Any number of things could be causing this car to "diesel".

Dear Car Talk

Dear Car Talk | Jul 01, 2001

Dear Tom and Ray:

I have an older car (1966 Jaguar) that diesels, or keeps running, for several seconds after the engine is turned off. Then it finally dies with quite a clatter. Is there anything that would cause this besides carbon buildup in the combustion chamber? If it is carbon buildup, is there any cure other than to remove the head and scrape off the carbon? -- Thomas

RAY: It probably IS carbon buildup, Thomas. But there are also several other possibilities.

TOM: If the engine is running too hot, it can diesel. Even after you turn off the ignition, there can still be enough residual heat in the engine to keep combusting fuel, even without the benefit of a spark.

RAY: Timing that's too advanced can make the engine run too hot and too fast. That combination can also cause dieseling. So both of these possibilities should be investigated.

TOM: But when you determine that it IS carbon, you have to get rid of it somehow. One technique that's worked for years is to trickle some water into the carburetors. You don't want to use your garden hose here, but if you pour a very slight, continuous trickle of water into the throat of the carburetor while the engine is revving at 2,500 or 3,000 rpm, you might steam enough carbon off of the pistons to make a difference.

RAY: Of course, you might also hydro-lock your engine and ruin it. So be careful not to use too much water at one time.

TOM: If the water doesn't do it, you might have to try one of the products specifically designed for this purpose, like Chevron's Techron (available in stores) or 44K from BG Products (800-961-6228 or

RAY: And if you still have no luck, then you have to take the head off and scrape the carbon from the pistons with your false teeth ... or some other blunt instrument. Good luck, Thomas.

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