high idle problem on a 4.0

mailboxman · Nov 18, 2009

i searched these forums and cant seem to find the answer--my idle is 1500 cold and drops to 1200 after warming up.

I reset TPS to .96 volts---I cleaned MAF with MAF Cleaner
I replaced coolant temp sensor
I replaced IAC valve today
I checked for vacuum leaks with carb cleaner and propane
I did selt tests tonight and only codes were KOER
412Cannot control rpm during KOER high RPM check
411Cannot control rpm during KOER low RPM check
538Insufficient RPM change during KOER dynamic response test/ Operator error

I dont know guys but something is screwin with me---I now have the new IAC
bolted on but unplugged--I have the old iac plugged in but not bolted on and my idle is around 650, but it goes down to 500 with ac on and will not idle when cold

any suggestions?? It seems like the computer is telling the IAC to idle fast, because if i disconnect the iac it slows down but check engine light come on

any help would be appreciated

Nhaz · Nov 18, 2009

I would say check the temp sensor but you said you have replaced it. I believe there are 2 temp sensors. ones for the dash the other for the computer. Is it possible you replaced the wrong one?

A more exotic problem is one I had where the computer itself was a corroded mess and had pretty much the same issue my IAC refused to turn off. check the computer behind the passenger side panel to see if its getting wet.

mailboxman · Nov 19, 2009

Thanks for the response Nhaz---I even swapped back to the original temp sensor and still the same thing--I did check the computer and like the rest of the truck--it is really clean(i just got this truck in Sept)

Unless I can get a better answer I am going to throw some more money at this and try replacing the ATS sensor on the intake although from what I read, it only affects WOT???

I consider myself a pretty good mechanic but this high idle has me stumped--presently i am running with the new AIC on the truck but unconnected, with the old aic strapped next to it and plugged in to keep the cel off

appreciate any more ideas anybody has

thanks

Rollin · Nov 19, 2009

What about the TPS, and are you sure the throttle valve is going back to close spot and not sticking open just a hair. Mine did that, some how it was binding up only when you used the gas pedal.

modelageek · Nov 19, 2009

as Nhaz said "there are 2 sensors are you sure you are changing the correct one"

Big Jim M · Nov 19, 2009

Prolly!

modelageek said:
as Nhaz said "there are 2 sensors are you sure you are changing the correct one"

My exact response! I betcha you are changing the temp sensor for the gauge and not the one for the engine computer.
Big JIm :icon_thumby:

modelageek · Nov 19, 2009

On my truck a 99 2.5 both sebsors are 2 wire. On other trucks the gauge sensor is 1 wire and the puter sensor is 2 wire

Nhaz · Nov 19, 2009

mailboxman said:
Thanks for the response Nhaz---I even swapped back to the original temp sensor and still the same thing--I did check the computer and like the rest of the truck--it is really clean(i just got this truck in Sept)

Unless I can get a better answer I am going to throw some more money at this and try replacing the ATS sensor on the intake although from what I read, it only affects WOT???

I consider myself a pretty good mechanic but this high idle has me stumped--presently i am running with the new AIC on the truck but unconnected, with the old aic strapped next to it and plugged in to keep the cel off

appreciate any more ideas anybody has

thanks

It took me a long time to work out what my problem was. My redneck fix was to install a toggle switch for the IAC so i could manually control it.

creepinjeepster · Nov 19, 2009

Did you reset your TPS before or after changing your IAC? If it was done before this could be the reason for both your high idle still and your CEL when the new IAC is plugged in. (NOTE: Not all TPS's are set to .96V. There is actually an acceptable range for them to be positioned at. There is a tech artical in the library here about it.) I tried adjusting mine to the .96V I kept hearing everyone talk about on here after replacing my IAC. I ended up with CEL light and lost throttle response and irratic idle. Ended up adjusting my TPS to a .87V. CEL went out, started responding to throttle and started idleing correctly.

mailboxman · Nov 19, 2009

Thanks for the great responses--I thought I was alone on this one--the temp sensor I changed is the one on the front of the intake manifold--i had to take off the pipe from the air filter to the intake to get at it--it is definitely two wire--does this sound like the right sensor??? If not, i will gladly replace the other one!!

I have checked the throttle body and the butterfly is not hanging up--when I disconnect the IAC my idle goes down so I also believe no vacuum leak??? I sprayed a whole can od MAF sensor cleaner and a can of carb cleaner trying to find a vac leak--i even resorted to propane with my plumbing torch (unlit of course) and no idle change.

Just for shi** and Giggles I will pull the wires off of the second temp sensor and see if my gauge quits working--this should tell if I replaced the correct sensor or not--other than the idle, this truck runs great--

I will go the toggle switch route if i cant fix this correctly, but does your check engine light come on when you shut the toggle off???

I am also considering changing the act sensor on the intake--has anybody any experieince with this one?? When you look into the butterfly on the throttle body, you can see the end of the act sticking into the intake--i cant see how this would affect idle though???

You guys really got me thinking that maybe I did replace the wrong temp sensor--what concerns me though is no codes???

Thank you all for the comments--got me thinkin!!!

Big Jim M · Nov 19, 2009

Just for shi** and Giggles I will pull the wires off of the second temp sensor and see if my gauge quits working--this should tell if I replaced the correct sensor or not--other than the idle, this truck runs great--

I should have suggested that!
Big Jim

Nhaz · Nov 19, 2009

The IAC on a toggle shouldn't pull a code if you use it like its supposed to.

As in don't forget to turn it off when the engine warms up some=> If the engine is warm you shouldn't even need it. The computer is hard wired to use the IAC in the start cycle even if its not needed.

mailboxman · Nov 19, 2009

Thanks again all for the responses--i did change the correct sensor--I am getting discouraged so I walked away after that--it would be great if thats all i had to do was change another sensor to fix this thing--but thats not the case---but i had another idea while out walking the dog--if it was the temp sensor---it would be stuck at either 1000 rpm(cold idle) or 700 rpm(hot idle) but the idle is 1500rpm cold and 1200rpm (hot)????????? so it seems that the temp sensor is working, just something else is out of whack????

I am thinking about going out and starting it up and disconnecting TPS first, and just see if the idle remains high or goes down--if this has no reaction i will plug the tps back in and unplug the MAF and see if the idle goes down. If one of these disconnected makes the idle go down--maybe i have found the problem and will replace---if neither affects the idle maybe i will have to look somewhere else (EEC)????

What do you think??? Also, i have read a couple of different ways to clear codes and reset computer---does disconnecting the battery for 5 minutes do it???Please let me know.

Nhaz · Nov 19, 2009

Disconnecting the battery for awhile will clear all codes.

mailboxman · Nov 19, 2009

You guys are all great with some really good answers--I think we may have all missed the best one from creepin jeepster!! I went out tonight and started the truck--rpm's went right to 1500 (cold) I disconnected the TPS-nothing, I reconnected the TPS and disconnected the MAF-nothing, so I left the MAF disconnected and also disconnected the TPS-Idle went right down to 600 which is telling me either the MAF or the TPS is suspect??? I also notice the TPS is Gray which probably means it is not a motocraft????
If you loosen the tps screws you can maybe get a 16th of an inch movement--a few weeks ago when I initially tested it, it was showing .86 volts, i loosened the screws and turned it as much as i could, retightened screws and it was reading .96 volts, and this didnt make it any better or any worse.
I really dont want to put a toggle switch on--i would really rather have it working correctly--If anybody can add to this I would love to hear your thoughts--I think I now need to either replace or test the maf and tps.

high idle problem on a 4.0

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