Dino92

I have. 92 f150 4.9 5 speed. Truck idles to high. I don't have a tach but it screams at idle. I have installed a new iac valve and cleaned throttle body and installed a new gasket. When running I can unplug the iac and it idles normal, I plug it back up and I can hear the air pulling through the throttle body and the idle raises. I have check the tps with volt meter .94 volts at low and 4.97 at high. What inputs does the PCM look at to activate the iac. Any help would be appreciated.

mark392001

Could easily be a vacuum leak which could be caused by a bad canister, vacuum lines, or intake gasket.

Dino92

I check for leaks and plugged off the hoses going to the canister , and the vaccuum block on intake including brake booster,

Dino92

I don't know why the iac valve is being activated . The idle speed is already to high. I thought when the tps read throttle movement it stopped controlling the iac unless the engine was holding you back like going down a hill. But while it is idling high I can move the throttle and the iac valve doesn't close, if I unplugged the iac while the throttle is moved the engine idles down to where it should be.

mark392001

Mine had idle issues and the previous owner had put a rubber grommet over the idle control screw which held it artificially high. I think he did that to avoid fixing real issues, which after I fixed everything was fine.

Dino92

Wow, the idle screw is ok on mine, I did notice that when I cleaned the throttle body the butterfly didn't close all the way, I could see daylight under the butterfly, I don't know if that would cause my problem because when I unplug the iac it idles normal

mark392001

When IAC is unplugged the truck should die if I recall correctly.

Dino92

I have researched some threads and they do say it should almost if not kill the engine, that makes me wonder if the throttle body is worn at the butterfly's causing airier air to enter.

tanman90

The throttle plates should be just about fully closed with closed throttle. I could see just a tiny bit of daylight through mine after a thorough cleaning.

mark392001

Forgot to ask, have you run codes on the truck? Even if the CEL isn't on you may still be generating a code that can isolate the cause of this without tearing all connected systems apart to check.