Idle Hangs at 1500 rpms

[93gtmustang]

I have a new Gt-40 Ford Racing Crate Motor with B-303 cam in my 93 Gt Mustang.
I'm running a Edelbrock Performer Intake, 70mm throttle body, C&L CAI with C&L 76mm MAF, 24# injectors, under drive pulley's , elec. fan, MDS Distributor, MDS TFI Coil, stock ingnition.
I have the idle set at almost 1000 rpms, that's where the motor sounds like it likes to idle at. Tps set between .96 to .98.
While driving, if you let off the throttle, or if you take it out of gear, the idle hangs at 1500 rpms. It goes back to the set idle of about 850-900 rpms after you stop and sit for a moment.
What would cause the idle to hang like this?
If I back the idle down too much, it sounds like it's struggling and about to fall on it's face.
I know it won't be perfect without a tune and chip.
I just want to get ride of the idle hang.
Any help would be greatly appreciated.
Thanks

 

[Bullitt347]

The idle air bypass valve (idle speed control solenoid) bypasses air around the throttle plate when you close the throttle so that the sudden lack of air into the engine does not kill the engine. This valve or solenoid also will increase engine rpm when the A/C turns on to compensate for the increased load on the engine. It also provides for a fast idle when the engine is cold as EFI has no choke like a carburetor does. If this is a brand new combination then you may need to do a couple of drive cycles so the PCM can adapt to the new combo and provide better driveability. It is not uncommon for this to happen with aggressive cam profiles as the PCM searches for a "standard" idle quality. If after a few days of driving the car the symptoms do not get better then we may have to dig a little further into the tune and such to figure it out.

 

[jrichker]

You guys with idle problems could same a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first post has all the fixes, and steps through the how to find and fix you idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :

 

[93gtmustang]

I spent some time reading the Surging Idle Checklist. Great information.

I've done many of the trouble shooting tips that are on the check list.

My TPS is not defective, because the idle will come down when the car is not in motion.

The only thing different in my car is the new motor and the larger injector size. And I think that a higher idle setting is making the idle hang like it is. I also think that it has something to do with how the MAF Sensor is reading air flow when the car is in motion and when it is still with the current idle setting that I have.

I just want to find a good all around setting. Basically, if I set the idle at just under 1000 rpms (between 850 and 950) , I have a good idle for the motor but then I have the hang at about 1500rpms while in motion. If I set the idle to the lower base idle setting, I'll have a floppy idle for the motor, but I'm pretty sure that I won't get the idle hang. My old motor would do this also. It took some time to get it just right. But my old motor was more stock, so it was a little easier to set.

I know a lot of people don't really like C&L MAFS. My C&L came with the car when I bought it. Also my MAF Sensor went bad last year, so I have a new sensor that I bought from Ford for $189.00. So to be cost effective, I bought the C&L sample tube for the 24# injectors for $40.00 instead of dishing out $280.00 for a Pro-M. I'm starting to suspect that the C&L MAF may be adding to or may be the problem.

I'm going to to try a few things today. I'll let everyone know how I make out.

 

[93gtmustang]

This link was listed in the Surging Idle Check List.
From first hand experience, I don't agree with the below sections listed.
Especially, Cleaning the MAF Sensor!!!

http://www.muscularmustangs.com/iac.php

Cleaning the IAC and Throttle Body Section
I would use O2 Sensor Safe Throttle Body Cleaner instead of the reccomended brake cleaner. That's my opinion.

Cleaning the MAF Sensor / DON'T DO THIS!!!!

Once you have the sensor out you need spray the carb cleaner on the end of a Q-tip and gently....no very very gently clean the wire (picture 4 "A") on the sensor with the Q-tip. Be super careful because this wire is very fragile.

The only way to clean a MAF Sensor is gently with alcohol and a Q-tip. This information is from a A-1 certified mechanic that I know. I ruined my MAF sensor with either brake cleaner or throttle body cleaner. It cost me $189.00 for a new sensor through Ford.

I have heard that electric contact cleaner, the quick dry type only, will also work well..
I have not tried it, it makes sense though.


Resetting the Base Idle

1. Disconnect the battery's negative terminal to reset/clear the computer's memory. Leave it disconnected for 30 minutes. WARNING: Whenever removing your battery cables ALWAYS disconnect the negative battery cable first and reconnect it last. Otherwise you could destroy your computer or cause a battery explosion.
2. Disconnect the plug going to your idle motor which is located on the front of your throttle body.
3. Reconnect your battery's negative terminal.
4. Start the engine, and set the idle to the rpm you want with the stop screw on the bottom of the throttle body.
5. Turn off the engine.
6. Reconnect the plug on the idle motor
7. Make sure all accesories (radio, blower motor, a/c, lights, etc) are off and start the engine.
8. Let engine run for two minutes.
9. Turn engine off and wait two minutes then restart engine and let idle for two minutes with all accesories on.

