How to Replace Your Diesel Glow Plugs

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Diesel engines do not have spark plugs or ignition system of any kind, so it is up to the glow plugs to get it going when the engine is cold or it's cold outside. As a result, Diesel glow plugs live a hard life and thus must occasionally be replaced.

Diesel glow plugs are subject to extreme temperature changes and high combustion pressures. Since a diesel engine may have as many as 10 glow plugs, one for each cylinder, you may not notice when one goes bad, but if three or more go bad, you will notice the engine has become very difficult to start.

Some vehicles have PCM's that monitor glow plug operation and report the full functionality of each plug separately; however, most just use a Glow Plug Relay so you may not know you have any bad glow plugs.

In any case, if you need to replace your diesel glow plugs, you'll need a few tools including a ratchet wrench set with deep sockets and universal joint, screwdrivers, six-point combination wrenches (1/4", 5/16" 3/8" 7/16 and 1/2"), a J 39083 Glow Plug Connector Remover and Installer for GM vehicles, a glow plug chamber reaming tool, valve cover gaskets, and penetrating lubricant.

How to Replace Diesel Glow Plugs

Before you start, gather all of your tools and supplies and make sure to read all the instructions carefully so that you fully understand them, making sure to allow plenty of time to finish the job so you won't rush and miss any steps. Also remember that these are general instructions that apply to most diesel engines, for more detailed instructions pertaining to your specific vehicle, consult an appropriate repair manual.

Safety is important whenever you're working around machinery; beware of hot objects, sharp instruments, and hazardous materials. Don't substitute tools unless you're sure you won't compromise either your safety or the performance of your vehicle. Also, since there may be fuel and fuel vapors present, do not smoke or allow open flames or sparks in the work area; it would be a really good idea to have a fire extinguisher rated for gasoline fires handy as well.

Now that you've properly reviewed safety instructions and consulted your vehicle's owner's manual to determined the locations of your diesel glow plugs, follow these steps to replace them:

  1. Remove the valve cover (Ford or if required).
  2. Remove what's needed to gain access to the glow plugs.
  3. Disconnect the electrical connector and remove the intake manifold glow plug from the cylinder head.
  4. Using a deep socket or combination wrench, remove the glow plug from the cylinder head.
  5. Screw the glow plug reamer into the glow plug opening all the way in then out.
  6. Install the new glow plug.
  7. Reconnect the connector to the glow plug terminal.
  8. Replace the valve cover with a new gasket (if required).
  9. Reinstall anything removed for glow plug access.

That's it! It is as simple as replacing a spark plug. On some engines it will take about an hour, on others it may take up to five hours, depending on what is in the way, or in the case of some Ford diesel, valve cover removal. A good project for a Saturday and you won't have to worry about your diesel not starting when it starts to get cold again.

What Does a Diesel Glow Plug Do?

On a diesel engine, combustion is affected by self-ignition of the fuel sprayed into the highly compressed and thereby highly heated combustion air, but in a cold engine, the self-ignition temperature is not attained by compression alone so a pre-glow system is therefore required.

The pre-glow system serves the purpose of increasing the temperature of the compressed air to facilitate the firing of the cold engine by the use of a glow plug; the duration of pre-glowing depends on the temperature of the engine and ambient temperature.

Pencil element glow plugs consist essentially of housing with screw-in threads and a pencil element pressed into the housing. The single-pole connecting pin is glued to the housing by means of a non-releasable round aluminum nut; the pencil element glow plugs are designed for a current of 12 volts and are operated in parallel.

On some older diesel engines, the glow plugs operate on a current of 6 volts and a dropping resistor is used to reduce the voltage to 6 volts. After a glowing period of 9 seconds, a "Quick-Start" pencil element temperature of approximately 1,652°F is attained, after 30 seconds the maximum temperature amounts to 1,976°F.

The pencil element is heated indirectly by means of a heater element. This heater element, a coil made of resistance wire, is embedded and insulated in a ceramic compound. When the glow system is switched on, each glow plug is subject to a current of approximately 20 amps, peak impulse of approximately 40 amps. Under the influence of increasing heat, the inherent resistance of the glow plug increases and will limit the current to approximately eight amps.

After a glowing period of approximately 20 seconds, a heater pencil element temperature of 1,652°F will be attained, after approximately 50 seconds the maximum temperature will be 1,976°F.

Testing Glow Plugs

Testing glow plugs is easy and can be done with them still installed in the engine — just disconnect the wire going to each glow plug.

Connect a test light to the POSITIVE (+) battery terminal and touch the point of the test light to each glow plug terminal. If the light lights, it's good. If it doesn't, it's bad and needs to be replaced. Do you replace just the bad one or all of them? My opinion is that if one went bad, then the rest are not too far behind. So I recommend replacing all of them at the same time. I would replace, at the very least, all of the glow plugs on the same side.

Some diesel engines, Mercedes-Benz diesel, for example, have a Pre-combustion Chamber that houses the glow plugs. This Pre-combustion Chamber helps slow down the combustion process and aids in cold starting. They do have a tendency to get carboned up and thus rendering the glow plugs ineffective. So when the glow plugs on engines equipped with a Pre-combustion Chamber are replaced, the Pre-combustion Chamber should be reamed out to remove any carbon build up.