Hey..posted about this before and tried what was said but still having same problem..1984 c4 80,000 miles..All stock and car runs perfect except for this issue..When the car is cold this problem is worse..When i turn key off car stumbles and runs on a few seconds after key is removed..I have replaced the alternator to avoid backfeed..Replaced connector to alternator and power connector to distributor..New battery and full tune up..Checked timing 6deg..Dont know what to look at next..Any advice really appreciated. ..thanx

 

Any chance the fuel injectors are dribbling fuel into the manifold after shutdown?

 

I havent noticed that but as long as key is turned there shouldnt b any spark to ignite the fuel right?

 

That is not always true. If you have nice layer of carbon on your piston crowns or even a speck attached to the spark plug ground lead it can glow red and pre-ignite the next charge prematurely. The carbon doesn't have to be on the piston, it could be anywhere inside the combustion chamber. Old air cooled VW's "ran-on" when they had too much carbon in them. I remember watching people buy premium fuel for old air-cooled beetles because of the carbon issue.

How often do you really push the engine in this Corvette? Do you drive passively? (Sorry, I didn't know how to ask that politely) My in-laws used to get carbon build-ups in their cars from driving them too gently and using terrible quality gasoline didn't help. I used to take their cars out and drive them like you stole them, I cought all kinds of heck for it but it cleaned up the engines. It worked even Better when I would pour a can of BG 44K Fuel Systems Cleaner in a new fresh tank of good gasoline (Sorry, Aklim). Carbon deposits are a real problem on low compression cars and older low compression piston-engined aircraft. I used to remove the heads and do a mechanical de-carbonizing on my Continental 0-300, they would not allow any additives that were not approved by the FAA.

I have a major "run-on" issue with a very High Compression C3 but I know why it is doing it. Dieseling does not require ignition, I turn off the ignition on my C3 and it will diesel (only on the Hottest days) for several seconds until I let out the clutch. My problem is that I have vacuum sucking gasoline out (of the carburetor) as the engine diesels and this enables it to continue. Until I have an EFI on that car there is no way to shut off the fuel enough to starve the engine. In your car you probably have some source of gasoline being drawn into the engine while dieseling. Like mentioned before I would also check the fuel pressure and one of the throttle bodies for a leaking injector.

As mentioned earlier, it is likely that you have a bad fuel injector and carbon deposits igniting this gasoline dripping from one of the injectors into the engine. Just thoughts..... One last thing, are you using the correct temperature range plugs for the Corvette?
Good Luck and I hope you solve it quickly and easily!

 

He says it does it more COLD than it does HOT. -Not likely glowing carbon build up.

OP Did you ever check voltage at the coil when the symptom occurs to ensure that the ignition is actually turning off power to the coil?

Did you observe the injectors to ensure that they're cutting off and not dripping? Even if the injectors cut when they're supposed to (and don't leak) the CFI intake is pretty big and will supply fuel as the engine coasts down, IMO.

 

I did check voltage at distributor and it goes right to zero when ignition is cut..I have seen one of the injectors squirt a shot of fuel out immediately when key was turned off..But when i turn key on without starting the engine the injectors do not leak..Also the plugs are ac delcos..They were originaly bosch but swutched because of this problem.they are only week old same as the rest of the tune up parts..Just last sumner i rebuilt both throttle bodies also..Just for more info if i go outside in morning and start car and immediately shut it off it runs on the worst..Dont think dieseling is the issue..
thanx for your input.
I would like to know is there anything besides ignition switch that would allow voltage to distributor when turned off?