Ford IAC valve replacement:

Below are some pictures of the original idle air control valve from a 1998 ford crown victoria. In this case, one day the vehicle mysteriously refused to start when the ignition key was turned. The starter cranked, but the engine would "flood out" and the exhaust had a slight gasoline odor to it. Pushing the gas pedal down a little while turning the ignition key started the engine and kept it running, but the vehicle would stall out as soon as the accelerator pedal was released.

The symptoms were consistent with a defective idle air control (IAC) valve. Base idle on this vehicle should be around 800 RPM in park,
but engine crankshaft speed appeared to be dropping so low that the engine stalled out. Since this vehicle did not have a factory installed tachometer, a scantool was connected to the obd2 diagnostic port under the dashboard and revealed that no error codes were present and the idle was very smooth if engine speed were manually raised to around 800 RPM via the accelerator pedal. After hitting the IAC with the plastic handle of a screwdriver a couple times, the low idle speed problem "fixed" itself. But to prevent repeat occurences of this problem in the future, the IAC was replaced with a new service assembly.

Some other points of intrest about the IAC:
Another point of intrest, the IAC valve basically creates an electrically controlled air leak on the throttle body. So if you have a vacuum leak elsewhere, it can exhibit symptoms similar to a bad IAC valve. When looking for vacuum leaks, don't overlook:
Do keep in mind that the larger the vacuum leak is, the more it will impact engine idle quality. For example pulling the vacuum hose off the brake booster or pcv valve will cause the engine to run really lean or to stall out immediatetly. But you'd likely need to monitor fuel trim and other block learn values with an electronic scantool to notice a vacuum hose that was pulled off a climate control system mode door actuator.

Also keep in mind that model year 2005 and later crownvics have electronic throttle control (ETC) and no longer use an idle air control valve anymore. The drive-by-wire powertrain control module (PCM) can command the throttle body butterfly plate to any position desired. So a valve to allow a little air to bypass the throttle body butterfly valve at idle speeds is no longer needed.


Note: IAC valves are sealed non-serviceable items and not designed to be "cleaned" with solvents such as carberator cleaner. So any results that may be realized as a result of "cleaning" are likely to be short lived.

Additional pictures of a crown victoria throttle body, an iac valve, and an air intake plenum are avaliable by clicking here.