Constant high idle

[88MercWagon]

Hey guys, I'm in the process of diagnosing 3 problems at once here. Recently, my car began constantly idling high...very high, as in ~1500 RPM, like when you first start it on a cold day. This is killing my driveability and gas mileage, and I need to fix it, ASAP. So far I have:
-Removed and cleaned both the IAC and EGR valves. IAC appears to be functional, the plunger moves freely through the full range. Also, on a side note, I know it is opening since there is plenty of air flowing through the bypass at the throttle body

-Unplugged IAC with engine running. No change, still racing along at high idle.

-Performed a vacuum test. 22in Hg at high idle, 17 inHg in gear, which brought the idle down to ~900 RPM or so.

Now, my question is, is my IAC appearing functional and shot? I'm beginning to think so since it ran like this with it unplugged. Or, is the computer keeping the car at high idle for whatever reason, and the engine's vacuum managed to keep the IAC from closing when I unplugged it? Any answers/thoughts are appreciated. Thank you.

[85crownHPP]

Smack the IAC with a screwdriver handle and see if it changes.

[88MercWagon]

Tried that, no change.

[Merc302]

ive got the same problem with my 85 Grand Marq. with the TBI whatever the shit is on the passenger side of the throttle body, with the teeth that catch the throttle. does NOT work right. i think its some sort of choke related item

[mrltd]

There could be a vacuum leak. could be a physical blockage from a cable or somethng on the TB also.

Could be bad IAT or ECT sensor

[Merc302]

I checked the throttle cable, its all "good" the butterflys seem to be operating normally.
you say intake air temp and whats ECT?

[Lincolnmania]

have you checked for codes?
check the spout terminal connection too

[bigblackmariah]

trans cable

my high idle issue ended up being a broken trans grommet and the cable end fell and fouled the return of the throttle body arm, of course i had trans symptoms too, but when it first happened a few blocks from my house, i thought IAC. upon inspection i found the cable keeping the TB open.

[sly]

hope you didn't drive it any more until you fixed the trans grommet. AODs like to die a horrible death when that plastic POS breaks.

this is what you need for the fix: http://www.lincolnsonline.com/tech/00052.html

[91waggin]

ive got the same problem with my 85 Grand Marq. with the TBI whatever the shit is on the passenger side of the throttle body, with the teeth that catch the throttle. does NOT work right. i think its some sort of choke related item

I checked the throttle cable, its all "good" the butterflys seem to be operating normally.
you say intake air temp and whats ECT?

You have several threads of your own about your issues. Use them. Don't foul up this guy's threads with your own problems. This guy has SEFI, and will likely need a completely different set of answers than you with your CFI. Please refer to the following message by the site owner:

http://www.grandmarq.net/vb/showthre...hreads-in-here.

I count 7 threads started by you on the first page of the performance section, when almost nothing you've been talking about has had anything to do with anything except your stock engine.

[87_crown_vic]

the tv bushing ... aka AOD killer... glad i have a 1986... though need to replace fiance's tv bushing since she has a 1990... gotta dig up my brass bushing i have floatin around...

[Lincolnator]

I'm having the same problem on my '86 LTC with SEFI. I've been working to get it smogged and found the vacuum line to the MAP was collapsing at high vacuum. I replaced the line and tested the MAP, checks good. Start up and the idle begins surging wildly so I disconnected the battery to clear the computer. I reconnected it and that's when the high/racing idle began, exactly as described above. I checked vacuum and it was holding about 20 inches at idle, if there was a leak it wasn't significant. I checked the timing, off by one degree, engine idle drops slightly with the spout out but goes back up when I put it back in.

I just now stepped outside to double check the ACT and ECT, I started it up and it raced at about 2000RPM while I took readings. I dropped my meter lead so I shut off the engine to retrieve it and when I restarted the engine she settled down to a nice warm idle. Rev the throttle a few times and it comes back to normal. Shut her down and restart and she purrs like there was never any problem. Don't you hate it when you fix something but you don't know what you did?

I don't know if this helps but my 2 cents

[91waggin]

After the battery is disconnected, sometimes it takes a minute for the computer to figure out exactly what kind of duty cycle it needs to run the IAC on in order to maintain a proper idle. So you probably fixed your problem when you fixed the hose to the MAP sensor, and it just took awhile for the computer to figure out what the hell was going on.

[Lincolnator]

I drove it for two weeks like that