UPDATE 12-16-11 (read post 11) Bad IAC or something else?

[boat]

This on the blue 94 mtx.

Just replaced the IAC about 2-3 weeks ago. Been having some issues getting it to idle correctly with all things turned on, lights, heat, radio, etc.

I disconnected the negative battery, not really following a procedure to reset the idle. So, I do this and turn on all accessories, and allow it to run for a little bit, then take it out for a drive. All is good. The next day go out to drive it, driving down the road, have the heat on and the rpms start dipping down to 500 and bouncing back up to 12-1300. I turn the heat off, and all is fine, no more fluctuation of the rpms.

Fast forward 2 weeks to yesterday. I found this reset procedure. I followed it to the "T" and all seemed like it was okay. Until later in the day, the rpms started bouncing again from 500-1200 with the heat on, and stop when turned off.

It didn't do this before changing the IAC, it was more of a really slow return to idle before. Could it be bad out of the box? Or did I uncover something else by fixing that? The IAC looks longer and bigger than what I replaced, almost like one I had on my 95 ATX.

 

[rubydist]

I have found that when doing the idle reset, there are a couple of potential "catches"

1. make sure the battery is disconnected for at least an hour before attempting - some pcms will reset quicker, but my red atx would take at least a couple hours before it would reset.

2. when you do the idle reset, and it runs for the 5-7 minutes, be sure to turn off the car before you attempt to drive it. for some reason, if you start driving it w/o turning it off, it can get confused about what idle speeds should be.

 

[msteiny]

Since you said it is bigger than what you replaced. Are you sure theres not a vacuum leak, I know they suck to take on and off, but what your describing sounds like a vacuum leak.

 

[Rubix]

I've had this problem before. With my SHO it happened when I was moving and not on the throttle, like coasting on a residential street usually between 25-35 mph; didn't matter if heat/accessories was on or off. I researched and figured it was a vacuum leak or IAC, as you have also found. I pulled the IAC, blasted the insides thoroughly with carb cleaner and put it back in; seemed to fix the problem. That was last fall. Problem just started happening again about two weeks ago. I know I have a vacuum leak somewhere (you can hear the hissing when in Park) but I think in my case it's the IAC. I'm going to either buy a new one or pull a good looking one from the junkyard in the next week or so. I'll be sure to report back with the results.

 

[boat]

I will try what rubydist suggested.

It mounts the same way. No air escaping around it. But the first time I changed it I have learned that the IAB doesn't like to be put in backwards. Flipped it back to the way it should be, and it ran fine. It idles fine with no accessories on. The rpms just dip up and down with a load on the engine.

Since you said it is bigger than what you replaced. Are you sure theres not a vacuum leak, I know they suck to take on and off, but what your describing sounds like a vacuum leak.

Good luck. Mine was a brand new one. Got tired of putting used stuff on, then not really sure how the old used stuff is going to work or for how long. I would buy the new one if you can swing it. I think I paid too much to get it "now".

I've had this problem before. With my SHO it happened when I was moving and not on the throttle, like coasting on a residential street usually between 25-35 mph; didn't matter if heat/accessories was on or off. I researched and figured it was a vacuum leak or IAC, as you have also found. I pulled the IAC, blasted the insides thoroughly with carb cleaner and put it back in; seemed to fix the problem. That was last fall. Problem just started happening again about two weeks ago. I know I have a vacuum leak somewhere (you can hear the hissing when in Park) but I think in my case it's the IAC. I'm going to either buy a new one or pull a good looking one from the junkyard in the next week or so. I'll be sure to report back with the results.

 

[rubydist]

what I have found w/ factory iac is that they typically don't close all the way, so if you have a vacuum leak of any significance somewhere else, you cannot get the idle speed down to where it should be (unless you mess with the throttle plate stop, which is an invitation for all manner of difficult-to-drive-at-small-throttle-openings issues), so my advice is to find and fix the vacuum leak before you mess w/ the iac.

 

[Rubix]

Here ya go, a poor cell phone video of the rpm bounce I experience. Took this just last week while thinking about this thread. My foot isn't on the gas, just coastin down the road; watch the gauges.

[youtube]NPVSURmLSCk[/youtube]
http://www.youtube.com/watch?v=NPVSURmLSCk

 

[kevinspann]

Jamie, I still think your VSS is dying/dead.

 

[rubydist]

Rubix, my experience is that cleaning the iac doesn't last long, so I would guess yours is sticking again.

the rpm should not change that much due to either the vss signal being inconsistent or the a/c kicking on/off - it takes a sticky iac to change that much.

