Here's some good information
There are several possible causes for the surging.
- An owner of an early car says:
Does your engine have a vacuum limiter? It's a saucer shaped thing on the US passenger side of the throttle body. Has two large hoses on the throttle body side, and one small hose on the outside.
It can become overly sensitive with age, and it will give the variable idle symptoms you describe.
You can replace it with 5/8" and 3/4" rubber heater hose blockoff caps, without any problem. Porsche stopped installing them in 83 or 84.
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Vacuum leaks in general can cause surging. Use a four-foot piece of rubber or plastic hose stuck in your best ear to try to locate any hisses. Stay out of the moving parts!
The Temp II sensor, on the top front of the engine can cause problems if it is saying that the engine is cold after it warms up.
Improper ignition timing can cause surging.
If you do not have the Factory Workshop Manuals, it will be very difficult to maintain the 928. Contact Jim Morehouse and buy the complete set of 928 tech info CDs that he has prepared. Jim had every scrap of 928 info that he could buy or borrow professionally scanned, converted to pdf files and burned to CDs. There is a HUGE amount of info in the set - this will be the best money that you will ever spend on your 928. Jim's email is
jim928@ptd.net (We have no commercial connection with Jim.)