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Mustangs stalls when clutch pushed in @Higher RPM

16038 Views 18 Replies 5 Participants Last post by  Warghost
I've had an issue with my 2004 Mustang GT 5-speed for a while now. Heres a scenario. I'm driving down the road accelerating in third gear, the RPM's get to 3000rpm and I decide to shift to fourth gear. If I hold down the clutch without actually engaging into 4th gear, the rpms drop (like they should) but the needle will fall to zero and the engine dies. Normally I would engage the clutch and down shift or whatever, but the engine should only drop to its set idle right? This problem normally only happens at speeds above 40mph. The car idles fine, and starts fine (most of the time). The car only has 23,000 mile on it and and everything else seems to work fine. But sometimes the engine stall catches me off guard, and since the power steering and pwr brakes shut off, it could be dangerous if I can't pop th eclutch fast enough to get the engine going again.
Do you have any suggestions or anyone had this problem before?:headscratch:
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Have you checked.....

Have you checked the mechanical idle stop on the throttle body? It almost sounds like it may be adjusted all the way closed or close to it. Perhaps the previous owner didn't like the idle hang that the IAC causes and reset the mechanical stop in an attempt to drop the idle faster. I recently installed a 70mm throttle body on my 02 and it required some adjustment of the mechanical screw to get it to idle correctly. In my case I found that the air for idling appears to be supplied both by the butterfly and the IAC. Be aware that if you adjust the idle screw you may need to adjust the TPS sensor to have the correct setting at idle.
If the butterfly is incorrectly adjusted maybe the IAC cannot respond properly under the shifting conditions????

Good luck
Hello Dionysus,

Sorry for not responding sooner....too many chores! I am going to try to attach a photo for the 1st time so I hope it makes it. I have drawn a pointer to the adjustment screw and Throttle Position Sensor. Looking at it from the view in the photo you would turn the screw CCW to open the butterfly and raise the idle air through the throttle body. When I installed the Ford Racing 70mm throttle body on my car I found that the IAC is supplying part of the air at idle and the throttle body the remainder. If I disconnected the IAC the idle would drop and vice-versa. What I also found was if I adjusted the screw on the throttle body the idle would remain the same through a range. If I increased the idle at the throttle body the IAC would close down to maintain the idle RPM specified in the computer and conversly if I closed it down the IAC would respond and supply more air. As mentioned this only happened over a limited range of adjustment. If I opened the throttle body too much the IAC could only close down so far and the idle would rise at some point. At the other end it if I closed the throttle body down too much the IAC had problems supply enough idle air. What I propose may help in your situation is to adjust the screw so that the throttle body is supplying a larger percentage of the idle air. Maybe under your circumstances the IAC
is closing down after you shift and the throttle body isn't open enough to keep the idle up. This is offered as a guess only, I have not experienced your particular problem, obviously a fortunate thing!

If you choose to pursue this avenue the first thing I would suggest is too look at the screw and see if there are any signs that it has been moved. When I adjusted mine I had to use some pliers to turn it as the threads fit snugly so it can't move and lose adjustment. I would also be wise to check the TPS voltage setting at idle to see where it is set (key on but engine not running). On my car (02 GT) this was done between the Gray/Red and Gray/White wires. The instructions I got from Ford said this voltage should be between .96-.98 volts although my Haynes manual allows the setting to be 0.50-1.0 volts. If this is off then it could mean that the throttle body adjustment screw may have been moved because the TPS voltage will change with an adjustment there. Or possibly it could indicate a failing TPS. I would also make a mark on the screw and the throttle body so if all else fails you can return the screw to the original setting. At this point you could try adjusting the screw so that the throttle body is supply more idle air. If your car reacts as mine did you should be able to adjust it open slightly more and maintain the same idle RPM. Just remember that if you choose to keep the throttle body screw adjusted at a new setting it will change the TPS idle voltage setting and it will need to be checked. The TPS setting can be adjusted by loosening the two mounting screws and rotating the TPS. It's a little tricky because a small adjustment goes a long way and it tends to move when tightening down the screws. Enough patience and swearing will get it done. If you run out of adjustment in the holes the info I got from Ford says you can drill the holes out slightly for more adjustment. If you adjust the TPS setting you then are supposed to disconnect the battery for 15-20 minutes. When you reconnect apparently the computer "relearns" the TPS setting.
I hope this was not too long winded and helps. I am a home mechanic only and if any Pros out there see a problem with these suggestions please comment!


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I had mine running when I made my adjustment, a shot in the dark otherwise. I see you checked your TPS voltages and they look like they are in spec. I also see you installed a SCT tune, did you check to see what idle RPM it is loading? My Diablosport has a place where you can adjust the idle RPM. It might be good to check on this and see if the car is holding the programmed idle RPM for starters. If they are in sync then try adjusting the screw up a little. Do it when the engine is warmed up but the radiator fan is off....don't need another variable in the mix!
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