Glow plug knowhow: The key to a fast-starting diesel engine.

Glow plug knowhow: The key to a fast-starting diesel engine.

May 14, 2020 1 By Ray Bohacz

Whereas a gasoline engine uses the arcing of the spark plug to begin combustion, a diesel needs a heat source in the cylinder to start running. This occurs by the mechanical compression of the air in the bore while the engine is cranking. It is further aided by the employment of a glow plug.

A general rule (this will vary by engine design and displacement) is 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 degrees F from compression of the molecules.

Depending on the ambient temperature, that may not be hot enough for the diesel fuel to self-ignite, so a glow plug is used to aid in starting.

A typical glow plug will have a tip temperature of just under 1,900 degrees F. The liquid fuel (atomized by the nozzle/injector) can then combust, and the engine starts when it comes in contact.

A common misunderstanding is that the glow plug heats the air in the cylinder or prechamber (indirect injection engine); it does not. Its purpose is to provide a high-temperature area (the tip), so when the atomized fuel encounters 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 does not need to be fitted to the limited space in the combustion chamber as a traditional glow plug does. It is also easier to service.

For the glow plugs to function, they need to be supplied with electricity. Thus, for the engine to start correctly, the circuit must be fully functional. It may consist of a relay, wiring, buss bar, and a glow plug controller. If any of these are 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 with the resistance of each individual unit. Often an engine is only starting on as few as one glow plug, and once the weather turns colder, it will be challenging to get running.

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

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

Glow plug condition when removed:

Tip dented or creased: Relay stuck on, wrong glow plug was installed, engine jump-started wrong (in series instead of parallel).

Tip melted or broken off: Injection pump timing too early, worn, carbon laden or leaking nozzles/injectors, seized piston rings.

Heat damage/discoloring: Injection pump timing too early, over-tightened during the 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. It will show will no external physical damage.

Take caution

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 with the engine warm and after soaking the glow plug threads with good penetrating oil. Still, you need to be careful since the shear torque on an 8 mm glow plug is only approximately 15 ft-lbs. If the glow plug breaks off in the cylinder head and it cannot be removed, then the cylinder 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 anti-seize compound.

Glow plugs work great, but if they break in the cylinder head, you will long for the old ether days very quickly!