need help with idle issue

50 Proof · Oct 27, 2009

I recently picked up a 94 Mustang 5.0, 5 speed. It ran fine until last night. At idle, the RPMs hang around 2,000. If I rev the RPMs to 3,000+ it'll still only back down to about 2,000. It has about 136k miles on the odometer, everything is stock and original.

I checked vacuum and I'm getting about 20-21 pounds of vacuum, so I don't believe I have any leaks. I unplugged the IAC and it made no difference. I plugged the IAC connector back in and unplugged the TPS and the RPMs climbed a little.

I've been searching this forum over and over again and found the idle reset procedure. I'm going to try and reset the idle tonight. Is it reasonable to believe that the idle would just randomly need to be reset on a stock Mustang that hasn't even been tampered with?

I'm thinking the IAC is probably stuck open but I don't want to just pull the trigger on a new IAC unless I know for sure it's bad. Ford dealer wants $180 for a new IAC, I can get one from rockauto.com for about $100.

Any thoughts/suggestions?

Chythar · Oct 27, 2009

I would stick with a Ford/Motorcraft IAC. I've had bad luck with off-brand IAC's failing quickly, the Motorcraft ones last a long time.

You can clean the IAC and see if it affects your idle. Take it off the throttle body and spray throttle body cleaner inside a few times. If your idle comes back down, your problem is definitely the IAC. In my experience, cleaning the IAC doesn't last long but it is a good way to troubleshoot.

HISSIN50 · Oct 27, 2009

Don't reset your idle. If it ran properly before and for no reason it all of a sudden went haywire, it's a component or mechanical issue, not an adjustment issue (unless you know your idle stop screw walked or the bleed screw walked or cracked its O-ring).
Adjusting the idle would only create more variables.

What codes is it throwing? It should throw an idle limit code at a minimum.

I like the idea of cleaning the IAC. Just a caution (though you're prepared): Never clean an IAC if you're not ready to replace it. Sometimes cleaning does more harm than good. Matt is totally right about using Motorcraft for your IAC. It's one of the most finicky solenoids on the car (it uses PWM, so it needs to function precisely).

Good luck.

50 Proof · Oct 27, 2009

HISSIN50 said:
Don't reset your idle. If it ran properly before and for no reason it all of a sudden went haywire, it's a component or mechanical issue, not an adjustment issue (unless you know your idle stop screw walked or the bleed screw walked or cracked its O-ring).
Adjusting the idle would only create more variables.

What codes is it throwing? It should throw an idle limit code at a minimum.

I like the idea of cleaning the IAC. Just a caution (though you're prepared): Never clean an IAC if you're not ready to replace it. Sometimes cleaning does more harm than good. Matt is totally right about using Motorcraft for your IAC. It's one of the most finicky solenoids on the car (it uses PWM, so it needs to function precisely).

Good luck.

I'll check for codes tonight when I get home. The check engine light isn't on though so if there is a code, it's not triggering the check engine light to come on.

I stopped home on my lunch break this afternoon and tried cleaning out the IAC. When I reinstalled it and fired it back up, the RPMs were around 1700-1800 initially until I blipped the throttle a couple of times, and then the RPMs went back to sticking around 2000-2200 again.

Chythar · Oct 27, 2009

50 Proof said:
When I reinstalled it and fired it back up, the RPMs were around 1700-1800 initially until I blipped the throttle a couple of times, and then the RPMs went back to sticking around 2000-2200 again.

Unless you get a code that implies something else is wrong, I would say that this result says the IAC is gunked up inside. It's likely stuck open a bit and letting more air in than there should be.

If you get any codes besides 111 (Tests passed OK), report 'em back here.

HISSIN50 · Oct 27, 2009

I'd also add that it takes me about an hour to clean an IAC. I let it marinate with cleaner in it for quite some time (ten minute cycles). I rinse and repeat till I no longer have sludge pouring out. Then I clean it manually (cotton swabs, etc). It takes a while.

You might want to give it another shot if you think it could still be gummy.

50 Proof · Oct 27, 2009

Ok, so I pulled codes. Don't know why I didn't think of doing that in the first place. I guess I just (wrongly) assumed that if the check engine light isn't on, I won't have codes. Anyways, with engine running I got codes 411 and 412 -

EEC-IV Computer Codes Page 3

411 (R) Idle speed system not controlling idle properly (generally idle too high) -

412 (R) Idle speed system not controlling idle properly (generally idle too low) -

So, I guess that explains it. I'll take another stab at cleaning this one out but I'm still going to order a new oem Ford IAC from rockauto. If I can get this old one to start working again then I'll keep it for a spare.

Thanks for the input guys.

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