strange Idle

stangman16 · Feb 7, 2007

Okay, this only happens every so often, and I'm not sure if it's related to the outside temperature or not but it was 71° outside. I think I've had this problem last summer, and it went away. No CEL displaying. It's been a little colder the previous couple of days. Anyhow, on my '99 GT, I was driving home from work and I press in on the clutch, the idle goes to 2800-3000 and stays there while I'm coasting and maybe drops 500 rpms (to be ~2500) when I come to a dead stop. If I turn off the car and restart it - it idles just fine ~700 or so. Once it had idle to 3200 and went back down to 1800 in a smooth transition while coasting...

I continue to drive home, next stop light it happens again, go to the gas station where the car sits for 3-5 mins tops, get in and drive the rest of the way home and it would idle just fine the next 3 stoplights to my house. I just don't get this - it's variable, doesn't happen often but when it does it's irritating to be at a stoplight and have the car idle at 2800.

Perhaps it's related to when I come to a stop - at times, the car will surge to 300-400 rpms and bounce right back up to ~700. I've been through 2 IAC's and I have a new TPS. Could it be the MAF sensor going bad? I have about 95k miles on the engine.

merc123 · Feb 7, 2007

Before reading your whole post I was going to say IAC or throttle position sensor. You might need to clean your MAF, not replace it.

HoustonGT · Feb 7, 2007

Hmm, intersting. My car never revs up like that but it does do that crap where it will idle all the way down to 300-400rpms and then come ack up. It has never stalled though. I'm hoping that its the IAC in my case, but for you it could be a fould MAF. Use some electronic contact cleaner and carefully clean it. See if it helps

stangman16 · Feb 7, 2007

Will do, I will clean the MAF sensor, but if that doesn't take care of it - what else could it be? I cleaned the MAF sensor sometime last year as well, but I don't remember if it was before or after this problem had occurred.

ADRENLN · Feb 8, 2007

let us know how it turns out.

sgarlic · Feb 8, 2007

From the way I'm interpreting your post, it doesn't sound like you're idling at 2800 rpm, it sounds like your idle hangs until you get to a dead stop. Now I'm going to say clean your IAC on that one.

I also want to mention, my '04 has done the same thing since the day I drove it off the showroom floor (not quite 2800 rpm) where it will not drop down to idle rpm until you're fully stopped. I can push the clutch in coasting from 30, and it'll drop down to maybe 1300 or so, but just flat out won't drop to idle til the car comes to a complete stop.

stangman16 · Feb 8, 2007

sgarlic said:
From the way I'm interpreting your post, it doesn't sound like you're idling at 2800 rpm, it sounds like your idle hangs until you get to a dead stop. Now I'm going to say clean your IAC on that one.

I also want to mention, my '04 has done the same thing since the day I drove it off the showroom floor (not quite 2800 rpm) where it will not drop down to idle rpm until you're fully stopped. I can push the clutch in coasting from 30, and it'll drop down to maybe 1300 or so, but just flat out won't drop to idle til the car comes to a complete stop.

I haven't got a chance to clean the MAF sensor yet, I will do that after work today.

It hangs even when I'm at a dead stop, but drops only by 500 rpms (if that). Let me just round up and say coasting it idles/hangs at 3000, when I get to a dead stop it will be at 2500 rpms and hangs there. If I turn on the A/C it will either jump up more and hang or go down some and hang. It's variable. It was running just fine for 10 miles to work this morning until the last 3 stop lights. It was doing it again, but this time when I would turn off the engine and wait it would start up and hang again almost exactly where it was.

I will try and see if I have time during lunch to clean it, but I doubt I will. 95k miles, is it time to get a new car yet?

stangman16 · Feb 8, 2007

Cleaned the MAF sensor and it still does the same thing. Sometimes the RPM's don't drop at all when I come to a stop. But the car idles fine when I first turn it on, just after going over 2500rpms it'll hang there when coasting and coming to a stop.

