Hello, I have 96 f-350 5.8 with very high idle, replaced tps from ford and iac from advance auto.When i unplug iac when idling it seems to idle close to normal. Anyone have any ideas that might help? Thanks.

 

check for vacuum leaks. the most common leaks are the intake gaskets but check everything.

 

Yea thanks for your reply, I have checked for vacuum leaks around hoses and intake with carburetor cleaner didn't find any. I was wandering if it could be the coolant temp sensor, or should i check vacuum leaks some other way?

 

The place they leak is the gasket between the plenum and the intake manifold. That big *** plenum ruins the gaskets. Propane works good one of them little torch's. Its hard to get inside that overhang of the plenum maybe if you used some hose...

 

Hello, Thanks again for your post checked everywhere no vacuum leaks found.No cel and i couldn't check it anyway because the wires to it have been cut now what?

 

You have to check with propane (easy to do). - Have you done this ? Carb Cleaner can make them leak lol.

 

As jethat posted earlier is the best way. Here's how, - you can use a propane torch , like you use for sweating copper. Attach a section of hose to it / open the valve , 1/4 turn or so and hold the hose next to gasket seams. Run the seam while checking , slowly (as best you can). - Take a breather if gas starts to get heavy, - let it clear and give it another shot.

If their leaking anywhere, your engine will let you know (do this w/engine running of course). It's the easiest and quickest way to check the intake gaskets. Hosing them down with chemical isn't a good way IMO. It's not accurate enough and with the engine running, it's also a PITA. Plus, applying what you are, isn't good at all lol.

You can check hose connections with propane as well.

BTW- Sounds like your IAC is trying to work, but you outside of idle rpm margins/parameters, making it unsuccessful.

Is this a edelbrock set up by chance?

 

Don't use flammables to check for a vacuum leak - use a stethoscope; either a ~$12 one from a parts-store, or a 3' section of garden hose. The usual spot for the plenum gasket to leak is under the center on the R side near where the 52 arrow is pointing:



The CEL has nothing to do with your ability to pull codes. Whether it works or not; whether it's on or not, you need something like this:

 

High idle problem solved it was a stuck egr valve. Thanks for all of your help.

 

When this happened did you have less power when warmed up then cold it seems as though I'm having a similar issue