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TOPIC: Idle Issues

Idle Issues 03 Apr 2019 14:51 #20767

Hello all,
I have a 1986.5 928S and it has been having intermittent idle issues. When cold and even after it warms up it occasionally wants to fumble and stall. Other times it idles perfectly for a while and if sitting at a stop light for too long it will start doing it again. Any insight would be appreciated.

Thanks!!
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Idle Issues 06 Apr 2019 16:18 #20772


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The first question is how long has it been since your 928 had a tune-up with replacement of spark plugs, distributor caps and rotors? The spark plug wires are also a potential cause of poor running, have they ever been replaced? OEM replacements are quite expensive but if the wires are original spark can jump especially in humid climates. These are the most common sources of stalling problems.

The 1985 - 1986 Porsche 928s require the setting of the base air/fuel ratio and idle speed. Most owners don't know this and most shops have the necessary equipment to "sniff" the exhaust CO% before the catalytic converter to make the necessary adjustment. If the base idle speed setting is too low, the automatic idle speed regulation may not be able to cope. If you send me a note at This email address is being protected from spambots. You need JavaScript enabled to view it. I can provide a PDF with the factory workshop manual instructions.

The LH injection module on the 1985 - 1985 928s has a narrower band where the lambda regulation (air/fuel ratio) can be kept in the allowable range, therefore the engine is more susceptible to an out of calibration mass airflow sensor (MAF). The MAF uses a hot wire as part of the airflow measurement technique. The wire glows red hot after each running cycle to burn-off contaminants. After several decades, the wire thins and the resistance changes. The wire is also more prone to become brittle and break. I provide a test service (no charge to 928 OC members, only shipping charges) and can advise if your MAF is within the necessary range. I stock rebuilt Porsche MAFs. Over 90% of the MAFs that I see are beyond the acceptable calibration range. Symptoms are what you describe and include lethargic performance and eventually will result in a no-start condition.
Rich
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1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast
Last Edit: 06 Apr 2019 16:19 by Rich928.
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Idle Issues 06 Apr 2019 20:37 #20773

Thank you Rich...tons of information and some things to check out for sure. The car had been sitting for 8+ years without running and I just got her road worthy. It's been such a blast driving it and most of the time the idle is fine but occasionally it wants to sputter and even stall...usually shortly after the car has been started and it's not completely warm.

I may be interested in getting a rebuilt MAF from you as I'm sensing this is an issue with many 928's. Are they difficult to swap out?

I really appreciate the help!!
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Idle Issues 06 Apr 2019 22:57 #20775


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It isn't too difficult to remove the MAF. Remove the air filter box, remove a rubber collar. The most involved issue is removing the connector retaining clip and not dropping it down into the V of the engine.

I have a PDF doc that outlines the steps.
Rich
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast
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