I have noticed that when I'm at a stop, if I press real hard on the brake the rpms drop by about 100 or so. Does this indicate a vacuum leak? I don't have a scan tool, how do you check for a vacuum leak besides a visual check? I did a visual and the green and red lines look fine. I replaced PCV 6 months ago.

Mike97gt · Feb 9, 2007

a vacumm leak would not cause this. When was the IAC replaced and with what brand?

stangman16 · Feb 9, 2007

svttech76 said:
a vacumm leak would not cause this. When was the IAC replaced and with what brand?

Sometime last year, borg warner is the brand. I took off the IAC and cleaned it, I also had the old one still and I shook both, the old one had an internal rattle to it - the newer one did not. There's not more than 15k miles on this current one. After cleaning it and test driving last night there was no change.

This morning I drove to work, 46° outside, kept the rpm's below 2500 (which is unusual for me ) and I had no problems. I was 2 turns away from my work and gave it almost full throttle to 4000 rpms and it immediately started hanging again. Even if I force the RPM's to go down to 1000 by keeping it in gear, slowing down, and then pressing in on the clutch it will raise to 2800. I think my next step is to try and test the TPS, although that was replaced last year as well. Gonna do a search real quick to find out how to test it.

stangman16 · Feb 10, 2007

Okay, I drove half way across town and when I got back this started happening again. I pull over & the car is idling at 2800, and disconnect the IAC and the car starts idling at 550 rpms and keeps running, I can rev it up a little and it'll go back down to 550. The moment I plug in the IAC wire it jumps back to 2800. Bad IAC ? I tried the one I had on before and it's having the same exact issue. This seems to only happen when the engine is FULLY warmed up and when I give it WOT. After I do the WOT 1 time, doesn't matter what RPM i go to - it'll shoot to 2800 for idle.

Is it supposed to run with the IAC disconnected?

stangman16 · Feb 12, 2007

anyone?

smasher41 · Feb 13, 2007

stangman16 said:
Okay, I drove half way across town and when I got back this started happening again. I pull over & the car is idling at 2800, and disconnect the IAC and the car starts idling at 550 rpms and keeps running, I can rev it up a little and it'll go back down to 550. The moment I plug in the IAC wire it jumps back to 2800. Bad IAC ? I tried the one I had on before and it's having the same exact issue. This seems to only happen when the engine is FULLY warmed up and when I give it WOT. After I do the WOT 1 time, doesn't matter what RPM i go to - it'll shoot to 2800 for idle.

Is it supposed to run with the IAC disconnected?

does your engine compartment make a wistling sound at startup or idle? If so its prob the IAC.
i had almost the exact issue you have now, except mine would randomly idle at 2000rpm.The problems i was told was the IAC, Vacum leak, or my MAF was about to take a dump. I tried cleaning the IAC, no luck, replaced PCV valve and hose for vacum lines, still did it.

With the IAC unpluged my car would stall, the rpm were simply too low, i set the idle base to 800 rpm without IAC and reconnected.

Start your car, pop the hood, and tap the MAF sensor with the handle of a screw driver, if the idle changes, it could be the problem. (I was told this, still havent tried so myself) I picked up a used MAF for $65 just in case it is.

Autozone had a Well's IAC for like $44 so i picked it up to see if it worked ( manager was stumped and told me try it, and bring it back if it wasnt it)

drove around almost all day today, seemed fine. idle was rock solid.

stangman16 · Feb 13, 2007

Welcome to Stangnet! & thanks for the reply, I'll give that a try and see what happens. I'm leaning towards the IAC as well, but I don't understand why it does this only when the engine is fully warmed up. I want to be 90%+ sure that the IAC is the culprit before spending over $100 at the dealership for an OEM one, this will be my lucky 3rd IAC within 3 years.

I'm half deaf so I can't tell if there's a whistle sound coming from the compartment, but I think my dad said he thought he had heard one when we were checking for a vacuum leak.

smasher41 said:
does your engine compartment make a wistling sound at startup or idle? If so its prob the IAC.
i had almost the exact issue you have now, except mine would randomly idle at 2000rpm.The problems i was told was the IAC, Vacum leak, or my MAF was about to take a dump. I tried cleaning the IAC, no luck, replaced PCV valve and hose for vacum lines, still did it.

