911sc engine runs-on after switch-off

Harvs11 · Harvs11's Garage

Can anyone suggest a reason why my 1980 911sc sometimes runs on for 1-2 seconds after switching off. The car starts & runs nicely otherwise. Any help would be great in trying to save some time ($$) in the shop sorting it out.

CMXI · 01-24-2008, 01:53 AM

Just a novice thought.

Can it be that the fuel vapors combust due to the heat inside the cyl?

Possibly in combination with the that fuelpump comtinues to rotate a couple of seconds after you switched off the ignition??

In theory if the altinator is running and you get a small amount of current bypassing to the ignition system the engine wont stop until there are a lack of either fuel or air..

The rule of thumb is.
Heat/Spark, fuel, air.
Take one away and the engine stops.

You can get this phenomenon on some diesel engines by keeping the engine-revs up and pressing the gas pedal when you turn off the ignition.
Than you can keep the engine running while not having the ignition on.
However this is a less complicated engine compared to the Otto engine.

Annyhow i would´n be to conserned as long as the engine performes well while driving.

Perhaps somebody has a more scientific explination....

Best Regards
Patrick Hansson
Sweden.

Mo_Gearhead · 01-24-2008, 03:50 AM

Gasoline engines can diesel (continue to run after ignition is turned off) if they have a 'hot spot' in the combustion chamber(s).

Sometimes this is caused by carbon deposits that can build up on the piston top or in the head/combustion chamber and a rough edge of the carbon gets so hot it acts as a spark plug to ignite the fuel ...thus inducing 'run-on' (dieseling).

This is rarer, but can also happen in some engines with domed pistons with valve 'relief cuts' that produce sharp edges in the piston top. One should always round off ANY sharp 'edges' on new piston tops/domes to reduce the chances of this occurring.

In older engines, with many miles, where carbon has perhaps built up, I warm the engine to operating temp., remove the air cleaner to allow access to the carbs/FI ports and use a spray bottle (like windex) filled with water; revving the engine by hand, I spray the water into the carb/FI body, several times.

I do this every few months on high mileages cars. The water, in effect, gives a sort of 'steam bath' to the combustion chambers and can help loosen and remove carbon edges that can be the culprit.

Try the water treatment, it may help.

Superman · 01-24-2008, 07:33 AM

This should be impossible. When a CIS fuel pump stops, so does the engine. Try this some time. Pull the FP relay while the engine is running. It stops the engine just as if the ignition is shut off. So....if a CIS engine diesels, the fuel pump must be continuing to run. The system that prevents this (FP running when ignition is off) is an important safety feature. Without it, your FP could spew fuel on you and your car following a collision.

Superman · 01-24-2008, 07:33 AM

This should be impossible. When a CIS fuel pump stops, so does the engine. Try this some time. Pull the FP relay while the engine is running. It stops the engine just as if the ignition is shut off. So....if a CIS engine diesels, the fuel pump must be continuing to run. The system that prevents this (FP running when ignition is off) is an important safety feature. Without it, your FP could spew fuel on you and your car following a collision.

db_cooper · db_cooper's Garage

Do a pressure check on the system. System should hold pressure when the engine is off for some time. I don't recall the specs. If the fuel system drops pressure with the engine off, you could have a leaky injector (s) or a fuel distributor problem. This would allow the engine to run on for a few seconds when hot.

Superman · 01-24-2008, 01:38 PM

I doubt it. When the fuel pump stops, fuel pressure IMMEDIATELY drops to below the level necessary to open injectors. Run-on in CIS cars is extremely rare.

rusnak · 01-24-2008, 01:57 PM

I wonder if it could be the ignition switch (electrical side)? If the windows, heater, accessories, etc are acting strangely then I think it could be the switch.

dshepp806 · dshepp806's Garage

electrical...rule it out.

Best,

Superman · 01-24-2008, 03:00 PM

Agreed. Another thing I'd look at real quick is the switch behind the air flow sensor plate. That is the switch that senses when the plate is displaced upward. If the switch sees the plate is displaced upward, it concludes the engine is running and allows the fuel pump to run when the starter is not being operated. If this switch continues to think the sensor plate is up, even after the engine has been shut off, then this could be allowing the fuel pump to continue to run.