Irritating idle surge

leakyfaucet · Nov 7, 2004

I have an idle surge in my 93 mustang 5.0. It's very irritating and doesn't happen all the time, just randomly, regardless of if the engine is warm or not, it seems. I can find no cause for it to happen. As far as I understand, if you have idle surge, it should happen all the time or while the car is cold or during a particular circumstance. I can't find what variable is causing it. I'll just be driving along, no probablems for an entire day, put my clutch in and my engine will rev up and down, sometimes stalling.

So far I have:

Purchased and set a new TPS.
Checked for vacuum leaks. (I believe I got them all, the car generates 17 hg vacuum at idle. Is that normal?)
Cleaned the hell out of the Idle Adjustment motor.

I'm running out of ideas. If I unplug the Idle adjustment motor on the throttle body, the car will idle at 700-750 fluctuating. Obviously it never idle surges like that. Could it just be a faulty adjustment motor? If it was, wouldn't it be fluctuating all the time? It doesn't, which makes me think the computer is telling it act up.

Anybody have any ideas? Thanks!

HISSIN50 · Nov 7, 2004

assuming you have done the proper idle set procedure, the fact that it stalls on you makes me think that though the IAC is clean, it perhaps is still not functioning properly. gotta spare one to swap on?

your engine vac sounds alright.

good luck.

leakyfaucet · Nov 7, 2004

I don't have a spare one to use. How much could I get a new one for? I'd imagine from ford it would cost quite a bit, which makes me hesitant to do it if it might not even fix my problem.

What really confuses me is that when you unplug the idle motor, it idles fine. So that means the motor stays open a little bit by default and when it stalls I'm guessing the motor closes up even more than if it had no electricity at all. Weird.

HISSIN50 · Nov 7, 2004

leakyfaucet said:
I don't have a spare one to use. How much could I get a new one for? I'd imagine from ford it would cost quite a bit, which makes me hesitant to do it if it might not even fix my problem.

What really confuses me is that when you unplug the idle motor, it idles fine. So that means the motor stays open a little bit by defealt and when it stalls I'm guessing the motor closes up even more than if it had no electricity at all. Weird.

does your TB butterfly have a small hole in it?

places like AZ sell IAC's. if you get one read the instructions - some of them mount upside down.

here is the deal. static (in your garage while testing), there is no load. i think you said you stall when coming to a light (clutch in, etc). the puter should keep the idle at ~1500 rpm to keep it from stalling (at idle but rolling). it does not appear to do that. so while the car may idle alright in the garage, under load is another issue. it does not appear to be compensating for all conditions.

i might try another IAC if i were in your position. the thing is that it is a bit tough to test one (i think one needs to measure the square wave, IIRC). i would hate to point you in a direction and have that not solve the problem. but that is where i would start.

good luck.

leakyfaucet · Nov 7, 2004

HISSIN50 said:
does your TB butterfly have a small hole in it?

places like AZ sell IAC's. if you get one read the instructions - some of them mount upside down.

here is the deal. static (in your garage while testing), there is no load. i think you said you stall when coming to a light (clutch in, etc). the puter should keep the idle at ~1500 rpm to keep it from stalling (at idle but rolling). it does not appear to do that. so while the car may idle alright in the garage, under load is another issue. it does not appear to be compensating for all conditions.

i might try another IAC if i were in your position. the thing is that it is a bit tough to test one (i think one needs to measure the square wave, IIRC). i would hate to point you in a direction and have that not solve the problem. but that is where i would start.

good luck.

I don't know if my throttle plate has a hole, and I can't check it right now. I'll have to check tomorrow. I doubt it does, though, because if it did then my engine would probably have enough airflow to maintain idle while the IAC goes crazy. The idle will surge even while stopped, speed doesn't really have an affect on it, although it does "seem" to do it more after I've been driving and come to a stop. I currently have my speed sensor gear removed, so the car doesn't even know I'm moving, although it did the same thing with the gear in.

What is AZ? If they sell cheap IACs, then I'm all for it!

Thanks again for the help!

HISSIN50 · Nov 8, 2004

sorry, AZ = Autozone.

from what i have read, the VSS is what tells the puter the car is moving and that it should assume a load (raise the RPM's) while at rolling idle. however, IIRC, Yount does or did not have his hooked up and his idle hung in rolling idle situations. im not sure i remember that correctly or that it was not an anomoly - i bet he replies in the morning.

i will also toss out that some folks have had luck with the IAC air bypass plates. they are ~10 bucks used and maybe 15-20 new, IIRC. works for some, not for others. just something to keep in mind if other things fail. i would prefer to fix the problem and not bandaid it instead.

good luck.

leakyfaucet · Nov 8, 2004

What is IIRC? Sorry, I'm really bad with acronyms. Oh yeah, I tried the plate the goes inbetween the IAC and the throttle body. I found all it did was raise the idle. The idle would still surge but since there was extra air coming in, it wouldn't ever go low enough to stall, but close to it. Didn't work.

I guess I'm going to have to bite the bullet and see if a new IAC fixes the problem. The fact that it works perfectly 95% of the time tells me a new one probably won't fix it.

HISSIN50 · Nov 8, 2004

leakyfaucet said:
What is IIRC? Sorry, I'm really bad with acronyms. Oh yeah, I tried the plate the goes inbetween the IAC and the throttle body. I found all it did was raise the idle. The idle would still surge but since there was extra air coming in, it wouldn't ever go low enough to stall, but close to it. Didn't work.

I guess I'm going to have to bite the bullet and see if a new IAC fixes the problem. The fact that it works perfectly 95% of the time tells me a new one probably won't fix it.

sorry - dont feel bad; i asked the same thing awhile back (about IIRC). IIRC = If I Remember Correctly.

so the idle plate did not help, but rather magnified the idle setting (raised the idle but still allowed the surging but at higher idle rpms). hmmm. im wondering if the pintle in the IAC is not oscillating as it should. but your guess is as good as mine as to whether a new one is the fix. stuff like that is hard as it is not so out of whack to toss a code or be obvious.

im outta ideas. good luck.

leakyfaucet · Nov 8, 2004

Well, thanks for your help anyways. I'll be sure to tell you if it fixes the problem. If not, I can always return it

Oh, my oxygen sensor harness is a little wacky. It might be disconnecting then reconnecting. I'm pretty sure it stays disconnected almost all the time. Is it possible that this connection could cause the surge?

eliot22 · Nov 8, 2004

Did you pull codes? You might be amazed at what you find. Autozone sells a code reader for about $20.

jerry beach · Nov 8, 2004

Drain the coolant into a clean container. Remove and clean the throttle body. Its free, and there is a decent chance this will fix it. I have read on many occasions that Ford created the adjuster plate to avoid cleaning the TB's. I have the adjuster plate on mine and it fixed nothing. Mine only surges a couple of times at startup then quits, so I dont care really. The correct idle setting proceedure worked to fix mine once. Then the battery was replace and it came back. The correct proceedure is different for my 88, I dont have the 93 instructions. I know for a fact they are different. Alldata.com is a good source for stuff, but you will have to pay a yearly fee for each car you add to your account. Professionals use this service, its way better than stangnet just not free.

leakyfaucet · Nov 8, 2004

I've already tried cleaning my throttle body, but thanks for the advice nonetheless. I have pulled codes and all I get are oxygen sensors failing to turn on. That of course is because of the harness.

Thanks for all the help so far guys.

Irritating idle surge

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