I'll do my best to get to the point:

Issue- High idle. My truck will not idle below 1600rpm and will go as high as 3400rpm. It surges a little but if I drive it the RPM will continue to climb and stay.

What has changed- I have been trouble shooting the pesky door ajar/ auto lamps inop/ door locks inop issue. I have tried a used VSM and I got a used instrument cluster. The used VSM was faulty as well (I’ll spare details). The instrument cluster had no relevance to my issue. I put the stock cluster back in but have the VSM is disconnected to keep the interior lights out. I also installed a Scangauge II.

All the truck wiring for the VSM pins out OK. The door switches are good. I have a VSM on order and hope to resolve my aforementioned issue. What do I think any of that has to do with my trouble? NOTHING. I can read schematics and there are no related wiring connections between the VSM and the engine PCM (with the exception of ground).

I need to know what can make the idle go crazy like that. Here is what I have checked.
-Throttle cable
-Throttle body
-Inlet Hose
-Obvious Vacuum leaks
-Connectors
-Engine trouble codes

I have come up empty. I fear I will need to take my truck to Ford and they will be more than happy to hold their hand out for 1,000 dollars as some incompetent boob replaces parts without actually doing diagnostics. (No I'm not bitter from previous experiences).
I could figure this out myself if I had diagnostic tools and an engine parameters list.

Thanks

 

i to have an issue like this on my '99 with the 5.4. we are lucky that we have mauals. mine will sometimes idle around 1250 or higher, but not over 1500 and it will do it when its warmed up. starts fine and idles up like it should when it starts. kinda an intermittent issure with mine as it doesnt do it all the time, but gets annoying.

 

This is very annoying.

 

Try cleaning throttle body, IAC, and MAF. 3400 RPM is a quite high though for a stuck open IAC.

If that doesn't work, maybe having the throttle position sensor (TPS) checked?

 

The 1st thing I would do is swap out the TPS. Had the same problem on the 01 Chevy I traded in. Swapped it out, worked great. AutoZone has it right now for $28.99. I just checked online.

 

I second this suggestion. And keep looking for air leaks into the intake tract.... The engine can't rev this high without getting plenty of air. If the throttle body blades are closing properly, it's getting in somewhere else.

 

Thanks. I agree about the air. I'll check the feedback on the TPS voltage. I can also clean the IAC as well. I checked the vacuum line to the brake booster but it is fine. Not sure where else there would be a leak. Thanks for the advice. I'll keep y'all posted so there will be history of the fix.

Any way (or what is the likelihood) a PCM / ECM could do this?

 

IAC................

 

Well, thanks to all that helped me. She is fixed and running like a top. The IAC was the issue.

I removed it and cleaned it. It ran OK until I tapped it with my screwdriver. The RPMs shot right up and I could hear the air rushing through. My scanner showed that the voltage to the IAC did not change. I pinched the hose that comes off the intake to feed the IAC and the throttle dropped to normal levels.

Easy fix - all it took was a little cash and time.

Thanks again!