Engine diesels after shut off. Why

My 1991 geo tracker has 115000 miles. New timing belt. Fresh tune up at shop. The engine will diesel when shut off but not every time. Engine temp is normal.
Any suggestions.
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On Wed, 12 Feb 2014 20:18:00 +0000, Donnie

Couple of possibilities. Carbon deposits in the cylinder that are very hot. Wrong heat range spark plugs (were they replaced for the tuneup?) Improperly set timing.
If it just started after the tuneup, I'd take it back and let them know.
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