View Full Version : high idle after m90 mustang swap

05-22-2015, 12:57 PM
Hey whats up guys. I swapped my built sc engine into my new edge mustang and its idling high. Don at lasota racing is tuning it and says we cant tune any further until its resolved.
What happens is on a cold start it does everything normal for about a minute then right when the idle comes down it shoots up to 2200 rpm and will not come any lower.
I have an FMIC and the 90 degree couplers suck in under vacuum. Could that be the cause? Or any other suggestions for what i can check?
Tried 2 used iacv's no change after switching them

05-22-2015, 01:24 PM
What happens on both cold/hot start if you disconnect the IAC and then start?

05-22-2015, 04:59 PM
I was told by don, my tuner, to follow the ford racing idle reset procedure, which is as follows;
-run the engine until you reach operating temp
-disconnect the iacv and turn the throttle stop open until the car idles at your desired rpm
-drill out the tps mounting holes so you can adjust the tps reading, then adjust to .97v
-reconnect iacv
for this reason, when I disconnect the iacv the car idles at 850, but runs rough, and lacks throttle response

I was told this is what needed to be done in order to get my car tuned right. but around the same time I did this is when my high idle problem occurred. its setting a code for "IACV % DC- RPM higher than expected" although the iacv is running 42% duty cycle and the pcm is not dropping that any to bring it down.

don suggested that my iacv was either wired wrong, or plumbed wrong. it couldn't be plumbed wrong cause it only goes on one way, and it cant be wired wrong because when I command a higher iacv duty cycle with a scan tool the rpm raises and when I command lower duty cycle rpm drops. so its wired right, correct?

tonight I am going to get a new tps, and undo the idle reset procedure by installing the tps in the factory position, then adjust my throttle closed until it reads .97v again. I did this before and it made no difference, although afterwards I found my tps was bad (when I opened the throttle plate to wot it maxed out at 2.4v rather than 4.6 or whatever spec is) so I am now going to be on my 3rd tps

05-22-2015, 05:11 PM
also some things you guys should know, the engine is moderately modified, 212/218 .520 lift 112* LSA cam, LT headers, custom FMIC, etc.
with a smoke machine I checked for vaccum leaks and the only one I found was a minor leak at the throttle plate shaft, but it is a low mileage magnum powers 85mm throttle body, so I figured it wasn't a big deal. I know mp wouldn't warranty It because I bought it used, and a new one is like $350, which I only want to spend if I know for sure it will fix it.

the engine was rebuilt and almost every sensor is new. we tried 25* spark advance at idle and then 35* spark advance (needs more advance cause of the cam) no change either way.

Don is confident the tune is not the root cause.
o2 readings look perfect, wideband shows 14.0-15.2 don't know if any of the oxygen sensors are reading correctly, I used engine break in oil which could have damaged the o2s but thought the voltage would be low in that case. but in this case the o2s are reading perfectly.

05-22-2015, 05:15 PM
tomorrow I am replacing the tps, both front 02 sensors and replacing the 90 degree couplers with cast elbows to eliminate the squishing. i'll keep you guys posted on the running condition of the car after I make the repairs. in the mean time, throw me some ideas on what to check. I work at a shop as a professional mechanic so I have all diagnostic equipment that may be necessary, but I am stumped. wish there was a dyno facility nearby that would take it and just get everything done for me and dyno tune it, as I have very limited free time to work on the car, yet its my DD and I am currently not driving it.

05-22-2015, 10:55 PM
Now i ain't no perfeshionul tuner but i have an anecdote along the same lines as your issue. After an m90 swap on a non sc motor i had high idle issues and it was not vacuum related. I was tuning with moates quarter horse and i lowered the idle air minimum airflow to something low like 0.010. It's been a few years but i vaguely Remer that setting the minimum idle airfllw to 0.00 would cause the motor to stall out. And .020 was too high. For whatever reason after a lot of trial and error changing only one parameter at a time i found that 0.010 was the happy spot to keep the motor from idling at 2200rpm. Take that with a few grains of salt though. We probably have/had totally different fuel/heads/cam combo. But i had a stock ford iac. So at least that much is in common. Now if you try telling this to your tuner this he will probably say "so have your forum tune your car for you!" And hand you the keys. At least that's what i would say.

05-25-2015, 12:35 AM
Sounds like a vacuum leak somewhere, for sure. I assume that you've done a spray or smoke test?

05-25-2015, 12:47 AM

05-27-2015, 11:31 AM
If the person tuning your car can't look at the data and tell you what the problem is he can't tune. That's what tuning is all about reading the data log and making corrections. If you have serious mechanical issues it can be difficult but he should point you in a direction.

05-27-2015, 09:35 PM
Which IAC and which EEC are you using? I went through a very similar issue years ago trying to run a OBD-I IAC with a OBD-II EEC.

05-28-2015, 06:44 AM
I'm running a fox body IAC on my SC motor and it required no tuning.

05-28-2015, 06:56 AM
My question was directed to the OP.