My 01 Expy suddenly surged forward a few times while stopped and my foot on the brake. Some guys from the Expy forum suggested that my IAC thing is bad...Now at the risk of sounding stupid. What is the IAC, and what is its correlation to my problem? I work on race cars for a living, not these modern day hi tech vehicles. Sorry!

 

Here's the easy version:

1. The Expy has a throttle butterfly in the intake, right? When the truck throttle is being used (i.e. truck is not at idle) the butterfly opens and closes to let more or less air in and control RPMs (along with the amoutn fo fuel injected).

2. When you're idling, the throttle butterfly is completely closed. The engine doesn't move the butterfly to regulate idle RPMs. Rather, the engine uses the IAC.

3. The IAC is a small valve, built like a needle valve (i.e a small hole with a plunger/needle that plugs the hole to stop airflow, and opens up gradually to regulate airflow through the hole), and controlled by an electronic solenoid. When idling, the PCM controls voltage to the solenoid to regulate how far the needle valve opens, and thus how much air goes into the engine. The IAC is a bypass that allows air to come through the air filter and intake, up to and around the throttle butterfly, into the intake manifold.

4. So when driving and not idling, the IAC is closed and the butterfly is open varying amounts. When idling, the butterfly is closed and the IAC is open varying amounts.

Over time, the IAC gets gunked up and the little needle valve part doesn't let enough air flow through...or the valve starts operating jerkily (i.e. instead of smoothly opening and closing, or smoothly varying the degree of openness, it catches and releases rapidly, causing bursts of airflow). This can cause RPM surges and can cause the engine to die unexpectedly while idling. Say, for instance, you're idling and the AC compressor kicks on...the engine needs to bump up the airflow to remain running with the extra drag of the compressor...so it tries to open the IAC, but it's gunked up and doesn't respond fast enough...and the engine dies.

Cleaning the IAC usually solves the problem. Sometimes, the IAC just needs to be replaced. Over time, the PCM 'learns' to use the gunked up IAC by overcompensating the amount it opens it...so when you clean or replace the gunky IAC with a new one, the PCM might overreact and try to operate the new IAC as though it is the old IAC...causing surging (because the new IAC, unlike the old IAC, responds rapidly to the PCM's demands).

Is that clear enough?

 

Crystal! Now, my only question is,can I use carb and choke cleaner on it without causing more damage? I use Berryman products at my race shop...Thank You very much for the assistance. T

 

Yes you may. It is located on the rear of the intake elbow going up to the throttlebody.

Jimmy

 

Make sure you pull it off of the engine before you clean it...then, clean it out really well, and blow it out with compressed air. Then, reinstall (don't forget the gasket!) and voila!

 

Thanks a bunch guys! I am going to try it within the next week.