Recently my '96 XLT 4.0L ohv with 90k miles has started exhibiting rough idle after what I used to know in my carbureted days as a "hot soak" (stop of approx. 20-30 mins after reaching normal operating temp). Engine starts fine when cold, fine when hot after short stops (gas stop, 5 mins) and fine after about an hour or more of cool down. But after a 20 min. hot soak it has started to stumble and rumble at idle, and even off idle with the throttle open slightly, for about 30 seconds (it could even be less time than that - maybe it just seems that long). After it has "cleared its throat" it runs fine. On one occasion it stumbled enough to cough and die, but after a restart it was fine. Check engine light hasn't come on, and have not tried to retrieve any codes from the engine computer (something I have never attempted on this engine). My impression is that it is running rich, and my gut tells me to check the evap canister operation - a feeling based on the previous similar symptoms on my old Chevy S10 would show when its evap canister went bad and it would dump the fumes at the wrong time - albeit that was a carbureted engine. The IAC is new but did not fix the problem (not to worry, it was already on its second leg after a cleaning about two winters back, which cured the cold hard starting problem and prolonged its useful life. Think I got a little too liberal with the cleaning spray this time and turned it into a trumpet - least that's what it sounded like at idle after my little "repair" ;-). It has done this through at least 4 or 5 tanks of gas now, and seems to be getting worse.
Didn't find much about this googling the groups. Any suggestions where to start looking? Other than this problem, it runs great.
Thanks, Steve