So ever since I bought this truck (99 4.6 SC) it runs fine during the winter months, but as soon as it starts getting warm outside the engine detonates, mostly at low RPMs. I've tried fuel injector cleaner, cleaned the mass air sensor, cleaned the throttle body and intake manifold, checked for vacume leaks, and even tried that seafoam stuff. No luck. I've been running mid grade gas in it because it runs good on it, but with the way gas prices are getting I would really like to be able to run regular 87 again. Any ideas of what I could try next? Thanks
-Casey

 

With a 99 you may have EDIS instead of COP ingnition. Get some new plugs. You ahould stick with motorcraft, however some guys here like Bosch and others hate them.

 

Yea, I forgot to add, I've put new motorcraft plugs in it also.