Hello FTE, been a little while since I have been on here and I have a situation I need help with. I have a 1992 Bronco 5.8/E4OD, the engine is a 1994 roller 5.8.
I have been driving it occasionally for the past few years with no real issues but a couple months ago it started idling around 2200-2500 RPMs. I unplugged the IAC valve and the idle immediately drops to right around 750 smooth as silk. After doing some reading on here and FSB I started looking for vaccum leaks. I figured it was time to do a little maintenance anyway so I replaced the gasket between the upper and lower intakes, went over all vacuum hoses (replaced all under hood with rubber a couple years ago) and also fixed a cracked pcv hose. None of this fixed it at all, I capped all vacuum ports on the manifold (except MAP) and no change. Tried cleaning the IAC valve and did not help. So here comes the kicker. I bought a replacement Motorcraft IAC and found out the original part number was replaced (superseded) with a new part number, I bought that one and It will not fit. It hits the heater hose nipple and will not mount flush with the throttle body. For now I cut a piece of cardboard gasket to totally block off the IAC ports and mounted the old IAC back on the throttle body. Truck starts and runs but a little cold natured of course. We haven't been able to find a different one that will clear the nipple. Has anyone ran into this? I am starting to think about building a new water neck housing out of plumbing fittings from the hardware store. I apologize for such a long post but I wanted to try to be as descriptive as I could about what all I have done. Thanks for any input.

 

750 RPM with IAC unplugged is too high. The engine should be on the verge of dying with it unplugged.

It looks like multiple issues going on. Vacuum leak or someone jacked with the throttle stop screw. Run the KOEO tests and CM code display.

 

Thanks for the reply Randy, I agree that it should die out, and that is what led me to suspect the IAC valve in the beginning but now that I have the IAC plugged in and the ports blocked off it is still running. As far as I know nobody has touched the throttle stop screw. Truck was running great up until it started this high idle. I have capped off all the vacuum ports with rubber caps, brake booster PCV and all except MAP sensor. still no change. Only place I can think it would have a vacuum leak that I haven't checked is the gaskets between the lower intake and the block/heads.
Pulled the KOEO codes and got some odd numbers that did not really fit, going to try again this evening. What I retrieved was: 12 33 27 62 86 34 2.
What is the CM code display?

 

I just re-ran the tests after letting the Bronco sit idling for 15-20 min. K.O.E.O. codes were 12, 31, 23. K.O.E.R. codes were 23, 81, 99.

 

Code 23 is for an out of range TPS sensor, that is the likely source of your problem.

 

Thanks Conanski, reading up on the codes lastnight I was thinking the same thing. And the 31 code make me wonder if the egr valve needs a little cleaning. If it is stuck open and the iac is totally blocked off that may be where its drawing enough air to idle. Does that sound crazy? I will be picking up a tps today.

 

There should never be any fresh air getting into the EGR system, if that valve is stuck open the motor will get more exhaust gas than it should which will make it run rough and stumble with a lower idle speed if anything. Code 31 suggest the EVP sensor is out of range which is a problem you should fix but in terms of significance it's a fair bit down the ladder from the TPS sensor.

The TPS sensor problem is the source of your high idle, the computer thinks the throttle is closed and commands the IAC to open more than necessary, this all about the computer getting bad information and not about a vacuum leak.

 

Sounds good to me. I will pick up a TPS and remove the blocker plate from the IAC and report back this afternoon. Thank you for the help.

 

Conanski, thank you. Replaced the TPS and unblocked the IAC. Bronco is running almost good as new. I still have a check engine light and will try to find time to pull codes in the next day or so. Bronco is drive able and I am happy.

 

My '92 wont even run with the map vacuum capped.