engine run-on; what causes it?

Hendog · 10-14-2007, 06:44 PM

I finished replacing my distributor cap, rotor, wires and spark plugs yesterday. I've taken the 911 out a few times now. This afternoon, after the car had sat for an hour, I did a quick trip (6-7 Km). When I shut the car off, the engine ran-on. It's never done that before. I just got it home and took the looong way. It shut off just fine. What causes engine run-on?

rodoredas · 10-14-2007, 06:57 PM

A quote from the net - not specific to 911 but none the less - "Engine Run on in older cars can also be caused by the fuel switch in zenith Carbs not turning off fully and dribbling a little fuel into the engine after turn off. Some higher compression engines (like my modified 3.0S
also run on, because of higher compression combined with reduced back
pressure of a free flow exhaust. The fix was a solenoid that pulls the
carburettor throttle link closed hard when the key is turned off"

cheers

Steve@Rennsport · 10-14-2007, 07:47 PM

Henri:

Run-on (or "dieseling") is caused by a hot spot in the combustion chamber that provides an ignition source after the key is shut off.

That hot-spot can be an overheated spark plug tip caused by using plugs of the wrong heat range, low octane fuel, or excessive carbon buildups on the piston and/or combustion chamber that leave a small glowing piece of carbon.

You didn't say what plugs you installed, what grade of fuel you use nor how many miles are on your car so I cannot offer more specific suggestions, but hopefully this information will help you.

Hendog · 10-14-2007, 08:17 PM

Quote:

Originally Posted by Steve@Rennsport

Henri:

You didn't say what plugs you installed, what grade of fuel you use nor how many miles are on your car so I cannot offer more specific suggestions, but hopefully this information will help you.

Steve, I'm running NKG BPR6ES, gapped at 0.7mm. I burn Chevron 92 octane and the car has 82,000 Km. It's never done this in the 30,000 km of time that I've owned it. As I said, the only thing that changed is the new ignition components mentioned.

Paulporsche · 10-15-2007, 06:54 AM

The new plugs may be igniting the carbon buildup Steve mentioned. Try running some Chevron additive through it, which will be more concentrated than your fuel, and doing a few high rev runs.

If your DME is operating properly then your mixture should be correct.

Was your engine fully warmed up after that 6-7km run?

Hendog · 10-15-2007, 08:23 AM

I can't see there being any carbon build up; I drive it like a Porsche 911

It may have simply been a one time thing. I will keep an eye on it of course but it was a little disconcerting. I actually put it in gear and let the clutch out to kill it

I'll have to try some Techron prior to changing the oil and see. Thanks for your input guys.

Superman · 10-15-2007, 08:40 AM

As usual, Steve's description is spot-on and eloquent. The thing I'm curious about is how fuel is getting in. Carbs can/will deliver fuel as long as there is negative pressure on the venturi. But fuel injection systems, in my experience, STOP delivering fuel the INSTANT there is no voltage to the fuel pump.

Hendog · 10-15-2007, 08:53 AM

Quote:

Originally Posted by Superman

As usual, Steve's description is spot-on and eloquent. The thing I'm curious about is how fuel is getting in. Carbs can/will deliver fuel as long as there is negative pressure on the venturi. But fuel injection systems, in my experience, STOP delivering fuel the INSTANT there is no voltage to the fuel pump.

Yes, that thought crossed my mind...and the question persists.

Also, is it possible that the old wires, rotor, cap did not run as hot as the new components? BTW, the last time I changed plugs, 20K Km ago, I didn't gap them, I just put them in. This time I checked the gap and adjusted to 0.7mm, which is a smaller gap than NGK factory. Plus I now have new cap, rotor and Magnacors. Could the previous setup have caused carbon build up, even with "faithful right foot pressure"?

dshepp806 · dshepp806's Garage

I would imagine that the ignition switch would be more suspect. Does it "diesel" or does it just keep staying on (that would be ALL 6 cylinders)?

Mine would do this VERY intermittently, after being up and going for a long run. Had a failed alt/reg that was repaired about 2 months ago and I've yet so see it reappear (could these be related?)

After Steve's post as to carbon builup, now I'm concerned about my #1 spark plug having been in there for a while now due to the fact it's crossthreaded. I asked the P-wrench about leaving it be until time to drop the engine for repair,...if there would be ANY negative consequence of leaving it in for now (and the next many thousands of miles: he said NO.

Could I be getting additional carbon buildup within #1 due to this situation? She's running fine (and always has).......no misses, etc.

Back to point: My car would run for just a few seconds after ignition cutoff (2-3 seconds seem like eternity)....run quite nicely, at that. VERY VERY intermittent....I assumed I had a flaky ignition switch.....now you guys' have me wondering about other things. Guess I'll wait to see it reappear before reacting......

Best,

Porsche_monkey · 10-15-2007, 12:17 PM

A dripping fuel injector wouldn't help either...