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The Key to a Fast-Starting Diesel

The heat source that starts a diesel may be compression of the fuel-air mixture in the bore. As a general rule (this will vary by engine design and displacement), the cylinder pressure during crank needs to reach a minimum of 450 psi. This will then heat the air in the bore to around 1,300°F. from compression of the molecules. Depending on the ambient temperature, that will not be hot enough for the diesel fuel to self-ignite.

Enter the glow plug, which typically (there are many designs in use) will have a tip temperature of just under 1,900°F. 

A common misunderstanding is that the glow plug heats the air in the cylinder or prechamber (in a direct-injection engine). Instead, its purpose is to provide a high temperature area (the tip) so when the atomized fuel comes in contact with it, ignition will occur. 

In contrast, many newer diesel engines employ an intake air heater (IAH) that raises the incoming air temperature that, when aided by the heating from compression, allows for combustion. In some applications, a combination of glow plugs and an IAH is used. The benefit of an IAH is that it doesn’t need to be fitted to the limited space in the combustion chamber like a traditional glow plug does, and it’s usually easier to service.

For the glow plugs to function, they need a supply of electricity. That supply may consist of a relay, wiring, buss bar, or a glow plug controller. If any of these is not working, the engine will be hard to start when cold or may not even run.

When it comes to the electrical portion of the system, the voltage supply to each glow plug needs to be confirmed along with the resistance of each individual unit. Often, an engine is only starting on as little as one glow plug. Once the weather turns colder, it will be extremely difficult to get running. 

Some newer glow plugs are designed to keep heating once the engine runs until reaching a specified engine coolant temperature or running time. This design limits cold-start smoke and combustion noise, and allows immediate application of load. 

It is important to understand that if the glow plug is good electrically but the engine is still hard starting or has an excessive amount of white smoke when cold, the glow plug may be electrically intact but physically damaged. For this reason, it is important to examine each glow plug when removed for replacement. The following is a condensed diagnostic guide.   

  • Tip dented or creased: The relay is stuck on, the wrong glow plug was installed, or the engine was jump-started wrong (in series instead of parallel).
  • Tip melted or broken off: This could be caused by the injection pump timing being set too early; worn, carbon-laden, or leaking nozzles/injectors; or seized piston rings.
  • Heat damage/discoloring: This may be caused by the injection pump timing being set too early or overtightened during previous installation.

If the engine is running correctly but the glow plug has failed, it will have the heating element burned out from thermal cycles and use, but there will be no physical external damage.

An obstacle with glow plugs is gaining access to removing them and their propensity to break off in the cylinder head when being changed. It is best to try to remove them when the engine is warm and after soaking the glow plug threads with good penetrating oil. Still, be careful, since the shear torque on an 8-millimeter glow plug is only about 15 foot-pounds. If the glow plug breaks off in the head and it can’t be removed, then the head needs to come off the block. 

Before installing new glow plugs, always use the proper dedicated thread chasing tap and coat the threads with antiseize compound.

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