[89KenneBellVert]

So heres the deal, I had a rough idle and sometimes the car just would not drop in RPMs after inbetween shifts or when i would push in the clutch right away. I have literally replaced everything in the ignition and fuel system on this car hunting down several problems (some not related here.) I mean there is nothing left for you to suggest in those two systems. I've cleaned the MAF several times with nothing but maf cleaner and i was very careful with everything. The engine is next to stock, the only difference is a CAI. There are no vacuum leaks anywhere on this motor and i've recently replaced the PCV valve and triple checked to make sure all lines are still hooked up. So, next step, replacing the IAC. It seemed to help out with the rough idle and rich smells i was getting before, but now the RPMS hover for a few seconds before dropping when i quickly throw in the clutch and it never drops between shifts. I'M OUT OF IDEAS. I've been hunting this too long.. please please help. (I've also done the surging idle/setting idle checklist, so thats been tried.. i'm not new to these cars by any means.)

 

[Jax95Cobra]

Looking for help also on this matter. After searching it seems to be that is just the way these cars act. I recently got a Cobra, basically stock with just basic exhaust bolts on and a K&N. I got the hanging idle during shifts and when you press the clutch in coming to a stop. Also when you are at a stop and you rev the engine, the idle hangs at like 1300-1400 rpm then eventually falls to 800 rpm after like 5,7,8 seconds. Engine idles, runs fine, no vaccuum leaks, TPS sensor voltage within specs, etc. Just the hanging rpm's.

 

[89KenneBellVert]

Yea exactly.. but its driving me nuts. Mainly bugs me because it got worse after putting a new, fully functional item on it. The TPS is in specs, and i actually lowered it a bit because in the past i've had good luck with running a slight lower % on the TPS reading.. but nothing.

 

[P63]

OK, this is not normal.

First pull error codes.

actually first, try disconnecting the battery for 30 minutes, just to be sure the computer resets AND to exhaust the free options first.

Since your car drops back to 800 when you're stopped, it sounds like all your sensors, especially VSS, are still functioning. Perhaps the IAC you got is bad, or the contacts could be corroded. Pull codes first, though.

 

[89KenneBellVert]

I haven't pulled codes off an obdI computer before, I just ditched al the previous ones I've worked with. Is there anything out there that can read the codes for it or an easy way for me to do it? I've tried lookin it up before but not sure if i comprehended.

 

[irwinner]

Try closing your throttle body in small increments and drive it after each change. It sounds stupid and there's no real factual science behind it but I fought with what you are talking about for a few years when finally I just decided to close the TB a little at a time, it made a world of difference. The iac screw is more of a fine tuning thing.

Example: You can open the TB up by using your screw a lot (within reason) and then start your car, it will idle at whatever its suppose to (IE 800rpm) since the TPS resets every time you start the car. But when you rev on it it will allow too much air through which will cause the rpm's to hang.

Like I said, its not some fancy procedure but it does work for me. That base idle reset never worked for me. Let me know if it works.

 

[89KenneBellVert]

Yea that's exactly what I was trying. I will have to mess with it a little more now. I wonder if a previous owner was having iac problems and just adjusted the screw higher then it was set originally to compensate. I dont know. Just an idea

 

[P63]

The iac screw is more of a fine tuning thing.

.

I can't argue with results, but a bad IAC will let the revs hang at 1500... with the throttle closed... so how does closing the throttle more lower the revs?



OP:
If the revs hang, then drop to "nearly normal" once the car starts, this is a typical profile of a bad IAC. Over the years people have proposed cutting the VSS line, so the computer thinks the car is stopped, and it lowers the idle again albeit to around 800 instead of the normal 600. Others have replaced the IAC with a blocking plate. Both approaches stike me as being a bandaid that might work today and tomorrow, but cause more problems down the road.


I highly recommend pulling codes before adjusting or shortcutting anything. Autozone might have someone who can do it for free.

Information is key to good decision making.

 

[Jax95Cobra]

I don't have any check engine codes. Can I assume the idle air control is functioning correctly.

 

[5.0stangaw]

I had this same problem and nothing worked until i finally just unplugged the iac valve and thats how i have the car now only complaint is that i have to give a little throttle while idling when its cold but once the engine warms up its not a problem

 

[wadesquad]

This eliminated hanging idle for me, they sell something jus like this but I made one out off a piece of sheetmetal and gaskets on both sides.

I started by drilling a really small hole and progressively made it bigger till it idled jus like I like


Sent from my iPhone using AutoGuide.com Free App

 

[64bit]

The number 1 cause of a hanging idle in SN-95's is a vac leak.

 

[89KenneBellVert]

Yea so I reset the computer. Unplugged the battery for 20 minutes. Took the car around the block for a bit, pulled it back in. Did the whole sequence. No codes. It flashed system pass for both key on and key off. And I'm playing with the tps. No difference at all.

 

[89KenneBellVert]

Tried unplugging the iac, deff working because the car wouldn't even start or stay alive and it's warm.

 

[89KenneBellVert]

And the tps is currently set at .95 volts

 

[Jax95Cobra]

For these 94-95 models brand new from the dealership, did the rpm drop steady, smoothly back to idle rpm or did it hang a bit?

 

[84_rustang_gt]

im going down the same road but with a fox, had the surging idle, then hung idle, then wouldnt rev past 4k, now back to surging idle!!! did all of the above new iac, iac spacer with diff size holes!!! im completly done with efi!!! switching to carb tomorrow night, ftw!!!! good luck with yours, im sure mine is something stupid like a vac leak or so simple i could burn the car to the ground!! but im going back to carb, and praying the guy doing my inspection next year doesnt look under the hood!:evilgrin: so after tomorrow i will have a cobra intake, 65mm tb, and 24lb injectors with pro mass for sale! lol

 

[5.0stangaw]

Tried unplugging the iac, deff working because the car wouldn't even start or stay alive and it's warm.

my car did the same thing until i took the little plug off thats near the top of the throttle body and used an allen wrench to adjust it where it would idle and the rpms come down at a good rate between shifts and when i lift off the throttle

 

[Torchy]

Try closing your throttle body in small increments and drive it after each change. It sounds stupid and there's no real factual science behind it but I fought with what you are talking about for a few years when finally I just decided to close the TB a little at a time, it made a world of difference. The iac screw is more of a fine tuning thing.

Example: You can open the TB up by using your screw a lot (within reason) and then start your car, it will idle at whatever its suppose to (IE 800rpm) since the TPS resets every time you start the car. But when you rev on it it will allow too much air through which will cause the rpm's to hang.

Like I said, its not some fancy procedure but it does work for me. That base idle reset never worked for me. Let me know if it works.

THIS

I had this same problem. I have a fully built 347, 11:1to1, pro M meter, 30lb injectors 94 cobra. Either Teflon the hell out of the adjustment screw on the throttle body to make it hard to turn or put a small lock nut on the adjustment screw but I spent half a day playing with my adjustment screw and finally got it to idle correctly and RPMS came down smoothly.

I know some person will come along and say that this is wrong for some reason or another but my car runs perfectly now for 8 years and still runs 10's as a daily driver.

Extremely small adjustments on the throttle body adjustment screw for the blade. That adjustment screw isn't there by accident. Mine idles between 700-750 RPMS. I know for a fact 800 RPMS was too high and made my car do what yours is doing.

Try it out and post back. Just remember: extremely fine tuning on the adjustments.


Sent from my iPhone

 

[P63]

For these 94-95 models brand new from the dealership, did the rpm drop steady, smoothly back to idle rpm or did it hang a bit?

It would fall back to idle.