high idle/ hanging RPM-'02 GT - Mustang Boards

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Old 08-15-2009, 02:41 PM
wildride02gt's Avatar
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Default high idle/ hanging RPM-'02 GT

here's the deal. '02 GT, w/bolt ons. mods are as follows-
Dragon Performance intake plenum
BBK 75mm TB
Steeda CAI
off-road H-pipe w/ MIL eliminators

Has ran great with all mods 'til about the end of July. It started running horrible out of nowhere. I thought it was bad fuel, but this proved not true. Ever since I've been trying to get it to run right, to little avail.
Initial problem was that it was stumbling, rough idle, no power, and misfiring. Went through all the plugs, cleaned the MAF, cleaned TB and intake plenum, and replaced the EGR. After replacing the EGR it began to run fine, except now it idles at about 900-1000 RPM, and a horrible RPM hang. In neutral, she starts hanging at about 2000, slowly comes down to 1500, then sits for about 30 seconds, then finally drops to 900-1000. When driven, it wants to stay at 1500-2000 all the time. Also, occasionally she stumbles and falls on her face, lacks power, or hesitates. Oh yeah, and NO CODES!!

I'm at a loss. Everything I've done and looked over should have been a fix. I can't drive it like it is, 'cause I don't trust it. I'm looking for help from people who know what they are talking about. Please don't shoot me a reply like "just go give it the IAC RPM hang fix." That's not a fix, its a cheat, a work around. There's a reason why its not running right. I need help finding that reason, and fixing it.
No disrespect to anyone, I'm just looking for hard answers.
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Old 08-15-2009, 04:03 PM
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had a similar issue with my 99 gt, i thought it was the IAC originally not operating properly then even though it had no codes i had my buddy who has an expensive diagnostic computer (not the bullshit autozone comp) to read cars plug it into my car- and it pulled up the throttle position sensor as the problem- went and bought a tps and the car went back to normal. Alot of people will argue that fords TPS are nearly flawless and never fail but im a proven case that the TPS do fail!
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Old 08-15-2009, 06:25 PM
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hmmmm....TPS? never thought of that. it was just replaced when the TB was installed about 12 mos. ago, but I could give it a shot. I'll check voltages on it tomorrow.
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Old 08-16-2009, 09:25 AM
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could it be a vacume leak from when you changed the egr? That would make it idle high and lack power. just a thought.
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Old 08-16-2009, 05:20 PM
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Originally Posted by bossman302 View Post
Ok, sounds like the same issue I had pretty much. Here is what I did to fix my issue.

Back the idle stop screw out till doesn't touch the bell crank arm. Use a 0.010" feeler gauge, tighten until gauge just drags between screw and bell crank arm. Remove feeler gauge. Tighten screw exactly 1 1/2 turns. Start the car and make sure it doesn't stall out.

Next, adjust the TPS so the voltage reads .98 or .99. With the connector attached to the TPS stick a paper clip into the rubber boot on the connector where the middle (+) and bottom (-) wires go into the connector. Set your multimeter to volts. If it is already at .98/.99 you are set. If not loosen the screws and turn the TPS till hit those numbers. If you can't you will need to take the TPS off and enlarge the holes. Make sure the TPS is nice and tight when you are done. Don't want it to move.

Last step. Disconnect the battery and let it site for a little while to reset the computer. Reconnect the battery and start the car but do not touch the gas pedal. Let it idle for about 10 minutes. This will let the car learn it's idle.

After I did this my problem went away. It may not sound like it but this is really easy to do. If you don't want to mess with the throttle that ok but at least check the TPS voltage and reset the computer. I redid my throttle body cause my tuner messed it all up.
OK, the above thread led me into the right direction. I did not have to mess with the TPS so much as reset the idle screw on my TB. Some research reveals that the TPS has a widely variable range for its idle voltage. A Chilton's or Haynes manual tells you that an idle voltage can be anywhere from .5V to 1.0V. Also, that the TPS is NON-ADJUSTABLE. So, that being said, a properly installed TPS can have a idle voltage range from 0.5V-1.0V, at a given specific idle RPM. for example....
car1-idle RPM=800, idle TPS voltage=0.63V
car2-idle RPM=800, idle TPS voltage=0.77V
car3-idle RPM=800, idle TPS voltage=0.98V
All three cars should idle and drive properly, as long as nothing is done to them. The TPS idle voltage vs. RPM is the BASELINE for the fuel/air tables. The computer just needs to know that the voltage input starts at X so it can adjust the table to make it go from X=idle to 5.0V=WOT.
Anyway, whenever any changes are made to the air or fuel system, you modify that air/fuel table a bit. Therefore, when people install CAI, TB, Intake Plenums, Intakes, Injectors, what have you, you are changing values for the computer. The stock table values won't behave properly. However, there is some play that the ECU's stock programming can work with. All you have to do each time is reset the ECU. My recommendation would be that ANYTIME something is installed in the vehicle, the ECU should be reset. That way it can re-learn the new values, and adjust accordingly. This should resolve any issues. If anyone is having issues with hanging revs, or high idle, or minor stumbling, I would try the following steps before purchasing any parts.
1. Check vacuum and intake hoses for damage.
2. Disconnect battery.
2. Clean MAF with the MAF cleaner, DO NOT use carb or brake cleaner.
3. Clean TB and Intake Plenum with carb cleaner.
4. Clean out IAC and EGR valve's with carb cleaner.
5. Adjust throttle body set-screw.
6. Re-connect battery.
7. Start vehicle, and let IDLE for 10 min. Idle should be around 800 RPM. adjust screw accordingly.
8. Idle with A/C on max for about 5 min.
9. Drive vehicle and observe. If there is still a problem, then proceed to troubleshooting EGR and IAC valves.

Hope this helps anybody else dealing with this problem!
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Old 08-17-2009, 06:56 AM
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You can adjust the TPS by drilling the holes for the bolts in the TPS sensor slightly larger so you can move it it around a bit. You want it to be at .99 volts at idle. That is the ideal place to be at. I would also pull the IAC off and clean it out with some carb cleaner to help with that hanging idle.
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