HELLO EVERYONE. I GOTTA PROBLEM WITH MY 1991 F-150 with 5.0 engine that it idles very fast, i have replaced the AIC but it diddnt help. any ideas on how i can slow it down?

 

You have a major vacuum leak. Hopefully, you did not spray any throttle body cleaner in the TB to clean it. If you did, the intake gasket is toast. But the probabilities are high that's the problem anyway. It was a common issue on these engines for the intake gasket to go south and the engine high idled at about 1800-2000 rpms. But look for any vacuum leaks before tearing it apart.

 

With the engine idling, pinch or plug the large vacuum hose to the brake booster. If the idle doesn't drop, use a 3' piece of garden hose as a stethoscope to listen all around the intake manifold for the characteristic HISS of a vacuum leak. The gasket Labnerd mentioned is #49 in this diagram, and it often leaks under the plenum (passenger side).



Also check the PCV hose (42 & 51), all the lines coming off the vacuum tree (52), & the coffee can on the R wheelwell. There's also a vacuum reservoir glued to the evaporator cover facing the R exhaust manifold, and if the foil-faced heat shield has degraded (they all do), it can melt or warp away.

 

when i unplug the AIC the idle slows down to normal, it is hard to keep running when cold but seems ok after it gets hot

 

is there any other sensor besides the AIC that i can replace?

 

have you even read the above replies? you have a vacuum leak somewhere. its why it stays running with the iac unplugged.

 

yeah i found one broke i replaced it but it diddnt help...i'll check the rest of them and replace them with a rubber vaccum line

 

have you checked the throttle body and intake gaskets yet?

 

not yet but i will....would it lack power?...because i can burn both tires

 

not really. a vacuum leak generally only affects idle.

 

That's not normal.Sure: all of them. You could replace the entire truck if you want, but it won't necessarily solve this hi-idle symptom. It has to be diagnosed.You asked for help; we gave you several suggestions; you don't even try them & ask for more help...???

 

oh im going to...thaks for the help