1988 Ford Lariat F150 - 302 V8 - Dual tanks - AC, PS, PB, LWB.

NOTE: Previous faulty TPS that caused dead low idle has been corrected. Removal of throttle body led me to discover this present problem.

Present problem: Idle is slightly rough... ever so slight. New s/plugs installed= no change.

When throttle body was removed to correct a previous problem (TPS), I found that the throttle butterflies were covered with a damp carbon soot like substance. A consistency that could be moved with ones finger.

Just inside the intake manifold entrance, the bottom hole contained just a very small layer of hard caked black carbon. However, the top hole contained a build up of hard black caked carbon about the size of a silver dollar in diameter and roughly 1/8" thick. In both cases, this is right at the joint where the throttle body attaches.

Leaking, burnt, or stuck valve? Could some other electrical/electronic component malfunction cause this condition?

Note that out of necessity, I did run the vehicle with that faulty TPS for a couple hundred miles. I don't know if that has any relation to this carbon buildup or not.

 

Joe,
If I remember correctly, which is not too often, the PCV system has a filter in the air cleaner ductwork and if a lot of oil fumes are getting into the air intake of the throttle body that could be where the carbon grime is coming from. There were many vehicles in those vintage years that had loads of grime in the intake and ran just fine. Sometimes the best of intentions like cleaning it out cause other problems. This could be one of those situations where if it runs good, don't fix it! Has the PCV valve been replaced , if not might be a good idea to replace it.