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What are the problems associated with running a diesel engine at temperatures cooler than those designated by the manufacturer? S.R., via e-mail
Running too cool can be as bad for a diesel as running too hot. Diesels rely on heat generated by compression to ignite fuel, so insufficient heat results in incomplete combustion. This can lead not only to poor fuel economy but also to the formation of varnish and other deposits on internal components, including pistons and piston rings. It also encourages carbon buildup on pistons, valves, and rings. If the diesel fuel is high in sulfur, sulfuric acid is more likely to form in a cool engine, hastening internal corrosion.

The most common cause of low operating temperature is a faulty thermostat. When you replace it, you may want to flush the cooling system with a product approved by your engine manufacturer to dissolve scale and other contaminants. Your engine service manual will give you the proper procedure for draining, cleaning, and refilling your cooling system.

My tender's two-stroke outboard seems to be misfiring, and my dock buddy said it could be something called four-stroking. I'm getting a mechanic to look at it, but can you shed any light on this for me? A.V., via e-mail
Four-stroking occurs when the air-fuel mixture is so rich that it cannot always ignite. It is called four-stroking because the engine often fires every other revolution--like a four-stroke engine--instead of every revolution as in a two-stroke. Have your mechanic check for excessive fuel-pump pressure, leaks at the carburetor's inlet needle and seats, as well as defects in the fuel-priming system. Another possible cause is a weak or erratic spark, something your mechanic can also check.

While mechanical failure, such as inadequate compression, can cause the engine to misfire at all speeds, it can also cause a cylinder to not fire at idle or low-speed operation. Once midrange and high speeds are reached, normal firing will be apparent. Have your mechanic perform a compression check on the combustion chamber.

Need help with a maintenance problem? Write to Maintenance Q & A, Power & Motoryacht, 260 Madison Ave., 8th Fl., New York, NY 10016. Fax: (917) 256-2282. e-mail: ken_kreisler@primediamags.com. No phone calls, please.

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