1998 Cobra High Idle Issue

[MaHomies]

Hi guys, so I just recently fixed the issue of my Cobra just wanting to die when idling by replacing the IACV and the CAI filter, and after startup it normally idles perfectly. Lately though, after driving for extended periods of time (30 minutes+) when I come to a red light or stop sign and it's either in neutral or the clutch is engaged, the idle will hang at ~1500 rpms. After the car is off for awhile and I restart it, it'll go back to normal.

I've seen some threads saying it could be the TPS, so should I start there or could it be anything else? Car has 121k and a couple mods from previous owner, including BBK upper and lower intake, and BBK exhaust with an X pipe, MSD ignition and coils, etc. It also might help to add that I'm getting the car tuned soon, but I don't believe it has one currently so the A/F ratio is probably off. TIA!

 

[me32]

All of those mods you mentioned could be factors. But do you happen to have any of the factory parts to test? With a stock tune and a bbk upper and lower intake could cause weird issues. How many miles on the coils? Dought that's the issue but tthought id ask.

 

[MaHomies]

0 idea how many miles are on any of the mods, and no, sadly I don't have any of the OEM parts to put back on. :/

 

[cobracide]

I had to adjust my throttle blade and monitor the tps voltage, Also check your IAC is functioning.

 

[98 svt]

Yup, TPS

 

[9397SVTs]

How does it have a BBK upper/lower intake?

 

[MaHomies]

This is the first mustang I've ever owned, but I've always heard people referring to the CAI's on Mustangs as an "upper and lower" combined since it's two pieces, but I could be wrong. The upper being if it was a short ram intake, and if it has the lower too going into the fender well, it's a CAI. I might just be using incorrect terminology. I'm used to imports.

Cobracide, is there places I can find out how to do both of those? I thought I seen where you could adjust the TPS on some vehicles, but I didn't look too far into it. I'm slightly mechanically inclined, but not a lot of experience. I love DIY things. I'm about 99% sure the IACV is functioning because I tested the old one by unplugging it and the car idled rough but didn't die, and the new one idles perfectly on cold starts and when it's warm but hasn't been driven for extended periods of time.

Some people at work have been giving input and some say it could be fuel related as well? Such as injectors, etc?

 

[cobracide]

This is the first mustang I've ever owned, but I've always heard people referring to the CAI's on Mustangs as an "upper and lower" combined since it's two pieces, but I could be wrong. The upper being if it was a short ram intake, and if it has the lower too going into the fender well, it's a CAI. I might just be using incorrect terminology. I'm used to imports.

Cobracide, is there places I can find out how to do both of those? I thought I seen where you could adjust the TPS on some vehicles, but I didn't look too far into it. I'm slightly mechanically inclined, but not a lot of experience. I love DIY things. I'm about 99% sure the IACV is functioning because I tested the old one by unplugging it and the car idled rough but didn't die, and the new one idles perfectly on cold starts and when it's warm but hasn't been driven for extended periods of time.

Some people at work have been giving input and some say it could be fuel related as well? Such as injectors, etc?

Did you miss the LINK?

Proper TPS and Idle settings 5.0 / Fix Idle Surge

www.allfordmustangs.com

The TPS voltage is a function of where the throttle blade is set with the trim screw. My Areoforce gauge directly measures the TPS Voltage so it made adjusting easy. You will need a multi-meter otherwise. Also check that the throttle blade is not sticking and/or the TB spring is not binding.

 

[cobracide]

Here are my notes I used as well:

ADJUSTING THE THROTTLE POSITION SENSOR



After the new throttle body is bolted on, the final order of business is to correctly adjust the Throttle Position Sensor (TPS). Adjusting the TPS to the correct voltage is not difficult. It will require a Digital Voltmeter and a correctly sized screw driver.



