Hi, my mechanic had the truck for about 3 months and could not figure out what the problem is. I just got it back and put a new (refurbished) throttle body and a different plenum, just to try and it did not help. When I first got the truck about a year ago, the high idle (1500 rpm) came and went, along with the check engine light. All smog equipment is still installed. I tried to fix the problem initially with new plenum gasket and throttle body gasket, new EGR and switch atop the EGR, new IAC and TPS. After that the problem was not resolved and instead of coming and going, the check engine and high idle would remain all the time. It was at this point that I brought it to the mechanic and he told me to buy a new ECM, so I did, and that did not help. He gave up on it eventually and recommended I convert it to carb. What do you guys think? Any suggestions before ditching the EFI? Should I remove the emissions stuff? I'd like to keep it fuel injected if possible but also willing to convert if that's what it takes. Any help is appreciated.

 

What are the codes? If the check engine light comes on, it will set a code.

I assume it's been thoroughly checked for vacuum leaks? What about the PCV system? MAP sensor?

 

I doubt the emissions equipment is giving you a high idle. I would suspect that the TPS isn't adjusted properly (you should get a reading of 0.9 volts on the signal wire at closed throttle) or the IAC is faulty.
Unless you have an intake gasket leak somewhere. The most effective way to check for that would be a smoke test. There's special equipment for that but you will just want to get creative. The idea is to pump smoke (shop vac ?) into the air intake tube and watch to see if it leaks out of the intake. Theoretically it should only go out through the tailpipe.
For testing the TPS and IAC, I'd recommend easyautodiagnostics.com
glc's suggestions are good too.

 

I have not been able to successfully pull the check engine codes in the past but I will try that again and let you know how it goes. As far as vacuum leaks, the mechanic said he hooked it up to the smoke machine and wasn't able to detect any leaks. I have since removed and replaced the plenum and throttle body so I guess it's possible vacuum leak. I have replaced the PCV valve and swapped the MAP sensor with another 5.0 f-150 but don't know how to test that. I have been suspect that the IAC or TPS isn't working properly so I will give those a test too. Thanks for the help guys.

 

Unplug the idle air control valve. If the idle slows down or the engine completely dies you have an electrical problem. Possibly the throttle position sensor. If the engine doesn't slow down expect a vacuum leak. This is a little tidbit I picked up on the internet from a very knowledgeable guy.

 

A thought - is there an idle stop screw somewhere that may have been adjusted improperly? Could the throttle cable itself be misadjusted?