Is it always necessary to disconnect the negative terminal when adjusting the idle, TPS, or cleaning the MAF Sensor? It seems like there is a lot of disconnecting of the negative terminal.
Steps 7,8, and 9. Are they really necessary?

My original Ford Owners Manual states the following:

If you ever disconnect the battery, you must allow the computer to "relearn" it's idle conditions before your vehicle will idle at it's best. To let the engine do this, turn off all accessories, and start the vehicle. Let the engine idle for at least one minute.(Engine must be warm in order to to "learn") Allow approximately 10 miles of stop and go traffic for your vehicle's engine to complety "relearn" it's idle.
If you do not let the engine relearn its idle before you drive your vehicle, the idle quality of your vehicle may be adversely affected untill the idle is relearned. Your vehicle will eventually relearn it's idle while you drive it, but it takes much longer than if you use the previous procedure.

This method sounds much easier, and is reccomended from Ford.

 

[jrichker]

This link was listed in the Surging Idle Check List.
From first hand experience, I don't agree with the below sections listed.
Especially, Cleaning the MAF Sensor!!!

http://www.muscularmustangs.com/iac.php

Cleaning the IAC and Throttle Body Section
I would use O2 Sensor Safe Throttle Body Cleaner instead of the reccomended brake cleaner. That's my opinion.

Cleaning the MAF Sensor / DON'T DO THIS!!!!



The only way to clean a MAF Sensor is gently with alcohol and a Q-tip. This information is from a A-1 certified mechanic that I know. I ruined my MAF sensor with either brake cleaner or throttle body cleaner. It cost me $189.00 for a new sensor through Ford.

I have heard that electric contact cleaner, the quick dry type only, will also work well..
I have not tried it, it makes sense though.


Resetting the Base Idle



Is it always necessary to disconnect the negative terminal when adjusting the idle, TPS, or cleaning the MAF Sensor? It seems like there is a lot of disconnecting of the negative terminal.
Steps 7,8, and 9. Are they really necessary?

My original Ford Owners Manual states the following:


This method sounds much easier, and is reccomended from Ford.

Sorry about your bad experiences, however I am only responsible for the information that I personally post. Since none of the material you posted was in any of my posts, there is nothing I can do about it.

When someone finds errors or omissions, and has good proof of them, I will be glad to make corrections. The change in information on setting the TPS reflects the data provided by Joel5.0 and others is a recent example of the management of the checklist.

 

[93gtmustang]

jrichker,
Yes I realize that the information from Muscular Mustangs was not yours.
I should have stated that it was a link in your post, not your information.

Your Surging Idle Checklist is great and very informative.

I just wanted everyone to know what I had experienced and to pass on the information that I aquired first hand from a very knowledgeable A-1 certified mechanic that I know that has been in the business for 30 years. I just don't want anyone damaging parts when it can be prevented.

I wouldn't reccomend using brake cleaner on a MAF sensor.
When cleaning a throttle body or IAC I would use only O2 safe throttle body and carb cleaner.


As far as the disconnecting the negative battery cable. I just wanted some comments and what everybody thought about all those step that were described in the Resetting The Base Idle Section. I thought that there were far too many unnecessary steps after reconnecting the negative terminal.

I just recently used the reccomended method in my Ford Owners manual and it worked fine.
Thanks

 

[93gtmustang]

Originally Posted By 93gtmustang
And I think that a higher idle setting is making the idle hang like it is. I also think that it has something to do with how the MAF Sensor is reading air flow when the car is in motion and when it is still with the current idle setting that I have.

And I think I was right! I had the idle screw advanced too far.

This is what I did, and it worked for me.

With engine at operating temperature.
I backed my idle screw all the way off and then slowly, brought it up until I had a good idle at around 750-800 rpms.
Turned the engine off.
Set the TPS at .98
Unhooked the negative battery cable and waited only a few minutes.
Reconnected the negative cable and let the engine idle for about 2 minutes.
Shut off the engine, restarted the engine within the time it takes to fasten a seat belt, and drove in stop and go traffic for about 10 miles. I didn't drive the 10 miles all at once either. I stopped at a few stores in between.

Results
My engine no longer hangs at 1500 rpms when moving and letting off the gas.
Instead, it drops down evenly to the set idle without hanging at 1500 rpms.

No more chasing my tail!!!!!