The TPS is in essence, a low voltage potentiometer or resistor. It provides input data to the ECM (“the computer”) regarding the position of the throttle. Without the TPS and without the correct voltage, the computer has no idea as to where your “right foot” is, whether it is off the pedal (such as at idle) or at part throttle or at full throttle. The TPS supplies a gradually increasing supply of voltage to the ECM as the throttle is gradually opened. How the ECM processes this data depends on many prevailing conditions, including engine speed, load, vehicle speed, engine and ambient temperatures, and so on. The information from the TPS is especially critical for proper start-up and idle, as well as smooth throttle response. An incorrectly adjusted TPS may be most notable by an idle speed that is either too fast or too slow, or an engine that will not return to idle quickly after your foot is off the throttle. The proper idle speed should be between 750 and 900 RPM, and will be dependent on different engine combinations, particularly with engines that may have non-stock modifications.



To adjust the TPS, follow these instructions.



The TPS is a small black plastic device affixed to the throttle body with two screws, and located on the opposite side of the throttle cable linkage. The end of the throttle shaft has a blade that fits into the TPS. The TPS is electrically connected (with a quick disconnect plug) to the main wire harness.



After you have replaced your original throttle body with your new one, remove the TPS from the original throttle body and install it on your new Accufab throttle body. If new screws have been provided in the package from Accufab, use these screws (they are shorter). If not, reuse the original or stock screws. Reconnect the TPS to the wire harness.



You will need to adjust the TPS to get the correct voltage at idle. Changes to the idle speed can ONLY be made by adjusting the TPS. DO NOT ADJUST THE STOP SCREW ON THE THROTTLE BODY UNDER ANY CIRCUMSTANCES. This has been pre-adjusted at the factory to insure a proper “air gap” around the throttle blade in the fully closed position. If by chance this stop screw has been re-adjusted in the field (by those that adjust first and read the instructions last), it may be necessary to return it to Accufab for re-calibration. Before sending it back, call first and we may be able to give you re-calibration instructions over the phone.



The TPS is connected to the main wire harness with three separate, color-coded wires. You will see a “gray & red” wire, a “gray & white” wire, and a “brown & white” wire.



The “gray & red” wire is the ground wire (on some TPS’s it could also be a black wire).



The “gray & white” wire provides the positive voltage information to the ECM (on some TPS’s it could also be a green wire).



The “brown & white” wire provides a positive 5-volt reference signal WITH THE KEY ON (on some TPS’s it could also be an orange wire).



If you have any problem identifying the different colors or if the wires are different colors from those described, check inside the plastic quick disconnect plug. The plug connection tangs should have an “A”, “B”, and “C” next to each tang. “A” is the “gray & red” wire, “B” is the “gray & white” wire, and “C” is the “brown & white” wire. Because of the different suppliers of TPS’s (and the potential for different wire colors), you may need to “poke around” on the wires in order to determine the positive and negative leads. An easy way to take voltage readings is to stick a safety pin through the individual wires, and then make contact with your Volt Meter probes to the safety pins.



To check voltage, THE IGNITION KEY NEEDS TO BE IN THE “ON” POSITION BUT THE ENGINE DOES NOT NEED TO BE RUNNING. With the positive (+) lead from your voltmeter, pierce the “gray & white” wire. With the negative (-) lead from your voltmeter, pierce the “gray and red” wire. The voltmeter should read no less than .98 volts nor more than 1.0 volts. If the voltage is too high, loosen (but do not remove) the two screws holding the TPS to the throttle body, and rotate the unit counter-clockwise, until the voltage is correct. Rotating the unit clockwise will increase the corresponding voltage. When the voltage is correct, retighten the two screws. The only adjustment is the “slop” between the inside diameter of the holes and the outside diameter of the screws, so we are dealing with just a few degrees of rotation.



Usually, the space between the inside diameter of the holes in the plastic TPS body and the outside diameter of the screws will allow a rotation (clockwise or counter clockwise) of three or four degrees in either direction. This is usually enough to get the correct voltage setting. If its not enough, a small rat-tailed file (like the kind that hobby stores sell for working on jewelry) may be used to file a small slot into the holes to achieve additional rotation.



The Ford ECM’s are very “unforgiving” when it comes to ANY engine modifications from stock. The addition of an Accufab throttle body should not have any negative effect on idle speed, off-idle performance (tip-in) or the RPM returning to idle speed (within a second or two) when the throttle is closed.



