I'm posting regarding a 1990 F150, V6 with a manual transmission.

Lately, the engine sounds a little odd under certain circumstances. The sound is kind of hard to describe. The closest thing I can think of is when an engine is idling low, and one point is a little lower than the rest.

It only manifests after driving around for a while. It appears most prevalently after a rapid deceleration like getting off the highway. If the truck is allowed to sit for a while it goes away.

Based on this behavior I'm thinking a collapsed or leaking vacuum hose are likely culprits.

What do you guys think?

Thanks,
Tim

dirty or bad IAS?

Sorry.. IAS? Not familiar with that.

Incoming Air Control solenoid? Pick up a can of "Seafoam" and use as directed. Works great & fun to use! You can remove it and clean it but use the Seafoam also, to further improve throttle response, idle & remove carbon build up in combustion chambers.