1991 Ford F-150 High Idle

ZOBIE

MEMBER
1991 FORD F-150
V8
2WD
AUTOMATIC
140,000 MILES

I've got a high idle problem with my 91 F150 V8. My idle is constantly high, anywhere from 1200 - 2000. It doesn't matter if the engine is warm or cold.

I'm also having problems with the truck idling up on its own as I drive it. If I'm cruising down the road, I feel the engine surge and the factory tach shows the engine reving up and down. Also, I've had the engine rev up several times at stop lights, etc.

The trasmission also shifts hard as well. It all started with a high idle, but has gotten worse as described above. It's a 3spd automatic transmission with push button overdrive. My gas mileage is terrible now and the constant high idle is hard on my brakes.

Could this be a vacuum leak. I'm gonna check my vacuum reservoir and see if it's metal and if so if it's rusted.

Any help would be greatly appreciated.

2 Replies

RASMATAZ

MECHANIC
75,992 POSTS

1991 Ford F150 High Idle

Too fast an idle speed. If an engine without computerized idle speed control is idling too fast and refuses to come down to a normal idle speed despite your best efforts to back off the carburetor idle speed screw or air bypass adjustment screw (fuel injection), air is getting past the throttle somewhere. Common leak paths include the carburetor and throttle body gaskets, carburetor insulator spacers, intake manifold gaskets, and of course, any of the engine's vacuum fittings, hoses and accessories. It is even possible that leaky O-rings around the fuel injectors are allowing air to leak past the seals. Another overlooked item can be a worn throttle shaft and a defective idle speed speed control motor/valve stuck in the extended (high idle speed) position/throttle position sensor. Also the throttle plate could be binding in its bore and kinked accelerator cable, coolant temperature sensor might not be operating properly misleading the computer that the engine is still cold and computer throwing fuel at it raising the idle speed.