motor revs after ignition is turned off

[Mustang Boy]

ok this is how it goes usually when my mototr is warmed up say i park it turn it off and about a second later it revs itself vvvrrrrrr with no encouragement (ei. hitting gas) why is it doing this the other thing it does sometimes is backfires through carb when shutting it off but only does once in blue moon



i would like to know why my car is doing this any help would be appreciated

 

[Anonymous]

I've seen cars that sounded like they were dieseling when you shut them off. Just for a few seconds. A couple of times I just lowered the idle speed and the problem went away. I think other times it was due to overheating.

 

[weeds]

Every time this has happened to me, it has been a vacuum leak. Last time, the PCV valve had broken.

 

[wallaka]

Heat soak, vacuum leak, hot spots in the cylinder (carbon or spark plugs), low octane fuel, throttle plate leakage, it could be a lot of things. Basically anything that makes the engine run too hot.

If it only happens every once in a while, I wouldn't worry about it. Because I'm ghetto like that.

 

[hasa68mustang]

mine does thst when my idle is set up high, after setting it down it stopped...

 

[Mustang Boy]

well i found out it only did this when i was running on mid grade fuel so i stopped doing that and i havent had a problem since

 

[Anonymous]

Mine did that when it was overheating, after replacing the radiator problem went away

 

[tetraethyllead]

because of previous owners mine has carbon on the pistons. This causes it to ignite the mixture after I turn off the key. It's been doing it on and off for years. I just got in the habbit of releasing the clutch as I turn it off. (car is in reverse since it's a col. 3spd) This way if decides it wants to diesel on it will just stall anyway.

 

[Anonymous]

because of previous owners mine has carbon on the pistons. This causes it to ignite the mixture after I turn off the key. It's been doing it on and off for years. I just got in the habbit of releasing the clutch as I turn it off. (car is in reverse since it's a col. 3spd) This way if decides it wants to diesel on it will just stall anyway.

To get rid of the carbon, rev the engine up to about 3000 rpm and hold it there. then either hookup a vacuum line (from the carb) to a bottle of water or sit there with a spray bottle. I reccomend the spray bottle as it is easier. With the vacuum line you have to manually meter the amount of water going into the engine and you also have to worry about hydrolock (basically the death of your engine). Just spray the water into the carb and the carbon build-up will literally be steam cleaned away. About a cup of water should do it.

 

[JackFish]

I think Ford's solution was to put those anti-dieseling throttle stop solenoids on later carbs, like my 1946. Mine had been removed and occasionally, depending on conditions it will run-on a bit. Once I get the car tuned-in where I want it I may put the solenoid back on and use it the way it was intended.

 

[tetraethyllead]

To get rid of the carbon, rev the engine up to about 3000 rpm and hold it there. then either hookup a vacuum line (from the carb) to a bottle of water or sit there with a spray bottle. I reccomend the spray bottle as it is easier. With the vacuum line you have to manually meter the amount of water going into the engine and you also have to worry about hydrolock (basically the death of your engine). Just spray the water into the carb and the carbon build-up will literally be steam cleaned away. About a cup of water should do it.

Been there, done that. Also used GM top engine cleaner. Stuff is on there very subbornly. Although I admit I was a little shy on the water and used a spray bottle.

 

[66 Fastback]

The reason the factory started equipping them with anti-dieseling solenoids was they just about tuned the cars to the point that they would not run at idle. They leaned out the idle and they retarded the spark at idle by placing the vacuum advance on a ported vacuum source. The leaner, retarded mixture burned raised combustion temps and reduced emissions at idle. In order to get the thing idling decent and to lean out the mixture, the throttle plates were open more than previous years. These two things caused the engines to diesel upon shut down because the combustion chamber was hotter and the throttle plates were openned, allowing the engine to continue to draw in air and fuel if it did diesel. The anti deieseling solenoid shuts the throttle plate fully, shutting off the air / fuel supply.
So as mentioned, lean mixtures, hot temps, poor cooling, high compression, insufficient timing at idle, or a weak/leaking vacuum advance canister may be the cause of the dieseling,
Doug