DIAGNOSING LIGHT-DUTY DIESELS

Diesel engines are real misers when it comes to sipping fuel. They’re also known for their pulling power and rugged durability. That’s why diesels continue to be a popular option in many pickup trucks today. But diesels are also known for their idle clatter, black smoke and cold-weather starting woes. So let’s start with the ill-effects of cold weather.

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 these days, may become too thick when temperatures go below freezing or plunge to zero or below. Straight 30- or 40-weight oils would definitely be too thick. 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” symptom, therefore, is the dipstick. If the oil is thick and globby, it may not be the correct viscosity for winter driving. What kind of oil is in the engine? When was it last changed? 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 switch to 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.

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