I bought a new iac from rockauto this summer and the new one was worse than the old one in terms of being sticky, so I had them warranty it and now the one I have works fine - so just 'cause the one you have is pretty new doesn't mean its not sticky.

 

[Rubix]

Yea I remember reading that they'd get stuck again after being cleaned. I just didn't want to buy a new one when I cleaned it last fall. I've looked at them on rockauto before, out of curiosity which one did you go with?

 

[boat]

UPDATE:

I followed the idle reset procedure to a "T" that Sd Patt put up a while ago, followed what rubydist told me to do to the letter and I still have the same issue.

It seems like mine takes a while to reset the computer. So, I left the negative off for a few days while I wrapped up my school semester.

This morning, I re-attach the negative battery cable, start the car turning on everything I can with in a minute of starting (light switch doesn't work due to bad wiring in the dash, tried to force on by covering up the "eye", running lights and dash lights never did turn on. Just the headlights and brights came on). Let it idle the 7 minutes, shut the car down. Then restarted, turned the heat on full blast and went and drove it. All seemed fine, the idle was not dipping at all.

So, I thought it was fixed.

Took it through the carwash, the headlights came on, had the heat on full blast, idled just fine with no dipping down. I pull out and leave it running (with heat/defrost on 90) while I dry door jams, etc. Kept thinking I heard it dip a little bit, but wasn't sure if that was what I heard or not. I get in to drive away, had to turn around and then the RPM's dip and the car dies on me. I get it restarted and it won't idle for anything with a load on the engine. I turn the defrost on and the RPM's dip, I turn it off and it runs fine. Now this has all happened since I replaced my IAC.

Now one thing it is doing different after today's reset attempt.

If I am stopped at a stop light after all of this has happened, and I turn on the defrost, it does the same thing, the RPM's dip up and down, and doesn't want to idle. If I put a load on the engine while driving down the road and not stopped, turning the defrost on, it doesn't dip. But the next time I stop (or coast in neutral), the RPM's are stuck at 1500, otherwise with nothing putting a load on the engine, it idles at 1000, nice and smooth.

I suspect a bad "new" IAC out of the box. This car also has a Ted B LPM tune. Could it be the tune? Could the computer not be accepting a full reset? I am going to try putting in a used IAC that I have here in the garage, see if that fixes anything.

Not sure what else to do here.

 

[NoSlo]

This is a classic IAC symptom, but I'm going to guess throttle position sensor. Are there no engine codes set?

 

[SHOspazz92]

My dad had similar drive ability issues when he had a bad motor mount in the car. The engine had so much movement it pulled on some wiring, and even disconnected a sensor! The fix was installing the Motor mounts that are installed now (SHOnut extreme rear, Crazy ass Mount in the front that came from Tyner). That fixed everything. I would just take a look at everything, make sure its all connected....etc.

Just a thought.

-Sam

 

[rubydist]

my 92 seems to take a couple of weeks to completely 'learn' to control the idle properly, even after doing the idle reset properly. yours may have the same issue.

otoh, I have had a bad iac new out of the box, so don't assume its good.

 

[boat]

There doesn't seem to be any engine codes. I believe I ran them before and there was a "111". Basically means all is good.

This is a classic IAC symptom, but I'm going to guess throttle position sensor. Are there no engine codes set?

I won't be doing the reset today with a different IAC. Its snowing out and the blue one is staying right where it sits. If I have to, it will wait until spring.

I suspect it is bad out of the box. Too late to return it and no way to prove it is bad. I think I may have just eaten about $70

 

[boat]

Solved!

I think I had a bad IAC new out of the box. I put one I had laying in the garage in, reset the idle and then drove it, found to have that one actually be bad. The RPM's would go up to about 2k and just hang there, then eventually drop. Press in the clutch pedal and the RPM's would go up and down rapidly. Mainly between 1k and 1500. This one is bad. Found another one in the garage went through the same reset process, and this (Ford) IAC seems to be good. So far so good, I can put a load on the engine with turning on heat, lights, and all seems to be good.

I took this SHO into Ford this morning to diagnose some things I wanted to be sure of before I started throwing parts at it. (24.95 is better than money thrown at parts I don't need). I asked Chuck (Can't remember his screen name here, I want to say SHO-n-Go, I don't think that is right), about IAC's this morning. He told me that the aftermarket IAC's and MAF's are known more often known to come out of the box not working properly or to be bad.

 

[SHOspazz92]

Glad to see its going well for you. Enjoy the **** out of that car come spring!

-Sam

 

[rubydist]

excellent!