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In many ways, the combustion engine using diesel and those using gasoline are the same. They have the same general layout in design and operation, but with some important differences that belie their similarity of appearance.

The primary difference between the two is in the way that fuel is delivered and burned, which changes the dynamic of the engine's overall maintenance strategy and need. Unlike gasoline, which is sent into the cylinder and then ignited with a spark, diesel fuel is ignited with pressure instead of spark.

In the end, this makes diesel engines more robust in terms of longevity because their high-pressure workings mean a lot of heavy-duty parts are used to build them.

For the owner of a diesel car, the primary maintenance points for the engine are:

Engine oil changes
Air filter changes
Fuel filter changes
Water separator changes

Other points should also be taken into account, but they are roughly the same as the gasoline equivalent and, even better, many of the gasoline needs don't exist on diesels at all.

Engine Oil Changes

While engine oil on a conventional gasoline vehicle needs to be changed often, waiting too long or doing it infrequently often only results in losing some engine longevity and perhaps a need for a flush or full fluid change. In a diesel engine, however, losing track of oil changes and not doing it often enough can mean the entire engine seizing and becoming useless.

Engine oil in diesel engines usually requires much less frequent changing than do gasoline engines. Conversely, though, not changing it when it's needed can mean oil becomes so thick and useless that it plugs up the engine and causes failure.

Air Filter Changes

Air filters on diesel engines are put through pretty rigorous work in comparison to their gasoline counterparts. The fuel:air mixture in the diesel is extremely precise and the purity of the air can severely effect how well the engine is running. Whereas gasoline engines often lose some efficiency, but still burn away most of the particulates that might enter into them, diesel engines can misfire or clog recirculating filters (for cleaning exhaust) if the air coming in is too dirty. Consequently, air filters need to be well maintained.

Fuel Filter Changes

Of all of the systems on a diesel engine, fuel is the most problematic in terms of maintenance and troubleshooting issues. Nearly every problem a diesel engine might have is probably due to something in the fuel system.

Fuel delivery is under pressure, so if pressure is lost for any reason, the engine does not receive fuel. Diesel fuel also must be very clean to run well in an engine and so filters are employed down to micron levels to clean impurities from the fuel and make it run well. These filters require regular changes and unlike the gasoline filter, diesel filters that don't get changed can lead to failure in the engine, not just loss of efficiency.

Water Separator Changes

To go along with the fuel filter change is the water separator. On many diesels, this is a separate filter and needs changing more frequently in some cases than does the fuel filter. This is because of all elements that could be in the fuel, water is the most detrimental to diesel operations. Just a cup of water in the diesel fuel can bring a huge engine crashing to its knees. Let alone your small car.

Water filters are usually located in convenient places on diesel cars and often have in-line shutoff valves to make it easy to change them without bleeding the fuel lines. Everyone who owns a diesel car should know how to change their water filter and have new ones on hand for doing so if needed.

Anytime a diesel engine begins coughing or running roughly, 90% of the time it's probably the water separator.