A vacuum leak/idling problem
In the October 2010 issue of Auto Restorer I read the item about a 1990 Crown Victoria station wagon that won’t idle. I had a 1992 Ford F-150, also with a 5-liter engine, that had the same problem. When it was about five years old, the idle increased one day and stayed there—around 1200 rpm. This was dangerous, especially when I had to stop on ice in Minnesota! The error codes only said that the idle was too high, and it’s not adjustable. I took off the idle air control valve (same part that you mentioned as a possible cause for the Crown Vic) and it wasn’t shutting all the way. I got a new one and it was the same way—they are designed to not shut all the way. Maybe this is a default limp home feature?
Installing the new idle air control valve did not cure the problem, so I went to a dealer and asked if they had seen the problem before. They let me talk to a mechanic that said he would bet money that the gasket between the intake plenum and the intake manifold had been sucked partially into the intake between a couple of bolts on the right side near the front. I bought a gasket on my way out and found that he was right. Note the intake plenum is Ford’s name for the chamber between the throttle plate and the intake manifold. The gasket being partially sucked in created a vacuum leak, which has the exact same effect as opening the throttle a bit. The engine sensors then tell the computer to give the injectors more gas to get the fuel/air ratio right, so it ups the idle without any other effects. The broader message here is that a small vacuum leak anywhere can cause this problem.
Thank you for taking the time to share your story with us. There is likely not quite enough clamping force at the point where
the gasket between the intake plenum and manifold gets sucked in. It’s probably wise to use strong gasket adhesive on the replacement gasket to discourage the problem from reoccurring.