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TOPIC: High Idle

High Idle 07 Oct 2018 15:03 #20621


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i was wondering if any of you "928 Brains" out there could lend me some advice. i have a 1994 GTS Automatic with 103Kmi. I don't drive it a whole lot anymore, maybe around 25mi. per month. The problem i have now is; for the past several monnths it idles at 1000 rpm and 1100 when the air compressor kicks on. Along with this high idle, i've noticed that the running temperature settles in at a little bit over the halfway mark on the temperature gage, and with the temperature and idle at these conditons i observe that if accelerate aggressively sounds like I hear valve pinging, which I dont't get when its running at slightly cooler temperatures. On Ford an GM engines I've built in the past, these would be present if the timing is too advanced, but with this '94 GTS there no end user means of adjusting the timing. Am i right? Has any one of ye ever experienced this?
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High Idle 10 Oct 2018 15:17 #20622


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The 1987 and newer Porsches do not have adjustable idle speed, so a problem must be fixed to get proper idle speed.

High idle can be caused by several potential problems:

1. failed Temp II sensor, an improper resistance causes the LH to go into cold start mode which includes high idle and rich air-fuel mixture.

2. a vacuum leak past the throttle housing ... the net effect is a wider open throttle. A smoke test is the best way to find vacuum leaks, most professional mechanics have the equipment to do a quick test.

3. a failed idle speed valve. The valve is controlled by the LH fuel injection module. It runs off a variable duty cycle 100 Hz signal. A failed open valve is basically a big vacuum leak.

4. a failing LH fuel injection module. One of the failure modes is inability to manage idle speed. The air-fuel mixture is also not well regulated.

I offer free LH fuel injection module testing to 928 OC members as part of my company's services.
Rich
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1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast
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High Idle 10 Oct 2018 21:39 #20623


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Thanks for these troubleshooting tips, Rich! I will start tackling them one by one this weekenf in the order that you list them.
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