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Car dieseling after shut-off

My 76 Lincoln Mark IV has a tendency to diesel after turning it off (probably does it half of the time). What could be causing this/how can I fix this? It has done this before and after converting to an electric fuel pump.


This was a very common problem in the '70s, not specific to any manufacturer.

Two of the first things manufacturers did to reduce emissions in compliance with the Clean Air Act of 1970 was to (1) raise engine operating temperatures a bit and (2) “lean out” the fuel mix. These two acts created cylinder temperatures that operated on the verge of self-ignition (“dieseling”). Add to that the fact that carburetors at the time left the throttle plate in the “idle” position when the engine was turned off and you had the perfect conditions for an engine to diesel. I clearly recall often walking through parking lots hearing one or two cars dieseling after they were turned off. As long as there was enough residual cylinder heat and the throttle plate allowed idle air to be drawn in, the engine would diesel until the engine drew the float bowl down some… as long as the float bowl had gas, the engine had air, vaporized gas, and heat, everything it needed to run.

In order to counteract this problem, manufacturers added “idle stop solenoids” to carburetors. These when energized (key ON) kept the throttle plate open to the idle position, but when the key was turned OFF and they were deenergized they closed the throttle plate completely, shutting off all air and choking the engine.

Another thing that can retain heat in old engines is carbon buildup. Carbon retains heat, great for charcoal briquettes, but not great for engines. Carbon buildup can retain enough heat to keep igniting fuel. If there’s a path for air (refer back to the previous paragraph), vaporized fuel being pulled in, and heat, an engine can keep dieseling.

So, in summary,

  • check the idle stop solenoid for proper operation,
  • check that the throttle plate idle set screw is set properly such that the throttle idles properly but totally closes when the ignition is OFF,
  • check that the throttle plate isn’t sticking in the idle position allowing air to continues to be drawn in,
  • and if this doesn’t fix it check for carbon buildup in the cylinders.

Post your results.
Your post has brought my memory back to the '70s. Thank you.


My '83 Chevy V6 started doing this at 135,000 miles. I could go in the house and come back out and it would still be dieseling. The only way to stop it was put it in drive to slow the RPM and then turn off the ignition. This condition is caused by a build-up of carbon (burned oil) on the valves, head, spark plugs, and piston top. The carbon heats up and acts like a glow plug. Normally when you turn the ignition off the engine will “coast”. Then every time the piston moves down it sucks in the idling mixture and on the compression stroke the hot carbon causes combustion. You can help stop this condition by running only the high octaine gasoline. I cured my engine by putting in a can of “Spitfire”. That helped clean out some of the carbon.

The mid 1970s idle stop solenoid was one of the required but somewhat stop-gap emissions controls. They frequently failed but were not a big problem for the manufacturers with the 12 month/12,000 mile warranty.

Fouled plugs is one common source, Tam and Ray advocated spraying water down the carb to blow out carbon build up, sea foam or high speed highway miles can help

A vacuum leak may be causing the dieseling. The gaskets under the carburetor are notorious for leaking on that car. If you leave the transmission in gear and trun off the engine you can likely eliminate the problem until you get it diagnosed and corrected.

Back in the day we would take and pinch a paper cup and use it to pour a thin stream ow water with the car running 2-3 thousand rpm. Keep doing that until you nave used a pint to a quart of water.

it seems like it would be impossible for an engine to diesel for any length of time if the electric fuel pump was completely shut off. Or do you have an electric and a mechanical fuel pump in your car?

If you sport a mechanic fuel pump, running a can of that carb-cleaner stuff through the system might also help to clean out carbon deposits inside the engine which cause it to diesel. I’m not sure you can buy that stuff anymore tho. I used to use it on my carb’ed cars years ago as part of a tune-up. the reason I think it may not be sold anymore is it produced a huge amount of smoke out the tailpipe. I’d run most of the can through by pouring it directly into the carb while the engine was idling, then before I had stopped pouring I’d ask my assistant to turn the engine off. Then I’d let the car sit overnight, then finish up the next morning, at which point, whew, a really lot of smoke would pour out the tailpipe. This process did seem to help make my carb’ed cars run better. Whether it would help w/dieseling, don’t know.

87 octane will cause that. Try going higher.

If the idle speed is too high any carbureted engine will run on. Check your idle speed.

You can still get carb cleaner. Gumout makes it.

I’ve found Seafoam works really well though. My old '88 carbed pickup was dieseling really badly when I bought it because in 200,000 miles no one had ever done anything about carbon buildup. I put 1/3 of a can of Seafoam in the gas tank, and poured another 1/3 down the carb throat while the car was running.

The neighbor thought I’d started a fire with all the smoke coming out the tailpipe, but it worked pretty well. It only very rarely diesels anymore, and then usually only for a second or two. One of these days I’ll get around to doing a second Seafoam treatment and I expect that will stop the dieseling altogether.

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Even with an electric fuel pump carbureted cars can diesel after being turned off if there enough heat in the cylinders and a path to draw air in past the throttle plate until the float bowl runs dry. The vacuum pulls fuel from the float bowl.

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