My cummins engine manual says to not idle for extended periods of time due to the diesel fuel entering the oil. It goes on to say that idleing for extended periods cools the engine walls and reduces the amout of fuel burned. This is on a 1990 manual.
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Old enough to know better, Too young to resist. 1990 w250 CTD LE reg cab. Straight piped 3.5''. A-727 tranny. 249,000 miles. All STOCK! Gauges - EGT, Boost, Tranny, Tach, and Fuel Pressure. Soon to experiment with, Waste Vegetable Oil.