Edmunds Answers



  • karjunkie 12/12/08 8:11 am PST

    When temperatures drop, several things happen that can make a diesel hard to start. First, the oil in the crankcase thickens. At the same time, battery output drops, reducing the number of amps available to crank the engine. The 15W-40 multi-viscosity motor oil, a popular warm weather choice with many diesel owners, may become too thick when temperatures go below freezing or plunge to zero or below. The increased drag created by the cold oil can reduce cranking speed to the point where the engine may not generate enough cranking compression and/or fuel pressure to light the fire. One of the first things you should check when diagnosing a "hard to start" complaint, therefore, is the dipstick. If the oil is thick and globby when cold, it may not be the correct viscosity for winter driving. Switching to a lighter oil such as a 10W-30 (never anything lighter in a conventional oil!) may be all that's needed to improve cold cranking. For really cold weather, you might be better off using a CG-4 rated synthetic motor oil. The next thing that needs to be checked is minimum cranking speed. The rpm needed to light the fire will vary according to the application, but General Motors says its 6.2L and 6.5L diesels with Stanadyne rotary injection pumps need at least 100 rpm when cold, and 180 rpm when hot. If the engine isn't cranking fast enough, check battery charge and condition, as well as the cable connections and the starter's amp draw. Problems in any of these areas can make any engine hard to start. If the battery is low, recharge it and check the output of the charging system, too. Unlike gasoline, diesel oil is adversely affected by cold temperatures. Diesel is made of heavier hydrocarbons that turn to wax when temperatures drop. The "cloud point" or point at which wax starts to form for ordinary summer-grade No. 2 diesel fuel can range from 10 to 40 degrees. If the fuel tank contains summer grade fuel and temperatures drop, wax crystals can form in the water/fuel separator, causing a blockage. Water in the fuel is another problem that can cause starting and performance problems. Condensation that forms during cold weather is the primary source of contamination. Water that gets into the fuel tank usually settles to the bottom because water and oil don't mix. The water is sucked into the fuel line and goes to the filter or water/fuel separator (if the vehicle has one). Here it can freeze, causing a blockage that stops the flow of fuel to the engine. So if the filter or separator is iced up, the fuel tank needs to be drained to get rid of the water.


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