If on the other hand, modifications to the stock engine may include additional supercharger boost with the use of different supercharger pulleys (particularly noticeable on supercharged 2003-2004 Mustang Cobra’s) or adding a “re-programming” chip or add-on computer module, the standard TPS voltage readings and other drivability issues may be noticeable, and could include an overly fast idle and a resistance of the ECM to “allow” the throttle to close properly, resulting in a delay between removing your foot from the gas pedal and the engine returning to idle.



These are not throttle body problems. The throttle body is a mechanical device with only one moving part. As long as the throttle body slams shut when you close the throttle (key off, engine not running), the throttle body is functioning correctly. Any other problem is associated with the ECM and the feedback it is receiving, either due to the increased boost at idle (more air supply) or the add-on computer chip sending incorrect (or unknown) data to the ECM.

 

[cobracide]

More.. Looks like an SCT xcal can monitor the PID.

Follow the steps below and you will have it working.
Ok, first, the lever MUST rest on the stop screw. Otherwise the throttle position is never the same and the computer don't know what to do , so it idles high.

Quote:
1. Warm up car
2. Turn engine off and REMOVE THE KEY
3. Disconnect negative lead from battery and turn head lights on( this removes the capacitive charge in the system)
4. Connect negative lead to battery
5. Back off the stop screw on the throttle body
6. Start car
7.While grasping the throttle lever with your hand, give it a small amount of throttle and let off SLOWLY.
8. Adjust the stop screw to just meet the lever and give it another 1/8-1/4 turn and, then lock it down.
9. Turn ignition off.
10. Turn key to "ON" position and check TPS voltage. Set to .980 Volts with a Digital Multimeter (Fluke Meters are the most accurate)
11. Turn key off.
12. Repeat steps 1-4 to reset ECU.
13. Turn off Adaptive Learning in your tune so you can make the proper adjustments without the ECU making them for you. (Your tuner should know how to do this)
14. Datalog ISC duty cycle: this value tells you how much the IAC valve is having to stay open in order to control idle. ideally you want to be around 35%.
15. Datalog Idle air integrator- this is a value that correlates with the idle air neutral table values in your tune. They should be fairly close to zero. If it’s negative then you need to take air out of the ISC tables, if it’s positive then you need to add air into the tables. It’ll never be exactly zero. Allow the car to be at operating temp and idling for a few minutes before making a decision on this. In layman's terms- the integrator could be telling your ecu that more air is coming in than what is actually coming in- causing your high idle.
16. Turn on Adaptive Learning in your tune and drive car as you normally would so that the ECU can relearn again.


Quote:
You say if my throttle body doesn't stick when the car is shut off then it's not the issue? I'm not following that logic, can you explain?

Thanks
With the car off, take your hand and manually open and close the throttle body. If it does not "Stick" or and comes to rest on the throttle stop, it is working properly.

Quote:
Originally Posted by encasedmetal View Post
OP- sorry I am late to answer your question
ISC duty cycle: this value tells you how much the IAC valve is having to stay open in order to control idle. ideally you want to be around 35%.
Idle air integrator- this is a value that correlates with the idle air neutral table values in your tune. They should be fairly close to zero. If it’s negative then you need to take air out of the ISC tables, if it’s positive then you need to add air into the tables. It’ll never be exactly zero. Allow the car to be at operating temp and idling for a few minutes before making a decision on this. In layman's terms- the integrator could be telling your ecu that more air is coming in than what is actually coming in- causing your high idle. even if your TB and your IAC valve are working properly this can cause high idle.
TPS- this needs to be inbetween 0.95-1.0 v or 15-20% whichever way your scanner reads as SCT xcal won't read this.

 

[MaHomies]

Terribly sorry sir, I must have missed the link, but that looks like it should be more than enough information for me to get this done after work. Thank you much!!!

 

[cobracide]

Terribly sorry sir, I must have missed the link, but that looks like it should be more than enough information for me to get this done after work. Thank you much!!!

Anytime.. Good Luck!