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1987 911 Carrera Dieseling With Modified Engine

Engine

Vehicle Information: Model: 911 Carrera; Year: 1987; Body Type: Targa; Engine (type, size, modifications): 3.4 L (3.2L with 9.8:1 Mahle pistons/cylinders), authority chip, SSI headers.; Total Mileage: 225,000 kms;

I have a 1987 911 with a 3.4L engine (3.2L with 3.4L Mahle 9.8:1 CPRpistons). Recently, the car would experience "dieseling" or "run-on" whenI switch off the ignition key. The problem is intermittent and onlyseems to happen if the car is driven slowly. I can drive the car hard atthe track for 25 minutes on a hot summer day and it shuts off properly,but if I drive the car for 20 minutes in the city (not stop and gotraffic) and turn off the key, the engine sometimes stumbles for up to 6or 7 seconds. I have a fuel pump kill switch, and if I flick this switchwhile the car is dieseling, it stops immediately. I installed new sparkplugs and wires (Magnecor) at the start of the summer. I have triedswitching fuel stations (Sunoco 94, Shell, etc), but this does not seemto make a difference. It seems that the fuel pump is not shutting offproperly, can this be the case?

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