Engine "Run-On"
 

Occasionally, my 1973 911 T "runs-on" when I shut the key off. What exactly is this caused from?

I have been told the following could be the cause:
1. Vacuum leak
2. Idle too high
3. Timing too high

Is this correct? I do have my idle speed set a little high, but I think my car idles best this way. Could incorrect fuel mix cause this problem too? Thanks for any comments...

Bill

Run on is usually caused by excessive combustion temps. The cylinders continue to fire, like a diesel, due to the extreem temps.

The timing could be too advanced. Or there could be carbon build up on the valves/pistons which acts like insulation.

Check the timing and run a can or two of Techron mixed with the gas.

Good luck

carbon build-up --That's the likely problem. carbon buildup acts like a glowplug. Which is why run-on is also refered to as "Dieselling" -- The combination of the hot carbon and the added heat during compression fires off the fuel.

Bill,

I doubt with an engine rebuilt as recently as yours that carbon is an issue. I suspect that the idle speed being elevated is the problem. Set it to 900 rpm and see if the problem doesn't go away.

it might be a coincedence but my 74 would run on now and then after turning off the key for a few days before one tensioner failed. I had never had that problem before. I'm thinking the timing would get off from the slack. in fact, the last time it was started and stopped before tearing it down was the worse time.

As Warren suggest, setting the idle speed down may help. Also higher octane rated gas is likely to help.
But no doubt something in the combustion chamber is hot enough to fire the air/fuel. Carbon buildup is most likely in my opinion. Keeping in mind Carbon buildup can happen early as the ring seat, and allow oil to pass, to then get pyrolyzed (cooked) on to the combustion chamber surfaces.

Bill,

What spark plug do you have in the engine currently?

You don't want anything hotter than a Bosch W4DC or NGK BP8ES this time of the year!

In order for run-on or dieselling to ocurr, you will need an ignition source. There will always be some left over fuel in the engine, but if there is no spark, it will not continue to run after the ignition is turned off.

Warren may have something there - the spark plugs may be getting too hot, and may be acting as an ignition source?

Your ignition switch may also not be working properly to shut off the car...

-Wayne

Good thinking Warren, on the plugs being too hot.
Of course if plugs are too "cold" they will build-up carbon easly.
Pulling the plugs for inspection would be smart.

Hello

ON MFI´s check the fuel stop selenioid but even if they don´t cut off your engine shouldn´t run on.
Sound like your ignition timing is far off or you have the wrong spark plugs.

Grüsse

My idle is between 800-900 rpm. From what I have read, this is a little higher than factory settings, but I feel this is the best idle rpm for my car.

I'm not sure about the spark plugs in the car. They were replaced when the engine was rebuilt. I think my timing might be the culprit.

It's strange that someone mentioned a faulty ignition switch. I have noticed that when I first try to start the car, the car will turn over, but may take quite a while to fire. If I turn off the key and then immediately turn the key back on, the car will usually start immediately. Could this be the result of a faulty ignition switch, or do you think that the problem may simply be the fuel solenoid that sits atop the fuel filter? I remember taking this thing out once and put some power to it. It appeared that the plunger was moving like it should.

Maybe that's another problem...

The car didn't do this before. The only thing that I changed was that I enrichened both the part load and idle mix with a Gunson Gastester to between 6-8%. This is higher than the factory settings I know, but the car used to pop on deceleration. Adding more fuel to the mix solved this problem though.

Just when I thought I had everything under control...

Bill

I almost forgot...

I have to agree with Warren on the carbon buildup issue, as I probably haven't driven the car enough to have carbon buildup.

Also, I have the updated pressure fed tensioners, so I doubt this is the problem either.

My next fill up will have a bottle of fuel injection cleaner in it as well.

Thank you all very much for your comments!

911s don't typically suffer from a dieselling problem. I would check your ignition swtich! Next time it runs on, don't touch the ignition switch. Instead run over to the engine comparment and see if you can hear the CD Box humming (high pitched whine). If you can, then it probably is your switch that is at fault...

-Wayne

Thanks Wayne! I'll try that next time I get the problem. I have always thought that my ignition switch had something wrong with it...I keep you all posted!

Bill

I did some checking last night and I noticed that my throttle microswitch was not being fully engaged when I let off of the throttle. My throttle arm linkage is a bit "sticky". Part of the problem is caused by a worn arm on my Warm Up Regulator. I am going to adjust the microswitch tomorrow when I get time and regrease my throttle body arms.

Do you think this caused my problems?

If not, should I purchase the electrical portion only for my ignition switch, or should I purchase an entire igniion switch with steering wheel lock?

Bill

If your ignition switch is bad, then replace it. If the key mechanism seems fine, then just replace the electrical part. The key part is pretty spendy I think. But I am relatively certain this is not your problem. Real certain. If the switch were not opening, then the car would not diesel as it tries to shut down. Instead it would simply keep running. And wiggling the key would change things. No, I am sure your ignition switch is not causing this.

It sounds like carbon buildup is also not the problem, though it is still possible. If the rebuild cause a metal shard or sliver to be present in the combustion chamber, then this could glow enough to cause the dieseling. Unlikely though, since this would be sloppy machine work. A real lean mixture can cause this, but that's not your situation I guess. Timing that is advanced too far can do this. One extra-lean cylinder can do this. Spark plugs that are too hot can too. That's my first guess...spark plugs.

At any rate, my real advice is this: Fix it NOW! Dieseling can hurt your engine terribly. It can bend rods, for example.

Quick update on the problem I was having:

I turned my distributor counterclockwise a tad and now the problem is gone. I guess my timing was a little too high. The car runs much better now.

My idle is a little high, but I can live with that until I get some time to readjust. Thanks for all the comments!

Bill