Engine run-on

I thought that I had this problem licked, but it has re-surfaced again. Occasionally, when I turn off the ignition switch, the engine will not stop. I have changed the electrical portion of the ignition switch, checked all grounds, etc. When this happens, the tach and guages all read 0 and the battery light comes on (so I figure that I'm cutting my run voltage to everything except the fuel/ignition.

The only thought that I have here is that the key is rather old (worn) and I'm thinking maybe it is not turning the tumbler all the way. But, before I plop down big$ @ the dealer, or worse, I'd like advice.

ANy guesses?? DME relay?, Key portion of the switch? Short?

Thanks to all in advance
Scott

Does the motor continue running as if the ignition is still on (smooth idle), or does it run rough (dieseling)? If the first, then I assume you turn the kay on and off until the ignition actually shuts off, and this is how you get it to stop.

If the second, then you might be having to wait for it to quit, or put it in gear and, wiht the brake on, physically stop the motor with the clutch.

If the first, there is some sort of electrical problem and needs to be found and repaired. If your car is not carburated, then it is surely an electrical problem. Dieseling can happen in carburated cars. With this, the problem is that something in a combustion chamber is very hot (usually a spark plug electrode. Hot enough to ignite gasoline without a spark.

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'83 SC

Superman, It is a Motronic 87. There is definatley some sort of electrical issue going on. I have an MSD on it, but that cannot be the problem because I have let it run-on for as much as 30 seconds so there must still be fuel being added.

I can pull the key totally out and it still runs. And yes, I have to stall it w/ the brake to stop it sometimes.

I know I gotta replace something, but the key tumbler is quite expensive and I don't want to bite that one off till I'm sure..

I have MSD on my car, and as I can remember the MSD manual says something about turning the key off and the motor still runs, check it on your manual.

Usual disclaimer: Most of my experience has been with non-Porsche cars. Bad tumblers typically won't turn the ignition switch (attached to the other end of the tumblers) far enough to do the job. You report that your tach and other stuff DOES go off? I doubt that your ignition switch/tumbler system would turn everything else off but leave the engine ignition and fuel system on. MMBRAZIL's clue is probably a better place to search, but you can test the switch. The ignition switch should come off the tumbler, and you can turn it wiht a screwdriver or fingernail. You can even drive it that way for a while, wiht thye ignitiin switch dangling under the dash. But I doubt this is your problem

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'83 SC

I've narrowd it down a bit.. Seems the DME relay is not releasing.. this would still provide power to direclty coil/injection. I am going to check the switched wire there this wknd.. let you know what I find. This should tell me if my switch or relay is sticking.

Oh, cool, that's a $15 part. Easy. Go buy one, plug it in. Problem solved.

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Mark Szabo
1986 911 Targa 3.2

With all of the problems I have heard about lately ... caused by just that particular relay, if it fixes your problem, buy another one to keep in the trunk with your tool kit as a spare!!! Pretty inexpensive insurance policy, and I don't think it is even as issue as to whether you will ever need another one ... so much as WHEN you will need another one!

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Warren Hall
1973 911S Targa

Ok, guys this is weird... After changing my voltage regulator on Wednesday, the car stopped it's "run-on" tantrums.. Could it have been excessive voltage??? I'll be on the lookout for it again, but for now.. problem solved.

Scott,

Check out page 36 of your MSD manual and it will show you how to install the diode that will prevent this run on. On Internal voltage regulated alternators it is different from external but the problem is still the same. You are keeping the MSD box energized by the tiny bit of the charing current that is bleeding back into the electrical system from your charging indicator light. That is why your symptoms seemed to go away after the reg. change but they will probably be back.

Use a (1A-100V) diode avail at radio shack and it should correct your problem. If you need a diagram or have lost your instructions check their web sight.

Good luck!

s

Mark: good luck trying to find a DME relay for $15-- they're $37 at Zims. It's not the standard black relay found in the fuse box it's a long square relay mounted next to the DME box.

Scott: this is probably a dumb question but why would you want to replace the old reliable Porsche ignition with an MSD unit?
Charlie '87 Carrera

Charlie, correct. I had mine replaced not too long ago, before I learned how to operate a screwdriver. It's easy to get that one confused with the fuel pump relay and some of the others - they're fairly identical. But the DME relay is a $40 part.

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Mark Szabo
1986 911 Targa 3.2

Sorry for the dealy guys, lost my DSL conncection. DAMN!!!

I put the MSD on for 2 reasons.
1) the chip I installed seemed to make the car idle rougher and stumble some when cold.

2) I had an extra $135 in my pocket and wanted to spend it on my 911.. (mostly reason 2, but rationalized to the fiancee with reason 1)

I can say that I am quite pleased with the drivability of the MSD system. Worth $135, probably not if it was $250.

By the way guys, we're on week 3 of no run-on.. Maybe the excessive voltage was making the DME relay Stick???