dazednconfusedj

I have a 2001 mustang cobra, and i have noticed lately that when im coming to a stop light and i take the car out of gear, the rpm's seem to drop to around 500-600 then springing back up to around 1k, then finally leveling out at a complete stop to about 750 which is normal. I dont get why the RPM's are dropping so low when i depress the clutch while driving. It does not seem to do it while shifting up, but only when i go to take the car out of gear and braking does the problem arise. I have a SCT tuner that i was using on the car, and a JLT cold air intake, but since the problem came up i have returned everything back to stock and the problem is still there. I have already replaced the IAC, the mass air flow has been cleaned, and the TPS is set at .98 volts. I do have symptoms of a throwout bearing going bad, but i am not positive if this would generate the problem i am having, also, i suspect that the tuner has messed something up in the pcm and is unable to return the car back to its stock values correctly. Also, when setting at an idle, if i rev the car, the rpms will go up like they should, then when the RPM's are coming down, it seems to sit at about 1k for around 2 seconds, then completely drop down to about 500-600 then level back up to 750. I have been around in circles with this issue, so if anyone can offer me any advice or tips, i would greatly appreciate it!!!

cliffyk

The IAC is sticking.

Some had reported success cleaning it with TB cleaner, which generally seems to be a short-lived temporary fix.

Ford says it "...cannot be cleaned or adjusted,", replace it.

Also for future reference, the TPS used in the EEC-V systems is not adjustable nor does it need to be measured and verified.

The "eek-5" PCM monitors the TPS voltage at start up and periodically during engine operation, and remembers the lowest voltage observed as representing closed throttle. This is called the TPSRatch value (Reverse latch) in the tune's inner workings.

As it also knows the angular rate of change of the TPS potentiometer's resistance calculating the "percentage" of throttle opening, based on current voltage and the TPSRatch value, is simple.

dazednconfusedj

Thank you for the quick reply. I have a hard time believing that its the IAC sticking, although im stubborn... But i literally just replaced the IAC with a Ford Motorcraft IAC from the dealer. Is it possible that a brand new one from the dealer is sticking?

0949er

yeah its a common problem with these mustangs it seems like. Replacing the IAC fixing the idle issues 95% of the time.

dazednconfusedj

Alright, ill go ahead and send it back to the dealer for a replacement. I would just assume a brand new IAC from the dealer would work correctly.

cliffyk

I missed that buried in the glob of text in your first posting--it would have been much easier on my 63 year-old eyes like this:

That said, have you checked to see if the control signal is present at the IAC?

With the engine warm and idling unplug the connector at the IAC, the engine speed should drop and possibly even stall out.

It is quite unlikely, but perhaps possible, that the tuner has altered something that it cannot return to stock. I do not know anything about SCT's systems, however such a thing would be impossible with the Delta Force tuning systems and I suspect that it is not possible with SCT's as well.

There's nothing about the TOB that could cause the symptoms you describe.

dazednconfusedj

I am sorry about my cluttered text lol..

So when unplugging the IAC while the car is running i see no drop in idle at all, and no DTC is displayed.

Although, When i unplug the IAC before starting the car, it runs like ****, and will idle roughly.

So what does that mean?

cliffyk

That's OK, it just gave my old eyes a run for the money...

Is it possible that the "idle screw" (which isn't really an idle adjustment screw) on the throttle body has been mucked about with?

It is really a throttle valve stop screw that is used to set the valve so that a predetermined amount of air is bleeding through the TB--the IAC then can ad or subtract air to/from that flow to control the idle.

Here's the procedure for resetting it to where it should be.

dazednconfusedj

No, it has not been messed with.

dazednconfusedj

Its a stock throttle body and i have never taken a pair of pliers or vise grips to it.