82 chev 3/4 ton 350 sb - engine overrun

[caccia]

I took my 82 Chev for a tune up because the engine overruns when I turn off the ignition. The mechanic told me there he didn't know why. Can anyone provide some light on this?

[txab]

It can be caused by excessive carbon build-up causing ignition of fuel to continue. It's called dieseling by some

[Jsdirt]

Yep - the Monte did that often. Usually towards the end of a long dieseling run. Just when you thought it was ready to stall, it would give out a big CLACK CLACK CLACK and run about 1,000 RPM backwards.

[Duelin]

All of the above is good info. What you can do is when you shut it off is keep it in drive with your food on the brake. Then when it stops you can put it in park and take the key out. That will help control some of the dieseling until you figure out what the problem is.

[Doug_Scott]

Yep - the Monte did that often. Usually towards the end of a long dieseling run. Just when you thought it was ready to stall, it would give out a big CLACK CLACK CLACK and run about 1,000 RPM backwards.

 

And give out the worst smelling exhaust you ever want to smell.

[Jsdirt]

Yep ... and with a large cloud of oil smoke too.

 

Starting to miss my Monte. It was entertaining!

[Daly]

I had an 79 GMC do that, it ended up being a timing issue… I replaced the timing chain and rebuilt the distributer, along with a tune up… Ran like a champ afterwards…

But the carb can also cause problems, a buddy ended up replacing his carb and it fixed the problem…

All I can say is start with the cheap stuff and work your way through the engine…

[Jsdirt]

Yep, as mentioned - carbon or timing.

 

I had an '85 Monte Carlo back in the day that I could turn the ignition off, and continue to drive it around, dieseling ...

 

I used to love the looks people would give the car as I pulled into a store parking lot, shut the door, and walked away as it continued dieseling for 5 straight minutes ...

 

Used to go through oil like hell. I'd smoke out an entire intersection at every light or stop sign. Thank God oil was .77 cents a quart in those days.

 

That car had hit everything but he lottery too. Was one hell of a prize! Believe it or not, I daily drove this thing for 3 years, and took several 300+ mile roadtrips in it with no second gear.

 

[Doug_Scott]

Something that I just recalled. Sometimes the engine will actually run backwards when dieseling. There is one big problem with that, if it does it for any length of time, the oil pump is also running backwards, so no oil pressure will be available during that time. If the timing chain is worn, it may jump a tooth or two when run backwards on shut down.

[Doug_Scott]

Reason for calling it "dieseling" is because that is exactly what the engine is doing.

 

As others have stated, carbon in combustion chamber, incorrect ignition timing, plus some other causes. Verify idle speed and mixture is set properly. A common cause is someone screws with the idle speed/mixture to correct some other issue instead of just fixing the actual problem. Check to see if your engine has an idle solenoid. What used to happen was that solenoid would pack it in, so the owner would just raise the idle speed with the screw.

An engine always needs the same three things to run. With a carb, the engine always has one of the three things, regardless of key position. There is likely not just one cause in your case. Things to check:

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Check how much play in timing chain.

Check idle speed and mixture

Check condition of plugs, if any plug is loaded with carbon deposits, then you likely have an oil burning issue was well.

Check the manual advance in the distributor, make sure it is not sticking or seized.

Check compression on engine. Look for one cylinder being high.

 

After that you will need to de-carbon the cylinders. Various ways to accomplish this, easiest way is take the truck to a quiet private area, and run it though the gears at wide open throttle. That will generally remove most big chunks of carbon. A word of warning though, you may get a piece of carbon caught between piston and head, and it will knock like you broke something. Don't panic, you can run a very slow stream of water through carb air horn to break that piece up. Slow stream is done via a vacuum line, not a garden hose. All you are after is for the water to turn to steam as it hits the cylinder, do not stall the engine with the water at any time. Some times ATF can be used to help remove carbon. Rislone works as well.

 

Also pay attention to the secondaries on the carb. Sometimes they will hang open slightly, not enough to really alter idle, but far enough to help the engine diesel.

 

I am pretty sure you just need to run the truck through the gears at wide open throttle a couple of times. Don't go nuts and try this on city streets with other traffic around, you will be leaving a large cloud of crap that will point right at your truck, so the cops know who it was. If you live near a highway, use the on ramp a couple of times. Pulling to redline in first and second will not get you way over speed limit on highway. You do not need to drive for miles like this, just through the gears a couple of times.

 

Judging by your mechanics comment, he must have zero experience with carburetors.

[caccia]

Thanks for everyone's suggestions. I think we have narrowed it down to timing issue.