I have a 1988 F150, 351W, EFI. It is idling at about 1200 rpm most of the time. Sometimes you can tap the gas pedal and it will idle down. Occasionally, it idles correctly approx 850 rpm. It seems to be better since the weather here has warmed up some, but still not right all the time. I have changed the MAP sensor, swapped the IAC motor from my other truck, and all the other sensors recently. My theory is that one of the new temp sensors is not working correctly. The question is, which one? or is it something totally different? Any help will be much appreciated.

 

Have you tried adjusting the throttle stop? maybe it's holding the blade open too far. Is there any build-up in the TB that could be holding the blade open?

 

Yes, I have looked at it and it looks like it just doesn't close down against it all the way sometimes. I have also cleaned the throttle body out with spray carb cleaner. I can't see anything that would cause it to hang open.
Could it have something to do with the air temp sensor in the intake? Also you should know that I had to replace the throttle positioning sensor awhile back. When I tried to adjust the voltage coming through the sensor by turning it in the screw slots, I was not able to turn it far enough to get the voltage down to what the book said. Seems like it said that the voltage on that circuit was supposed to be like .62 volts or something and the closest I could get was like .8 volts. I don't know if that could be a problem or not, but worth mentioning. I am not getting a "TPS Sensor out of range code" or anything like that.

 

I had the same problem with my '92. Drove me nuts. The first time, a few years ago, I replaced the TPS and the IAC and it fixed it......for about a year. Since I used to work at the store where I bought them, I got them replaced under warranty. Fixed it again for a couple years. About a year and a half ago it started the same crap again. It didn't make any sense to keep throwing new IACs and TPSs at it every time, so I figured it had to be something else. Cleaned the throttle body. Checked all the sensors; all good. Same problem, high idle. Then I noticed the coolant was low. Topped it off and the problem went away for a few months. Then last fall it acted up again, but the coolant was OK. Grrrrrrr.......
So I was standing there staring at the throttle body, and it dawned on my pea brain that the throttle body might not be getting coolant flow. I pulled the hose off that feeds it and tried blowing through it. Sure enough, plugged up. There's a teeny-tiny hole in the radiator neck that returns the coolant from the throttle body to the radiator, and it was plugged up. I never noticed it before 'cuz it's so small (maybe 1/16") and it's way down in the neck, even though the hose was right in front of my face. I cleaned it out with a pick, and the idle went right back to normal, and it's been fine for the last 6-7 months. After taking a good look at it, the hose that feeds the throttle body comes out of a goofy-looking tee on the lower intake. The coolant temp sensor sits on the tee above the hose nipple, so my guess is that the sensor wasn't getting any flow past it, and the ECM thought the engine was cold.

 

As long as the TPS voltage is below .99V, it will not effect the idle. That's where I keep my Mustang and it idles fine. Better throttle response too with it up there, so .8 is OK.

 

Thanks for the info guys. If the temperature sensor isn't working properly, wouldn't that also effect gas mileage since the computer could inject more fuel due to a cold condition? I am really hoping that my gas mileage improves since I am getting about 10.5 mpg. Granted, I have a C6 and 3:55 gears, which makes for screaming down the highway, but I still think it should be better than that. I had a 78 Bronco with a 351M and a carb and 3:50 gears with a C6 and it got 13.