idle increases after motor is hot (1-2 hour drive)

[porschpow]

ok,

So after my commute back home I have notice that when in gear, and at a stop light, I could feel it pulling, popped it in neutral, and the idle bumped up (a lot) can't est the rpm, don't have tach.

But I did fix a vacuum leak (the brake release diaphragm by the parking brake release-BTW, still doesn't release, maybe not enough vacuum), so I don't know what else. I cleaned the IAC, seems ok. I guess it's original so I guess I probably need to buy a new one

[88Vic]

In the meantime, when it idles up, cut the ignition off and on quickly, fast enough so it idles down but doesn't kill the engine. I had this problem (haven't tried to clean the IAC yet) and my old 87 Fox 5.0 did it too, and I use this trick to get it to idle normally, works most of the time for a while.

[DerekTheGreat]

Check that leak you fixed again and then check under the hood for leaks and such. Is the throttle sticking? What happens when you unplug the IAC? Idle should be down around 450 without it.

[porschpow]

I will double check the vacuum is there a way to test vacuum when you're under the dash. I know you cold spray TB cleaner around the lines with the car on

And I could unplug the iac while the car is on right?

[DerekTheGreat]

Soapy water from a spray bottle should do, it will bubble up around the leak. Honestly though, Ashleys car has a leak somewhere behind the dash and it doesn't affect idle, mpgs or drivability. Yes, you can disconnect the IAC while the car is running. I'd be tempted to disconnect it first, then press n hold gas slightly until it fires and then slowly back off the throttle, mimicking the IAC. Do this when the car is hot, might stall when cold.

[gadget73]

soapy water bubbles around pressure, not vacuum. A vacuum leak would just suck it in.

If someone has messed with the "idle screw", the IAC may not be able to control the idle properly. If the motor still idles high with the IAC unplugged, either the IAC is stuck partly open, or the idle screw has been screwed with.

[DerekTheGreat]

Yeah, that's what I said lol. Sucking water in would help clear out the carbon in the cylinders anyway, don't ya know...

[porschpow]

So, I unplugged the IAC with car on, Idle didn't change. So this means IAC is bad?

[gadget73]

either stuck partially open, or someone has jacked with the throttle stop screw. Often you can remove the IAC and flush it out with carb cleaner to make it go again.

[porschpow]

I tried cleaning it out twice, should I back off the throttle stop screw?

I mean it idles relatively low on a cold start

[gadget73]

i'd hate to suggest messing with that screw without solid reason. There is a way to reset it, but it mostly relies on having a known working IAC to do it. Basically you unplug it with it warmed up and set it to about 550 rpm, then let the IAC bring the idle back up. Only works if the IAC works though, otherwise its just all screwed up. You can always try it, just keep track of how many turns you move the screw. If the throttle blade tends to stick, its backed out too far. The screw is supposed to keep it from shutting all the way and jamming in the bore.

http://www.allfordmustangs.com/forums/5-0l-tech/340926-how-do-base-idle-reset-how-check-set-your-tps.html

[sly]

I wouldn't touch the screw without cleaning the TB of carbon buildup and knowing that there's no vacuum leaks. If there's something leaking air into the system when the engine is hot, I'd want to be sure that's fixed before trying to adjust the idle.

The procedure is pretty easy once things are sure to be sorted. A warm engine should idle at about 650-700 rpm. This is with 10 degrees BTDC, IAC unplugged and SPOUT jumper pulled.