cherokee.

 

[B.RAIDER]

Check your vacume connection to your MAP sensor from the throttle body.
I broke my plastic line and it ran like crap til I spliced it back together.
Just a thought.

 

[4xBob]

Vacuum leaks can cause surging and can bump up the idle, but don't cause the idle to slow down. The EGR can stick open, causing the engine to slow or almost stall at idle. My 1990 is on the second since new. (1990 is the last year that the 4.0 uses a n EGR.) It could be an electrical issue, since the vacuum supply to the EGR is controlled by an electrically operated solenoid located on the fender wall beside the fuel pump resistor (gismo by the airbox- has vacuum lines going into it and an electrical connector.) The TPS usually causes a high idle when it flakes out, often accompanyed by weird (mis-timed) shifting in the auto tranny.

 

[The Lure Washer]

Anybody think it could be the ignition coil?

 

[8Mud]

I'm thinking vacuum also, it sure won't hurt to retorque your intake/exhaust manifold bolts. A pain to do on the Renix (two people one on top to turn the ratchet and one underneath to put the socket on the bolts works), but I don't think I've ever checked them and found them all as tight as they should be.
The EGR can do this, make sure the power plug is connected to the EGR solenoid (the EGR defaults open), so it needs current to close.

I've had vacuum leaks, EGR, O2 and TPS ground issues mess with my idle. A tip off for an O2 sensor issue is it usually gets worse after it warms up, at least in my experience and sometimes causes surging or bucking around 2000 RPM. Intake manifold leaks can also get worse after it heats up.

I've never replaced a Renix coil, I even drove the recall coil in my 87 for 16 years. Ignition problems often show up as a miss at idle then again in the higher RPM band, sometimes hard starts.
I've had a bad CPS cause some serious issues at idle and hard misses at speed. The CPS usually had a really high ohm reading, near 200 ohms or so. A no start is often how a cPS manifests though, the crappy running motor thing is rarer.

 

[kubtastic]

...I am having a related issue on my 2000, 4.0/aw-4:

-difficulty starting (first start is unsuccessful, second is instantaneously successful)
-inconsistent idle and stalling (very rough, varying from 300 rpm to 1000 rpm in P)
-significant hesitation to shift to accelerate, causing a "bucking" sensation as it goes into and out of gear and rpm bouncing, sometimes at 2,000, sometimes as high as 3,000.
-gutless (top speeds vary between 40 and 15 mph)
-gets worse after it heats up
-constant TPS codes (on my 2nd TPS, no dice, still code-throwing)