99 w 255k. Ive got symptoms of a bad IAC, got that from threads about the IACs on here, but this would be the 2nd mopar IAC I've put in this thing. Granted since I put first one in Ive changed the entire exhaust, most of the cooling system and installed new power wires and ground cables, few other things including fixing many vacuum leaks. Anybody want to talk me out of more part swapping to figure out this bad idle. Maybe put a cheap one in it just to test, although I've also had bad experience with that on this jeep.
Thanks

 

Is your PCV or related puking sludge into your IAC? Is there something wrong with your air box so dust and grit is getting into your IAC?

 

I haven't found any dirt in there. Do have an aftermarket filter on there but I don't think its leaking.

 

ANY particles that get thru the air filter will go thru the IAC at idle. That's the only way the engine gets any air at idle except for vacuum leaks like the designed in CCV system. If the air filter was 100%, the IAC would stay clean.

 

Pulled out the IAC this morning, found an ohms check on here last night im going to try. I don't see any kind of like dirt or sand or anything on here, a little bit of carbon maybe but that's really it.


yea, small amount of carbon residue on the shaft close to the body of the sensor. Maybe running rich, which maybe suggests O2 sensor?

 

IAC ohms test within the specs I saw here. ****

 

Don't forget nothing goes thru the throttle body except air. There never is any fuel in it. The carbon deposits are things that precipitated out of the air passing thru it like dirt, moisture, and any oil vapor that was in the air a road level or backed up thru the fresh air crankcase vent to the filter. It's possible to get some of the intake charge air pulsing back at wide open throttle, but not likely. That ring around the pintle nose shows it is seating on shutdown.

The IAC operates as a stepper motor. In order for the pintle to move, it must be commanded by the PCM. There are 2 12v circuits in it, each moving the pintle in one direction, either in or out, and they are both energized at the same time, thus locking it in position. If the PCM commands the speed to increase, it breaks the circuit to the "in" coil, allowing the "out" coil to move the pintle one step. These are finely incremental, such that 125 steps relates to 1000 rpm. The PCM memory retains this positioning data from the last time it operated. Each time the engine is shut off, the pintle closes the passage. On startup, the PCM opens the pintle the required amount remembered from the last time it ran, based on engine temperature. Generally, if the engine is cold, it starts at a 1000 rpm setting, then drops as warmup occurs. Electronics is fascinating, isn't it?

Problems with the IAC can cause all kinds of starting problems like high speed runaway or having to add pedal to get it to start, then dying. Since it closes on shutdown, if the opening circuit doesn't work, it will remain closed. Likewise the other extreme.

 

Are you getting CEL (code?)? I know the computer latches a code for the IAC because just this morning I accidentally dislodged the IAC connector when doing some work (injector change) and got a CEL light when I started it up. I didn't bother reading what the code was because I knew what the CEL was for. I just reconnected the IAC and disconnected/reconnected the battery ground to clear the code.

My 98 had bad cold and hot idles. Replacing the IAC got me the cold idle back but I had to replace the front O2 sensor to get the hot idle. The Jeep had 197,000 on it at the time so it wasn't unusual that these things went south. Rule of thumb about sensors is to either automatically replace them at 100,000 miles or expect to from failures as they occur. The IAC was completely fouled with crap. I got the Jeep 2nd hand so I know it spent part of its life in a beach sand environment. It got carboned up due to a lack of routine filter replacements by the previous owner. Based on the rest of the condition of the thing I think I can call that a fair assessment. I cleaned it out but it was just shot and not responding to the position commands from the computer. I can't imagine having to replace the IAC more than once, maybe twice, over the life of the engine unless the quality of the air entering the throttle body is very poor.

 

We removed the A/C compressor from my WJ because the clutch was shot and the pulley was wobbling about ready to lock up. Now every time I use defrost the IAC has trouble deciding where it wants to idle. I can be sitting there idling waiting for someone and the idle will jump up to 1000 or 1200. If I shut the engine off and restart, it'll be fine for a while. I think the A/C plugs are just hanging around collecting dirt, so maybe I should isolate the circuits.

 

How can you isolate the circuit such that the computer bypasses the signal out to the IAC when using defrost (A/C clutch engagement)? Obviously, the computer is controlling IAC position(s) regardless of whether it "sees" the A/C clutch or not. The HVAC control in the dash is sending the defroster position to the computer so maybe that's where you make the change? Just thinking out loud here because I'd like to do the same thing. I'd like to use my defroster without engaging the A/C clutch or arming the computer. I'd have to take another look at the schematic.

 

IDK for sure, but I think the compressor relay has a feedback circuit that tells the PCM that it's engaged. The relay, not the clutch. It may be a simple matter of pulling the relay and any fuses in that circuit.

I'm getting ready to go to work, so not now.