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 info@electronikrepair.com 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
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast
