This has happened twice to me and once to the wife. Sitting at a light, truck is at full running temp and running normal, foot on brake, and all of the sudden it almost dies then goes to 2500RPM.

It's a 5.4L 2001 Expy. It happened before engine replace and again after engine replace for leaking head. Is it the IAC?

 

I seem to recall hearing about that happening before and it turned out to be the throttle position sensor. Don't quote me though.

 

That sounds more like a IAC than anything else. The TPS won't affect idle by itself. It will have an effect on the throttle response, but if you don't touch the throttle, a bad TPS won't cause the idle to be erratic. FWIW, if this ever occurs when you go from park to reverse, check your transmission filter. Trust me.

Take care,
-Chris

 

My 1997 4.2 will do the same thing a at a stop light it will
start to shake and die but it dose not rev up the dealer
replaced the engine and it still does this some one told
me that it was the IAC but i have not check this yet
to tell you if it is a fix or not.

 

having the same problem in my 90 w/ 5.0 . replaced egr , iac and did full tune up /oil change . doesnt do it as bad now but still runs rough,vacume leak would cause the same response. but i am thinking my problem is either fpr's or tps. ??