The diesel combustion system works on different principals than a gasoline engine's combustion system. In a gasoline engine, the air/fuel charge is moderately compressed by the stroke of the piston. The spark plug fires, burning the fuel in the cylinder. As it burns, the air/fuel charge expands, driving the piston downward.
The diesel is compression fired; that is, no external source of heat or spark is used to ignite the fuel charge. As any substance is compressed, it develops heat. If a flammable substance-fuel-is compressed sufficiently, it will burn or explode. The diesel engine uses compression within the cylinders to ignite the fuel charge, which again drives the piston downward as it expands. For reference, normal compression within a gasoline engine is about 8.5:1 with fully developed racing engines reaching around 13.0:1. The diesel engine uses a compression ratio of about 21.0:1. The act of compressing the air/fuel mixture to this level generates tremendous heat (about 1700°F) and it is this heat which ignites the fuel.
The fuel injection pump is the heart of the system, drawing fuel through the filter and delivering it to the injectors at the proper time in the combustion cycle. This mechanically driven pump must be kept in perfect synchronization with the engine.
The first signs of diesel trouble usually show up at the injection nozzles. An injector may fail or it may become blocked or stuck from dirt in the fuel. Some signs of injector trouble are:
A faulty injection nozzle can be located by loosening the fuel line joint at each injector while the engine idles.
If an injector is working properly, changing its fuel supply (by opening the line) will cause a change in the idle quality of the engine. The injector that does NOT cause the idle to change is the problem.