Car idles up randomly

[Mustang5L5]

This prob just started this summer.

Car will either idle normal at around 800 or idle high at 1500 rpm. It does not surge. It does it more when the car is first started and stops once it gets hot. Revving doesn't lower it and like I said...it's not a surge.

Weird part is I was at a light in neutral idling at 800 and the rpms just randomly rose up to 1500 rpm. So the iac opened up on it own by some signal

I've cleaned the iac. Biggest issue is if I stall or shut off when the idle is up....the car will not restart unless I push the gas pedal to the floor and crank it more than usual.

No codes stored....all 11s.

After car is hot it doesn't do it...so after 30 mins it is fine. But if I shut the car and restart...it sometimes goes back to the high idle. It's totally random, and like I said...not a surging idle at all.

It almost revs up like whe you do a koer test for codes.

Any ideas?

 

[Mustang5L5]

Btw...car is an aod that was converted to 5 spd. I added the Vss but kept all the aod wiring and EEC

 

[281pony]

i recall my car doing similar things to this with the sticky idle before my dyno tune. he went over a whole checklist of stuff real quick he was "turning off" for me to cure these issues.

have you ever considered a dyno tune? do you have anyone local? if it comes to me what he turned off with my tune, ill report back. i thought it was along the lines of smog iirc. it made a huge difference though when he did it even idling on the dyno.

 

[Celeb]

Possible failing TPS?

When mine was on it's way out idle would rise and hold but would quit when it was hot.

 

[2000xp8]

A few years back i had this issue.
It was the TPS.

 

[HISSIN50]

Another vote for it possibly being a TPS issue.

 

[Mustang5L5]

It could very well be the tps....I have suspected it of being on it's way out.


Good thing I have several spares.

Thanks guys

 

[jrichker]

The TPS is the #1 suspect for an idle that rises and then goes back down when you turn the ignition off.

See the "Surging Idle Checklist for help with all your idle/stall problems. Everything you need to find and fix idle problems is in there.

The first two posts contain all the updates to the fixes. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.

 

[Mustang5L5]

Well, tps replaced and still does it.

But...

New code appeared. Now I get a consistent code 21 (ect sensor) when I run the koer test after the car has been warmed up for a while. Hoewver, when I run a koeo test just before, I get code 11 (pass)

Is it normal for ect code to not appear in koeo but always in koer?

Anyway, got a new sensor...just need to install.

 

[Haus]

That ECT will fix you hot/warm start problem most likely

 

[Mustang5L5]

Replaced the ect and problem solved.

No more codes, car runs nice.


Got to do a cylinder balance test too...different story there.