My 04 ranger is back at it AGAIN.

It started about 3 years ago. Cruising along on the highway and then come into town. Idles at 2000 RPM

Shut it off. and restart it. Again, 2000rmp.
Shut it off and it idles normal

drive it 2-3 day and then it idles high again

I have clean and functioned the IAC and it works fine
Cleaned throttle body and I never can get it to stick.
Replaced throttle cable

it still was idling high

replaced ECM and it idled normal for about 1 year

replaced em again and it idled normal for about 6 months

it is back at it again


For what ever reason the EM is telling the IAC to open up and raise the idle

Suggestions?

 

Which engine, tranny & body does this puppy have??? Have you used your ELM scantool to look for pending, or set trouble codes, if so post All code Number clues.
Have the scantool run & post the STFT & LTFT numbers & any other suspect data it finds.

 

a bad pcv valve will cause a high idle and not throw a code..

 

4.0 5 speed. ELM? I have a Snap-on M2500 No codes ever. STFT? LTFT?


Has zero vacuum leaks and the PCV has been changed with zero difference

 

Ive got a 94 that had hi idle issues like you describe. I disconnected the idle solenoid and later had a guy with all the books do a system search and he came up with a short somewhere in the main wire harness. Have run with the solenoid disconnected for many ears now, did turn the idle stop up, but have no issues with idle with ac on or anything. give it a try and see if it helps.

 

Post up the short & long term fuel trim numbers. That should give us an idea if you have a vacuum leak some where that's not obvious.
When its at warm idle, what happens when you Disconnect the idle air control electrical connector???

 

it does Not have a vacuum leak. Been there and tried that with carb cleaner. And it seems to happen after the engine is warm(when it does happen). Vacuum leak would be consistent. or at least go away when it is warm .

Disconnecting the IAC does nothing when it has the high idle

 

Ok, disconnecting the IAC should cause the idle rpm to drop, maybe run rough, or stall, so with Nothing happening when its electrical connector is disconnected, suggests its not working properly. Its a common problem part & cleaning might not last, it didn't for me but was a good trouble shooting step before replacing mine with a BWD one & with a online discount code, from Advance Auto. Motorcraft, or BWD are recommended if you find its replacement time.
If your scantool will read fuel trim post up the numbers.

 

I know you believe replacing the PCV valve & spraying carb cleaner around under hood has eliminated the possibility of a vacuum leak & the cleaner put in the right place can help find a large leak, but it can't get everywhere a vacuum leak can occur & you don't need much of a leak to raise the rpm up to 2000.
So fuel trim numbers can tattle on minute vacuum leaks, as well as fuel injectors with a runny nose & can tell us which cyl bank may be the problem, so fuel trim numbers can be useful for other than detecting vacuum leaks & might be helpful in this diagnosis.
So if your snap-on can read the Rangers fuel trim, consider having it do so & post up the Numbers.

 

I have seen this on other Fords . Their fix was an IAC from Motorcraft . The aftermarket ones just would not work properly .
Just a thought ...

 

Ok, let me say it again a little differently

The problem started one day on my way home from work 3 years ago and 75k ago.

Ended up changing the ECM, problem went away for about one year and 25k.

The problem came back. Change ECM again, The problem went away for another year or so and 25k

I'll see if my scanner can see trim levels

 

With there being No change in idle speed when you disconnected the IAC electrical connector with the engine at idle, put the IAC high up on your suspect list. EDIT: Seeing as how you've gotten positive results after changing out the engine computer a couple of times, also put the IAC wiring & pins/sockets connections at the computer firewall electrical connector on the suspect list too.

 

Had problems on my 87 F150 years ago, RPM would not drop below 1700, intermittent. Problem was the plate in the throttle body was not closing completely due to gummed bushings. Made it real fun stopping and sitting at stop lights.

 

I will try an get the truck in the garage today before the snow
I am guessing a pinched or corroded wire somewhere

 

While your inspecting the IAC wiring on both of its run, also inspect the electrical connectors pins/sockets for pushed back/unseated, not locked into place pins/sockets, loose crimp, bent, or corroded pins/sockets, ect & let us know what you find.

Right now, with the idle speed Not responding to having the IAC electrical connector Disconnected, the IAC belongs pretty high up on your suspect list.