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'86 Chevy Won't Turn Off

My 1986 chevy Scottsdale pickup continues to run after I turn off the ignition…it rattles and sounds like a diesel, then finally shuts off in a puff of smoke. At the advice of a mechanic I added fuel injector cleaner to the gas tank, but to no avail. I have also had the timing checked and idle speed adjusted. Someone suggested it might be a carburetor issue, another thought vacuum seal…

What’s happening is that you have enough heat in the cylinder to cause some combustion and since the engine is a pump it’s continuing to draw in fuel until the pressure drops in the float bowl, which if the fuel pump were running it would be keeping pressurized. That was a common problem in the '70s when engine operating temperatures were raised to attain more complete combustion for reduced emissions and better fuel efficiency. A solution was the introduction of an “idle stop solenoid”. That was a solenoid in the accelerator linkage that allowed the throttle plate to completely close when the power was turned off, choking the engine to a stop.

You have here some possibilities.

You may have an idle stop solenoid that’s not functioning. A repair manual should tell you. Someone may have manually set the idle screw without the solenoid engaged, either because it stopped working, or out of ignorance in essence making it ineffective.

It’s also possible that you have carbon buildup in the cylinders. Carbon retains heat, and hot spots could be igniting fuel.