BMW Diesel Overview

October 7, 2018 | By | Reply More

Since the early 1900s, the diesel engine has been widely used in many industries to power vehicles and equipment. They have remained popular because of their overall efficiency and durability, although in the United States this may not be as apparent as in other countries. Worldwide, diesel vehicles account for 50 percent of vehicles on the road. In the United States, fewer than 10 percent are diesel!

BMW has been in the diesel engine club for quite a while. In the mid 1980s, they released their first diesel engine, the M21. This was based on the venerable M20 engine but with strengthened components to handle the increased forces from a diesel engine. It was available in both 3 Series models as well as 5 Series. It’s rare to see a BMW diesel from the 1980s in the United States. Any models you may come across are most likely gray market imports.

Thermal Efficiency

By operating at a much higher compression ratio than gasoline engines, diesel engines are able to compress the fuel/air mixture until ignition occurs. As the pressure inside each cylinder rises, the temperature also rises until spontaneous combustion occurs. The compression process lets the engine squeeze a little more energy out of the fuel. Diesel engines are known to have the highest thermal efficiency of any internal combustion engine design. This means diesel engines create the least waste energy [heat] of any internal engine design. Less waste equals more power and efficiency.

Glow Plugs

Diesel engines use glow plugs to aid in starting and smooth running. By increasing the temperature in each cylinder with a glow plug before cold start-up, proper combustion can be completed quicker, particularly when the engine is cold or in low ambient temperatures. In most modern diesel engines, the glow plug system is inactive until temperatures are below a set point. Most commonly, this is around 40 degrees F. The colder the ambient temperature, the longer that glow light will stay lit! On many diesel engines with bad glow plugs or glow plug modules, the vehicle may not want to start at all if the glow plug system is not operating correctly. 

BMW diesel engines have a primary driver for the glow plug system, known as the glow plug preheating control unit. These are failing more frequently as many engines reach higher mileage. Most commonly, the control unit sets glow plug circuit faults for multiple cylinders. Rough running is often noticed during cold starts if there are issues with the glow plug system. During diagnosis, don’t forget the basics!

Always start by ohming out each glow plug if bad plugs or the control unit are suspected. Correct resistance values may vary, but you should never find infinite resistance on a glow plug. Although glow plugs are easy to test with a multi-meter, the time-consuming part of testing is the necessity to first remove the intake manifold. This is the case with BMW’s M57 and N57 diesel engines. The glow plug control unit is located


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Category: Fuel

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