I’m having trouble with my idle air control

Question:

I own a 1969 L46 Corvette that you have helped me with in the past (thank you). My question this time, however, is with my tow vehicle—a 1998 Dodge Ram 2500 4x4 with a 5.9L (360 cid) gas engine and automatic transmission.

My problem is that within three to five minutes of first starting the vehicle and while still in “open loop,” the idle air control (IAC) motor completely opens, causing the engine to go into an extreme “vacuum-leak” type of behavior. In fact, the noise of the engine sucking air is unavoidable. However, as soon as the engine reaches normal operating temperature (closed loop), the IAC goes back to operating normally with the engine running as it should—with a smooth, steady idle, with reasonable throttle response upon driving.

The truck cannot be driven while the IAC is wide open, as you can imagine. I’ve replaced the thermostat (195°), the water temp sensor and the air temp sensor, thinking that the PCM (power train control module) was getting a bad reading and attempting to correct a “rich” condition by opening the IAC. But, no such luck.

My question is this: While in “open loop” is the PCM running a preset program rather than monitoring inputs from sensors? If so, does this mean that the PCM needs to be re-programmed in order to fix this problem? Is that even possible, or do I need to replace the PCM (ugh!)? If so, what are your thoughts regarding used PCMs?

Alternatively, if the PCM is monitoring sensors (in “open loop”) do you have any thoughts on which one(s) might be contributing to my problem?

Answer:

The idle air control (IAC) motor is essentially a motorized valve that opens and closes to regulate the amount of air going into the engine during idle (when the throttle plate is closed) in order to maintain smooth idle at the desired engine speed. The IAC is controlled by the vehicle’s power train control module (PCM). The PCM sends the IAC a signal to open or close based on information the computer receives from various sensors.

The only time the PCM should go into a “limp mode” is when there’s a failure in the system.

Have you put an OBD II scan tool on the truck to see if any diagnostic trouble codes are stored in the computer? If the computer senses that the IAC is not functioning correctly it will usually register and store a trouble code, which can indicate which sensor, if any, is not functioning correctly.

The problem could be in the PCM and this typically will not register a trouble code. A PCM is a computer and they can last a long time without any problems so I have no issue with buying a used one if certain common sense parameters are met. Make sure that the used PCM is out of exactly the same year and model as your vehicle. If possible, buy it from someone who guarantees that the used PCM is working properly.