dieseling
Possible ignition switch issue
Thread Starter
Intermediate
Joined: Jun 2025
Posts: 25
Likes: 7
From: Charleston,SC
Possible ignition switch issue
I started experiencing a problem with my vette where when I have driven it for a while and the car is up to normal operating temperatures, when I go to turn off the car it doesn't want to fully turn off but will idle very rough like it's about to choke down. I have to turn the key back to on (starts running normally) and off several times to get it to fully turn off. Sometime taking the key out will solve it, sometime not. I am thinking the issue might be a bad ignition switch but wanted to see what folks think this is.
Team Owner
Joined: Jul 2006
Posts: 22,190
Likes: 4,178
From: Kannapolis NC
No, I don't think it's an ignition switch issue. I think you have the timing set too high or the idle is too high and it runs on after you turn the switch off spark plugs could also be carboned or wrong, but I don't think you have an ignition switch issue. By far, the most likely is idle speed set too high.
we don't have any idea what car, what engine, what ignition system you have or what's your timing is set on or how fast your idle speed is. It's all just assumptions.
we don't have any idea what car, what engine, what ignition system you have or what's your timing is set on or how fast your idle speed is. It's all just assumptions.
Team Owner
Joined: Jul 2006
Posts: 22,190
Likes: 4,178
From: Kannapolis NC
Idle is not too high If 600 is accurate. When the issue is occurring, I'd disconnect one of the wires on the ballast resistor and see if it dies immediately. That ballast resistor is very hot. It will burn your hand, so don't touch it. Safest way would be remove the pink wire at the coil positive where it comes down from the ballast resistor. If the switch was the problem, it would die as soon as you pulled it off, but I've never seen that happen. just loosen the nut on the positive on the coil and pull both pink wires off and see if it dies instantly.
Are you certain that the idle is only 600 when this is happening?
Are you certain that the idle is only 600 when this is happening?
Team Owner
Joined: Jul 2001
Posts: 21,396
Likes: 1,011
From: Various places in Southern California.
Definitely dieseling. A common problem in the '70s when octane levels started to go down.
Likely cause is carbon build-up. Here are some ways to address this:
1. If you put a load on the engine after turning the key, that will likely stop it. In cars with automatic transmissions it was common to recommend shutting them off while in gear rather than in park. With a manual transmission, you can just let the clutch out a bit while in gear (but be careful that the car doesn't jerk forward).
2. Replacing the spark plugs can help if they are carboned up.
3. Switch to higher octane fuel, if possible.
4. I've successfully used the "Bubba water injection" technique to clean things up. Get the car running at around 2000 RPM and drizzle a small amount of water down the carb. Not much--you don't want to hydrolock the engine, but a small drizzle will often clean things up. If you've ever looked at an engine with a head gasket leak that allowed some coolant into a cylinder, it is easy to tell which had the leak because that piston is much cleaner than the others. Same principle here.
Likely cause is carbon build-up. Here are some ways to address this:
1. If you put a load on the engine after turning the key, that will likely stop it. In cars with automatic transmissions it was common to recommend shutting them off while in gear rather than in park. With a manual transmission, you can just let the clutch out a bit while in gear (but be careful that the car doesn't jerk forward).
2. Replacing the spark plugs can help if they are carboned up.
3. Switch to higher octane fuel, if possible.
4. I've successfully used the "Bubba water injection" technique to clean things up. Get the car running at around 2000 RPM and drizzle a small amount of water down the carb. Not much--you don't want to hydrolock the engine, but a small drizzle will often clean things up. If you've ever looked at an engine with a head gasket leak that allowed some coolant into a cylinder, it is easy to tell which had the leak because that piston is much cleaner than the others. Same principle here.
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Intermediate
Joined: Nov 2005
Posts: 25
Likes: 10
If it is dieseling as others have suggested, let clutch the out just a little bit so you feel some drag on the engine as you turn the ignition switch off. I have a 427 Cobra and a 427 Corvette and I do that on both of them to prevent "run on" after turning the key off.
Last edited by cj428cj; Sep 30, 2025 at 04:55 PM.
