1999 Porsche 911 Carrera Idle hangs when rolling

1999 PORSCHE 911
103,000 MILES • 6 CYL • 2WD • MANUAL
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FREDINTEXAS
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I have a 99 3.4L 911 (996) with a 6-speed that is suffering from an intermittent idle problem. Sometimes when I clutch, usually at speeds over 35 mph, the engine rpm hangs around 2000. If it's cold outside it might even hang as high as 3500 rpm. Everytime I clutch, it will continue to hang until the speed of the car drops below 10 mph. Once it hits the 10 mph threshold, the idle will instantly drop to about 800 rpm, which is the normal idle speed.

I can't find any rhyme or reason to temperature of the car or weather conditions when this happens. It seems completely random. Somedays it will do it all day long. Somedays it will just do it one cycle, and then quit the first time I come to a stop, which caused the idle to return to normal.

This only happens when I'm rolling above 10 mph. I cannot get it to happen at a stop.

The car gives no codes on both an OBD-II scanner and a Durametric scanner.

So far I have tried changing the IACV, changing the TPS, changing the MAF, cleaning the throttle body, cleaning the temp sensor connection, checking the throttle assembly for binding, checking the accelerator pedal for binding and searching for vacuum leaks.

My mechanic was no help as the car wouldn't do it while he had it, so that was a waste of $100. This is driving me crazy. Do you guys have any ideas?
Jan 12, 2009 at 4:55 PM
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JNOVACK
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here are some things to check please let me know what you find after checking these parts
Inspect EGR Valve Clogged, dirty or improperly functioning or defective egr valve.

Inspect Idle Air Control Valve Damaged, loose, or faulty idle air control valve.

Inspect Idle Speed Control Actuator Damaged, loose, or faulty idle speed actuator.

Inspect Idle Stop Solenoid Damaged, loose, or faulty idle stop solenoid.

Inspect Throttle Position Sensor Faulty throttle position sensor or throttle position sensor circuit.
Jan 12, 2009 at 7:27 PM
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FREDINTEXAS
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EGR Valve was replaced 1 year ago. It went bad just after I bought the car and threw a CEL. On inspection it seems fine and is not throwing any CELs. Idle Air Control Valve was what I believed to be the culprit. I started by cleaning it, but when that didn't help, I replaced it. Didn't make a bit of difference. Throttle Position Sensor tested good, but just to be safe, I replaced it as well. Didn't make a bit of difference. I have no idea where to find the Idle Speed Actuator or the Idle Stop Solenoid. They don't show up in my manuals. Here's a chart of some real-time values I recorded while driving home tonight. When the throttle position is at 0, I was clutching and letting the car coast.


https://www.2carpros.com/forum/automotive_pictures/300836_hangingidle_1.jpg

By comparison, here's another reading I took after the car began functioning normally again.


https://www.2carpros.com/forum/automotive_pictures/300836_regularidle_1.jpg

I can only record 8 fields at a time, so tomorrow I'm going to take more reading but track the 02 sensors, etc. Any ideas?
Jan 12, 2009 at 10:49 PM
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JNOVACK
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also you need to test the camshaft and crankshaft sensors
Jan 13, 2009 at 9:45 AM
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FREDINTEXAS
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I'm trying to log the info with my durametric scanner, but as this is an intermittent problem, I'm stuck waiting on the car to do it again.
Jan 14, 2009 at 7:53 AM
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JNOVACK
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also check your vacumm lines for any cracks or loose
Jan 14, 2009 at 10:02 AM
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FREDINTEXAS
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https://www.2carpros.com/forum/automotive_pictures/300836_test01_1.jpg

Here's some readings from the drive home last night. I pulled the screen showing the last time the RPMs hung while clutching and the first time it started behaving normal. I noticed that when the car is behaving correctly, anytime I hit the clutch, the 02 sensor voltage will momentarily drop to 0, at which time the engine rpms drop to 800. When the car is not behaving correctly, the 02 voltage never goes to zero unless the car coasts to under 10 mph. Is there a sensor or switch that causes this blip to 0 in the O2 voltage that I need to check?
Jan 15, 2009 at 11:14 AM
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JNOVACK
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I am trying to find the tests for O2 sensor I will get back to you with the test but also check cam and crank sensors also vacumm for leaks or cracks
Jan 15, 2009 at 3:15 PM
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DR LOOT
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I think I understand what is happening, if you look on the right side of the engine you will find the TPS (throttle positioning sensor) is on top of the right-head, you will need an om meter to check continuity, disconnect the sensor, the meter on the sensor and manually open the throttle, if there is no continuity the sensor is no good
Feb 13, 2009 at 3:04 PM
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FREDINTEXAS
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I've changed the TPS, ICS and Water Temperature Sensor. I had a Porsche dealership look at it. Their first guess was ICS, but when that tested ok, their next guess was the DME (ECU). Of course, they wanted almost $3000 to change it out and weren't even sure it was the problem.

I have found that if the car isn't doing it, I can recreate the problem by turning the car off and then back on. It happens when the car is started warm.
Feb 16, 2009 at 8:38 AM
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DR LOOT
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Sorry I had a little to much to drink that night, what I meant to say was not the TPS but the Idle Speed Actuator or the Idle Stop Solenoid, is a micro switch is on top of the right-head close to the throttle body, ISC is sometimes intermittent, you will need to check it when you're having issues, also as I read over everything that has been posted, I can't find anywhere where you check the fuel pressure, or change the fuel filter, fluctuation in fuel pressure can cause an erratic idle,
Feb 16, 2009 at 9:02 AM