It just started doing this. When I turn the car off and pull the key out, it stays running for about 4 or 5 seconds. I checked the volts and it gradually drops from 12 to 0v instead of dropping straight down. Is this my ignition switch going bad?

 

Sounds like a classic case of dieseling. I'd suspect the fuel and or air before blaming the ignition. My former Camaro did it every now and then on a hot day. Basically, the heat from the engine would continually ignite the fuel in the piston chamber, causing it to run after being shut off. The voltage reading is the alternator still turning with the engine.

I never found a way to fix this though, since I got rid of the car. I assume it has to do with the air/fuel ratio. I was a little thirdgen noob back then.

 

The only experience I had with this was on another car. A carbed car the Idle was set way to high and it would diesel after turn off. So I just dropped the Idle down and it helped.

Might not be related to your problem though.,

 

If the timing is off, the combustion chamber will get too hot, and it will cause hot spots, which will ignite the fuel without spark.

And as said above, if the throttle is open to far on a carbed car, it will run-on.

If none of the above fixes it, if your using low octane fuel, try premium.

 

don't pull the key out

you can pull the key out of my blazer but it still stays running

 

it isn't deiseling though. It continous to idle until the volts on the hot wire to the distribrutor drops to 0v, then it dies. When I shut off my key, I watched my volts to my distributor and they do this

12v-10v-8v-7v-5v-3v-1.5v-.3v-0v

and it takes about 4 or 5 seconds to drop.

Also if I hook it to another acc. wire, then as soon as you hook it up to the other 12v acc source it drops the volts to like 1v or 5v and it won't start. If you take off of that power source then it goes back up to 12v. However if you hook it up straight to the battery then it will start and run. I have no idea what the h3ll is going on, but I can't figure it out

 

if im understanding what your saying right, what it sounds like to me is you are powering up a fan through the ignition side of the
switch, if the fan isn't powered up but it is turning then it produces power which is then fed back into the circuit it is wired into.

 

For those with dieseling problems try this: Tha factory made a 12 volt solenoid that was used to open the carb for idle. You adjust the carb (mechanical screw) so the throttle blades are closed. You adjust the idle solenoid to the appropriate rpm. Back in the 70's dieseling was a huge problem and just about every vehicle out there had one of these solenoids on it. Power it off of the ignition side (but not the ignition wire) so that when you switch the key off the throttle snaps shut.
As for the other problem, there is a rod that runs down the center of the steering column that connects to the ignition switch. Make sure that that rod is clean and can move around freely. Then check the switch contacts and adjustment (download the wiring diagram). Basically, carefully, temporarily, wire around the switch and see if that works (isolate the switch). If so, then go for the ignition switch. If not then look for something else. Seems like something is trying to back feed the ignition.

 

I wired my fan off the ignition switch to run all the time. I'll pull it and then try it. Thanks

 

I've seen guys do this and IMO there is absolutely no need to run the fan all of the time. Pardon my opinion, but I think it's crude to do that. The wiring to the fan is simple and straight forward. There is a switch on the passenger side cylinder head between 6&8 cylinders. 9 out of 10 times that wire is fried or fallen off the switch. If you run the fan all of the time it will wear the fan out sooner, but more importantly the engine won't heat up to operating temperature in a reasonable amount of time. If you want the fan to come on sooner JET performance makes a switch for this.

 

Well, sorry but I don't have a switch on my head between the 6 & 8 cylinder.

 

Look again, it should be there, or a plug where it was removed. The switch (or sender) between 1&3 is for the temperature sending unit, the one between 6&8 is the fan switch. Look for a green wire near the area of the starter, the wire comes from the fender area, it's possible that someone may have removed that switch or moved it to a different location (like on the intake) it needs to be on the head, because that is where the problems with temp usually show up first, unless Chevy decided to put it somewhere else on your car. Check the wiring diagram. That wire grounds and makes the fan come on. That's also the way to check the switch.
If the heads were ever off they might be reversed.

 

I have a '67 327 that I put in 6 years ago. It only has 19k on a rebuild and I have always had the fan run the way it is. Maybe I need to be a little more open minded and go with the fact that it is dieseling.

on the other hand, One time when it was doing it I jumped out real fast and pulled the hot wire on the distributor and it went right off. I'll run it with my fan unplugged when it starts to do it and see if that is what it is.

 

Well, if the car continues to run normally with the key off, then it's something else. But if the car is dieseling, you can't really mistake that for running normally, it will run-on, missfiring and backfiring. If it's really bad you can hole a piston, so you need to geta handle on it.

 

Well, the culprit was my fan. Thanks for everyones help. I would have never expected it be the fan, but now I know.

 

the moment I read this I knew that would be it.. that was a great repsonse as well... i would have never thought of even suggesting that...

oh.. and pulling the coil wire off of the distributer would of course stop the run on.. there is no spark to ignite the mixture...

 

Unless it really was bad case of the diesels

 

Just bought a 91 firebird with a 383 that was doing the same thing i unpluged the fan and that fixed it would have never thought that would do it