Engine doesn't want to stop

I have a 1986 caprice brougham with the 305 and it recently started its old tricks again. When I go to shut the engine off the car doesnt completely shut off, it sits there chugging, barely running and really rough (almost like backfiring). I dont really know how to explain it, but if more explaination is needed I could try. The old owner said that it used to do that, she would have to turn the key on again then off and it would usually shut off. The car sat for probably 2 years. then I bought it about this time last year. The car was fine till now, when it started it again today. I tried her "trick" 3-4 times and it did nothing so a friend put it in gear and turned it off, but it worked. I was wondering what caused the problem? Thanks for any help, this problem is getting kind of annoying.

It's called "dieseling" or run-on. It happens for a number of reasons, some of the more common are vacuum leaks, hot spots in the combustion chamber due to carbon deposits, or high idle speed.
Sometimes, over-advanced ignition timing can lead to detonation (that rattling, pinging sound you hear under acceleration sometimes), which leads to hot spots in the combustion chambers.

Make sure all of your vacuum hoses are in good condition with no cracks or breaks. Make sure the engine idles in the 800-900 RPM range when the transmission is in park or neutral. Make sure the timing is set to factory specifications as listed on the emissions label affixed to your radiator fan shroud. Generally, this will be 8* BTC with the 4-wire computer interface disconnected from the distributor and the engine idling and at operating temperature (you WILL see a Service Engine Soon light during this, ignore). If the engine continues to diesel after doing this, there are safe and not as safe methods to decarbonize the combustion chambers. The safer way is to use GM's upper cylinder cleaner (follow directions on the can), and the not-so-safe way is to slowly pour water (1 cup should be all you need) into the carburetor air intake (the front, primary side - where the choke is). You want to do this again at operating temperature and with the engine maintaining at least 2,000 RPM as you slowly pour water into the carb. Failure to follow this exactly can lead to a hydrolock which can destroy the engine (spun bearings, bent connecting rods, blown head gaskets), which is why it's the not-so-safe method. After doing this, leave the engine running, put the air cleaner back on and drive to the nearest freeway. Clock that thing wide open up to freeway speed, then take your foot off the pedal and let the speed drop to about 40 or so (you obviously don't want to do this on a crowded freeway), then hammer it back to the speed limit - repeat this cycle about 5-10 times to dislodge all carbon not taken out initiallly by the water.

This should fix it - if not, then you might need to rebuild the carb itself.

I know the car has had some problems with getting stuck at a high idle lately, could that be a problem? also the old owner did say the carb should be rebuilt or replaced altogether

I know the car has had some problems with getting stuck at a high idle lately, could that be a problem? also the old owner did say the carb should be rebuilt or replaced altogether

Ding, ding! There you go. If the engine idles at, say, 1100 RPM or more, that's definitely enough to make it diesel. If it "sticks" at a high idle sometimes, then you probably have a worn throttle shaft bearing.

the car gave me some probs this morning too. It wouldn't start for awhile so I figured the carb was stuck, sure enough it was stuck shut, so I opened it and I noticed something under the motor to the side and it was on fire. so I quickly put that out. It kinda got me worried tho cuz thats how my dads went, engine fire. Newayz, after all that it started right up and I got home, but I guess that means i'm gonna need a carb or some carb work before I thought I would. Thanks for the help silicon

Nooo, the fire on the side of the motor had nothing to do with the carb. You probably had a short of some kind and it ignighted some oil on the block, which would explain the not starting issue.
The carb is supposed ta be closed when the engine is cold. Thats the choke. It pulls off once the car starts. I'm not saying you don't need a rebuild, but I'm saying the fire is a separate problem on its own.

I can vouch for the importance of vacuum. Of my 3 caprices, 2 had problems with engine dieseling.

The 1988 with the 305 was the result of incorrect idle. It was simply idling too fast, among a list of other problems. If you try to shut a car off at 1500 RPM it will diesel!

My carbureted 1990 caprice Olds 307 was dieseling, because the vacuum feed to the anti-dieseling solenoid had a tiny leak in it. The anti-dieseling solenoid energizes any time the ignition is on, and once you turn the ignition off, this solenoid is supposed to shut off, and vacuum to the idle load controller (front RHS of carb), which comes from a seperate vacuum chamber (similar to a cruise control vacuum chamber) is suppose to be cut off, so the throttle kicker drops back.

Not sure if this anti-dieseling solenoid is unique to the Olds 307, or if its something that began in the 1990s, but my earlier 305/350 didn't have it.

Because of a tiny vacuum leak, vacuum to the idle load controller wasn't coming off fast enough causing the slow release of the plunger, which kept the throttle open for a few seconds after the car was shut off.

Fixing a vacuum leak is so cheap and easy, you're best to start there.

thanks for the info, it did have loose vaccum lines before, but I put clamps on them just to be sure. I know theres a problem with some link on the carb, a guy came over one day and pushed it down, he said it kept the high idle cam up, so idk

Sounds like your fast idle cam was sticky. Free it up with some carb cleaner, check your return springs and it should be ok. The engine run-on can be damaging, if it happens again, just shut it off in gear.

Bob