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Discussion Starter #1
Having trouble with my 390. Seems like once the engine is hot and I shut it off, it keeps running for a little bit. It has an MSD, and I see in their literature that it may be cured by adding a diode near the voltage regulator. It just weird that it started recently. Could it be a timing thng? If so, how? Anything else you can think of?
 

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Probably carbon buildup. Try some Slick 50 or Seafoam fuel treatment. I like to run a bottle of the stuff in all my vehicles about every 5000 miles or so. If I go too long without putting some in the wifes car the fuel mileage drops.
 

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timing is off a tad. u can probally "fix" it by using higher octane fuel.... on my boat i can't run cheap gas..its been in and out of the shop since i got it trying to get it to quit dieseling and no one could ever get it to stop. when its hot (usually after pulling tubers around and going back to get them and shutting it off) it will run on/diesel on 87 octane fuel. it wont do it on 89 or 93.... its very common in the boat world for whatever reason for people to just burn premium fuel to "fix" it.... but its gotta be a timing issue to be honest.

if it just recently started, im wondering if theres been a change in the quality of fuel...perhaps a tad lower octane just enough to cause it? id be willing to bet if u ran a bottle of octane boost and some higher octane fuel it'll stop.
 

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Yeah could be any of those things or a combo. I've noticed my idle sometimes creeps up as it gets hotter and I have the issue....I have to back it down a tad, then it's to low until it warms up again (can't win)! Are you running vac advance or mechanical? I don't see what good a diode on your VR would do unless there is a known issue of voltage backfeeding through somehow - you would think that would have cropped up immediately?
 

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Discussion Starter #5
I'm using the same 91 octane that I've been runnning all along. Tried a bottle of additive recently and it didn't do any good. I have been running a vacuum advance. I recently disconnected it and the car runs-on like before (when it's hot). My initial timing is 13 and total timing is 40 all in by 3000rpm (with vacuum advance disconnected and plugged) MSD says that there is a possible issue with older cars, whereas they feed voltage back through a "charging light" I don't know what that is, but I do have a volt meter in my '68 XR-7's dash. I wonder if that could be it?
 

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I refuse to believe that timing could be the issue if the timing set set to where the car likes it. Ignitions do not continue to fire after the key is off, so timing could not play a role. The only way timing could cause it is if it is so far advanced that it is causing hot spots, or hot plug electrodes. If it is hot plug electrodes though, then the plugs likely need to be changed for colder ones. Have you changed plugs recently?

Some additives don't do what they claim. The 3 and 4 dollar bottles from Gunk and STP may clean injectors in new cars, but they do very little to nothing for carbon build up on the pistons. I have always used Slick 50 and it works great. It is about 10 dollars a bottle, but cleans much better than the cheap stuff.

I had a 70 Cutlass that would Diesel like hell when I shut it off. I ran a bottle of the Slick 50 in it and it cleared it up. From then on I would put a bottle in every few tanks of fuel. I also tore the engine apart a couple of years later and the tops of the pistons were almost all shiny aluminum.
 

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Years ago when this happened, we would JAMB the throttle to the floor at the same time we turned the key OFF.

That worked, MOST OF THE TIME, but such doesn't correct the issue.

Dale in Indy
 

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If it has an automatic trans, shut the engine off while still in drive.

Reduce the idle speed a little. This may help in any case.
 

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I think he is describing more than dieseling. The simple way to verify that the ignition is being back fed is by pulling the pwoer wire off the coil. If it stops running then you have found the cause.
 

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Friend of mine had a 69 Nova back in the day, it liked to diesel.
One night we were out raising he!! and he got pulled over.
Cop comes up says shut the car off, SHUT THE CAR OFF!!!!.
My friend says, I'm trying.lol

Don't remember what he did to fix the problem.
This thread reminded me of that night.
 

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on my boat, FWIW, the dealers say the timing being off creates the hot spots causing it to diesel.... if u google dieseling timing being off is the first thing that is known to cause it. but i'd do like is suggested above and run some cleaner through there...i personally like seafoam....run it through there, piss the neighbors off, do away with mosquitos at the same time and clean er out.
 

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Important question: when you say it continues to run, is it running normally and sound normal? Or does it sound clanky like a diesel engine and erratic? I think some are assuming your car is dieseling (a problem caused by hot spots/low octane fuel/high temp where the engine "runs on" and is actually self-igniting the mixture, and I believe turning the motor in reverse during this). OR is it continuing to run like normal like it's being ignited by the ignition system despite the key in the off position? When you turn the key off, does the motor spin down like it's shutting off, then sputter? Or not?

Kevin
 

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Discussion Starter #13
Important question: when you say it continues to run, is it running normally and sound normal? Or does it sound clanky like a diesel engine and erratic? I think some are assuming your car is dieseling (a problem caused by hot spots/low octane fuel/high temp where the engine "runs on" and is actually self-igniting the mixture, and I believe turning the motor in reverse during this). OR is it continuing to run like normal like it's being ignited by the ignition system despite the key in the off position? When you turn the key off, does the motor spin down like it's shutting off, then sputter? Or not?

Kevin
It sputters then dies. there was one time when it actually revved up a little, running fairly normally, but that was unusual. Normally it sputters.
 

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If it has an automatic trans, shut the engine off while still in drive.
What my Brother from another Mother said.

I blame it on the ethanol.
 

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Do you have a Pertronix Ignition?
 

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I had that problem once when I was drunk and put diesel in the coug I could not get her to stop running. I stoped srinking soon after and have not put diesel fuel in since. Sorry ,off topic, I could'nt resist . LOL.....
 

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PBCOUG, Your DRINKING has mess up your SPELLING, example, "srinking", should be DRINKING, and "could'nt" should be COULDN'T.

Try AAA, TOMORROW, and Don't Drink and Drive, you might hurt the Cougar. I don't care about YOU, just the Cougar.

Dale in Indy
 

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LOL - you guys are making my beer come out funny! I think Kevin's on to the right track, if it's not running smoothly after shutoff, you are likely dealing with non electric issues. Could be hot spots, or timing. Since it's vac advance - I'm wondering if you have it hooked up to ported carb connection? (i.e. only advancing under correct conditions?) It's not a constant vacuum port....try capping it off and see what changes/happens.....
 
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