SuperTeece

My problem began as low idle and rapid fall of RPMs when releasing the throttle. After looking around I found some great help in http://forums.bradbarnett.net/showthread.php?t=63226 concerning cleaning the throttle body. I did as the thread suggested and my idle evened out in the 900-1000 RPM range as normal. I was left with the rapid fall of RPMs when releasing the throttle which I found in http://forums.bradbarnett.net/showthread.php?t=13380. I cleaned the IAC valve and now have the exact opposite problem. Now, as I drive, the RPMs stick at wherever they are when I release the gas pedal. I do not believe it to be a linkage problem with the pedal because if I stop completely the idle will level after about 10secs. another symptom is that while shifting, once the gas is released and clutch engaged, the RPM will rise a tiny amount, maybe by 10 or so. While returning from my test drive, driving though base housing at the 20mph speed limit, I actually noticed it slightly accelerating while in 3rd gear with my foot off of the pedal.

-----edit-----
In a possibly related matter...

After driving for about 15-30min at average road speeds, my center console gets very hot. The most heat is directly beside the shifter. I also notice that the shifter itself is getting very hot. I checked the tranny fluid level and it is fine and doesn't look bad yet (maybe 10k miles on it). I thought I had an exhaust leak and it was causing the heat along with the smell and backfiring, though after cleaning the IAC valve the backfiring and smell went away and left only the heat to plague me.

Thanks!

TC

cliffyk

Other than the 10 second interval (are you sure, 10 seconds is a long time) what you describe is normal--it's called emissions control, thank the Feds for their overzealous approach to auto emissions.

Search here and elsewhere for "IAC mod", it can take a bit of tweaking to get it balanced however it will cure things to some extent. In hardcore tuning there's an electronic dashpot parameter that can be adjusted.

SuperTeece

I'm pretty sure. I think a better way of explaining it is it's like I'm changing gears or going into neutral and not letting off the gas. My main concern right now is that this might burn up my clutch.

I'll look around for the mod, thanks.

---edit---
So after looking around it does appear like this is normal... but it doesn't feel right. I guess this thing was just dirty when I got it and I ran it until it was too nasty to operate. Having never drivin a Mustang off of the lot I guess I just didn't know any better.

I still think it is wierd thoguh that I can coast in second, apply the brakes to drop below 10mph and the car will accelerate on its own back to around 15mph without me ever touching the gas.

cliffyk

Whenever the car is in motion at all, the engine will idle at 1200-1250 rpm, I have speculated that this might have something to do with the hydroboost braking system--to keep the power steering system pressure (used to provide power braking) up while the car is still moving?

In any event, 1250 rpm in second with the stock gears = 15 mph...

SuperTeece

In motion I will idle anywhere from 2-3k Does this make a difference?

cliffyk

That is high.

The way this all works is that the PCM uses the IAC as an "electronic dashpot" to in effect keep the throttle from closing rapidly and having the mix go overrich. The IAC is normally open, forced open by an internal spring, allowing additional metered air into the engine, but the computer varies the duty cycle of the IAC control voltage to use a solenoid "motor" to partially close the valve and allow only enough air to do what it wants done to the idle speed.

With the car stopped and accessories off, the IAC is mostly closed and the idle is around 750 rpm. Turn on the AC and it should jump to 1k or so as the PCM opens the IAC a bit more.

Between shifts the computer does the electronic dashpot thing and opens the IAC to keep the mix from going rich and destroying the earth (which I believe would actually be to the delight of Al Gore and most liberals).

It then gradually closes the IAC over a span of 4-5 seconds to let the revs drop in a controlled manner to around 1250 if you coast. The rpms will stay at 1250 or so until 4-5 seconds after the car actually stops moving.

BTW-the IAC restriction mod works by reducing the air flow capacity of the IAC in a way that the PCM doesn't know about. This is why size of the restriction is so finicky, the computer tires to do what it thinks needs to be done but then sees the revs drop faster than it likes and freaks out.

So, I would suspect that originally your IAC was stuck closed and cleaning it freed things up a bit, but that now it is sluggish when moving from open to closed. The solenoid eventually closes it, it's just taking longer than it should.

You can verify this to some extent by unplugging it when the engine is idling--the revs should jump to 2k or more. Then plug it back in and it should drop to norrmal idle speed rather quickly. If this is sluggish then cleaning it again may help, however it could be that the core of the solenoid "motor" is magnetically saturated and lost its ability to snap that puppy shut.

The good news is that a 4.6L IAC is only $55-$60 at your FLAPS, for the Miata they're $279...

SuperTeece

As soon as I took the IAC off, the operation and what it did was pretty clear. I am a hydraulic mechanic in the Air Force so solenoid operated valves and metering valves are thankfully familiar to me. I am just having to learn when, why, and by what they are operated. I thought the same about the the electromagnet in the solenoid may be corroded or some other way weak. I had not price checked yet but from what you said, a little swap-shooting wouldn't break the bank. I'll ride out to the parts store after payday and swap it out in the parking lot, see what happens.

cliffyk

Cool, I apologise for the dissertation but there are a lot of posters here that don't have much mechanical/electrical/electronic theory background.

One difference between the IAC and hydraulic valves is that the controlled fluid is compressible, so by varying the duty cycle of the DC wave applied to the solenoid you can control the flow rate of an operational circuit--if you tried that with oil (other than in a well-snubbed control circuit) you'd have a chattering, self-destrucing mess...

SuperTeece

I tried this the other day. The engine just began to run rough. RPMs dropped a bit, instead of increasing.