Howdy,

I’ve been having a rich condition and I checked my scanner’s data stream. All sensors are fine. Tune up is recent and plenum is fixed. There’s a whistling sound when accelerating/decelerating and I isolated it to the IAC valve. Upon replacing it, the truck ran very well…for a moment. It began surging and back to the same problem. Not enough air and you can hear whistling when modulating the throttle.

I would like to know how I can test the IAC connector for voltage, resistance, and continuity. Also, I know the PCM could ultimately be the problem.

Any recommendations would be helpful.

 

The aftermarket IAC valves are problematic, at best. I had one in my truck for a while, as the original failed.... and had nothing but problems with it, it just didn't react fast enough, so, stumbling idle, stalling, etc. Replaced it with a mopar unit, and all those problems went away. (I happened to have another one out in the barn that I swiped off another motor.)

The IAC valve is essentially two electric motors, that move the pintle in/out. So, two 12 volt circuits on the connector. PCM will apply voltage to BOTH sides, to lock the valve in place. Generally, it either works, or it doesn't. Both circuits are to/from the PCM. Nothing else in the mix there. If the PCM thinks there is a problem, it'll set a code....

Cleaning the throttle body is a good place to start. The IAC well tends to collect a fair bit of 'gunk', and it can cause the pintle to move slowly, not move at all, or choke off what the PCM expects for airflow. All of which, give you odd issues. The throttle body gasket is only a couple bucks. So, its a cheap and easy exercise. Disconnect the battery when you start on it, once it's all back together, reconnect battery, turn ignition to "on", not start, count to 10, don't touch anything.... THEN start the truck. It should figure out idle fairly quickly. This process gives the PCM a chance to re-learn the 'zero values' of various sensors..... which makes the learning process MUCH quicker.

 

Thanks for the answer! I recently cleaned the throttle body when changing the plenum gasket and made sure to clean the cavity where the IAC pintle goes.



So, when I first installed the replacement IAC and started the engine, it was surging. This was after I reset fuel trim and IAC setting with my OTC enhanced monitor. The next time I started the engine, you could tell it was stuck too far closed but no surging.

I just realized that the pintle may have needed to be adjusted out of the box. I did not adjust it, and it came with a generic instruction sheet stating to adjust it. Do you know what length the pintle should be and how that can be adjusted since it’s a new part?

Otherwise I’ll check voltages. Here’s a diagram from the 1995 service manual.

 

By definition the computer is adjusting the IAC, there is no pre installation adjustment that I'm aware of. In my experience Chrysler vehicles from that era tend to run like crap if the PCM loses all its adaptive information (battery disconnected, scan tool resets) and it takes a few drive cycles until the engine doesn't stall or has a high idle at a stop. If your scan tool has bidirectional features you may be able to set the target idle and see of the IAC can control the RPMs.

 

I see - the pintle’s not adjustable. I’ll definitely give it a long drive today and see what happens. One of the reasons I know it’s not getting enough air is because the PCV valve is constantly rattling. When the IAC used to be more open it wouldn’t force the intake to pull air from the crankcase. I have reset the fuel trim and IAC setting multiple times in the past to no avail.

Given there are no codes and all the sensors (and new NTK O2 sensor is switching) read normal, I’m having to suspect it’s a PCM or wiring problem. Should I always be seeing 12 volts at two of the pins at the IAC connector? There has to be some reference value.

Regards.

 

When the PCM is moving the IAC, there will only be 12 volts on one of the circuits. It energizes both circuits to lock the IAC in place. If one of the IAC circuits failed, that WOULD set a code. If I had to hazard a guess, at least part of the problem is the aftermarket IAC..... These trucks really don't like aftermarket electronics.

 

I'll give the genuine IAC valve a try.

 

Howdy! I installed a genuine Mopar IAC valve and it made the engine start more easily and idle better. The PCV valve rattle mostly went away. However, the engine starts poorly sometimes. You can hear a sucking sound momentarily and it goes away within 30 seconds and then the engine idles more smoothly. Clearly, it's the IAC motor adjusting. It leads me to question why this is happening. The only thing I have that is still original is the engine coolant temperature sensor. What confuses me is why the IAC closes before starting and then goes back to more or less normal shortly after starting.

P.S. I did reset the adaptive fuel trim and IAC position.

 

my snap-on scantool has a test for the IAC. It's not spelled IAC it's something like AIS(?). The test lets you step the IAC to produce different idle RPMs. Range is 800-1300RPM or so in RPM increments of 100 I think. The scan tool commands the RPM, which signals the IAC. Compare actual RPM to the commanded RPM and see if you are close.... you should be.
btw the 2nd gen IAC is quite different from common IAC design (ground and +V for driver, ground and data out for pintle position). We'll never get the IAC pintle position error since the PCM doesn't know the actual pintle position, we just basically get driver circuit electrical errors (ie: shorts) on the 4 wires.
You could attach a ground wire from neg battery to metal nearest the IAC, as the 4 wires are from the PCM driver circuits, there's no ground in the connector, see if that changes your idle any on startup.

For grins I'd start it with a battery charger connected so you got 14V immediately after you release the ignition key after it starts, see if supply voltage is a factor.

Otherwise might improve with time.

 

I had a similar situation with the engine and it took me a long time to do and solve this problem.