With the IAC unpluged my car would stall, the rpm were simply too low, i set the idle base to 800 rpm without IAC and reconnected.

Start your car, pop the hood, and tap the MAF sensor with the handle of a screw driver, if the idle changes, it could be the problem. (I was told this, still havent tried so myself) I picked up a used MAF for $65 just in case it is.

Autozone had a Well's IAC for like $44 so i picked it up to see if it worked ( manager was stumped and told me try it, and bring it back if it wasnt it)

drove around almost all day today, seemed fine. idle was rock solid.

smasher41 · Feb 13, 2007

stangman16 said:
Welcome to Stangnet! & thanks for the reply, I'll give that a try and see what happens. I'm leaning towards the IAC as well, but I don't understand why it does this only when the engine is fully warmed up. I want to be 90%+ sure that the IAC is the culprit before spending over $100 at the dealership for an OEM one, this will be my lucky 3rd IAC within 3 years.

I'm half deaf so I can't tell if there's a whistle sound coming from the compartment, but I think my dad said he thought he had heard one when we were checking for a vacuum leak.

Thanks for the welcome!

Have you tried cleaning the IAC by chance? if not I suggest checking this link out
http://www.muscularmustangs.com/maintain.php

The more i htink about it, the more i think it is the IAC

The IAC is the part that controls the idle speed, the valve moves in or out letting more or less air into the system, depending on engine conditions. When increasing idle speed, the valve allows more air to flow through it. To decrease idle speed it retracts the vavle to the seat, reducing air flow.

If the shaft that pulls the valve back or forth is dirty, scratched, or the coil is worn out, it would cause the valve to stick open before closing. Also, if the throttle body is set too low of rpm, then its cutting off too much airflow, forcing the IAC to try to hold open longer to keep from stalling.

Or possibly its the coil of the solenoid of the IAC burning out?
Check your IAC voltage (Haynes says the wires should give between 10.5-12 .5volts) if its not around there ar the plug then its electical issue.

Also, isnt a base idle of 550 rpm a bit low? I think 800 is the normal base idle.
I've read that if the base idle was either too high or two low( not sure which) then it will force the IAC to work harder for the entire time the car is at idle. eventually the solenoid weakens and quits functioning. When i changed my IAC i also reset my base idle to 800 rpm.

If your going through one a year, why get the expensive ford one? I got a cheapo Well's at autozone for less than half that, and it comes with a one year warrenty, unlike ford that willnot return or exchange ANY electrical components after install.

stangman16 · Feb 14, 2007

Yeah, already cleaned the IAC and it didn't make a difference. So it might just well be the coil burnt out in it. I've got the stock TB and haven't touched the idle screws, that one should be okay. Didn't even think to check the voltage there, I will do that afterwork tonight - do I just pull the plug off and check the prongs on the connector?

Well, the car was at a 2800 idle and I pulled over, popped the hood and disconnected the IAC and it went to 550 idle. I revved it up a little bit and it went right back to 550 with the IAC disconnected.

The reasoning for the Ford IAC this third time around, is the original IAC lasted for 48k miles and these last two have barely lasted half of that - if even that.

Thanks!

speedfreak98r · Feb 14, 2007

I would suggest checking your throttle spring. Just to make sure there's no gunk on, or around it, and the linkage

blkhorse · Feb 14, 2007

it is really hard to diagnose these kind of problems without a scan tool.need to check iac counts,maf,tps values etc.problem could be in computer,have you disconnected your battery and let it relearn?

stangman16 · Feb 14, 2007

Yeah, I agree - it's tough without the scan tool. I'm aiming to finish my '66 EFI by this summer, so most all cash resources go to that car. I can't fork up $400 for the scan tool.

I disconnected the battery when I cleaned the MAF sensor, but I can't recollect if I did that after cleaning the IAC. I will try that, and if it doesn't work I will go buy a new IAC.

speedfreak98r · Feb 14, 2007

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