Understanding Diesel Glow Plugs and How to Replace Them

diesel fuel pump
Your diesel engine needs glow plugs to get going in cold weather. Getty

Diesel glow plugs live a hard life. They 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 two, three or more go bad; you will notice the engine has become very difficult to start.

Some vehicles have PCMs that monitor glow plug operation; most just use a Glow Plug Relay so you may not know you have any bad glow plugs. It's always a good idea to do some basic testing like this before you jump to a major component replacement. There's never a point in your life that you want to waste time and money. 

How Glow Plugs Work

diesel engine
This cutaway engine shows the details of glow plugs and other engine components. Getty

On a diesel engine, combustion is affected by self-ignition of the fuel sprayed into the highly compressed and thereby highly heated combustion air.  There is no traditional ignition system on a diesel engine. In a cold engine, the self-ignition temperature is not attained by compression alone. 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 temperature of the engine and on ambient temperature.

Pencil element glow plugs consist essentially of a 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 diesels, the glow plugs operate on a current of 6 volts. A dropping resistor is used to reduce the voltage to 6 volts. After a glow 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.

Quick-Start pencil element glow plugs are usually used in passenger vehicles while trucks use the slower pencil element glow plugs.

The pencil element is heated indirectly by means of a heater element. This heater element, a coil made of a 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 8 amps.

After a glow 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.

Glow Plug Types and Testing

diesel engine diagram
This cross section shows a diesel engine's internal components and glow plugs. Getty

Except for the heater element, the design of a quick-start pencil element glow plug is the same as that of a pencil element glow plug. The heater element consists of a heater and control coil connected in series.

When the glow system is switched on, each glow plug will be subject to a current of approximately 30 amps. The glow plug is heated very quickly by heater coil. With increasing temperature, the control coil increases its resistance and limits the current to approximately 815 amps. This will protect the glow plug against overloads.

While there is no scheduled replacement interval for glow plugs, they are often forgotten until they go bad. That is why I, personally, recommend replacing them every 60,000 miles. If the winters get as cold as they do in here in Minnesota, you will like knowing that your glow plugs are not going to crap out when it's 40 degrees below zero.


Some Chrysler vehicles equipped with an optional diesel engine do not use glow plugs; they use an Intake Manifold Air Heater Grid to heat the air going into the cylinders. In the instrument cluster there is a Wait-To-Start lamp. The Wait-To-Start lamp gives an indication that the conditions for easiest starting of the diesel engine have not yet been achieved. The Powertrain Control Module (PCM) lights the Wait-to-Start lamp in the instrument cluster after the ignition switch is turned to the ON position.

One side of the Wait-To-Start lamp bulb receives battery voltage when the ignition switch is turned to the ON position. The PCM switches the ground path for the other side of the bulb based upon several inputs and its internal programming.

The Wait-To-Start lamp lets the driver know that the intake manifold air heater grid has had sufficient time to warm the intake air for a good quality start. The intake manifold air preheat cycle is controlled by an Electronic Air Heater Control Module. The lamp will be turned off by the PCM when the heater control module cycle is completed, or if the driver turns the ignition switch to the START position prior to the end of the heater control module cycle.

Glow Plug Testing

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 diesels 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.

Glow Plug Replacement Procedure

Gather together all of your tools and supplies before you begin. Allow plenty of time to do the job so you don't have to hurry.

  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 diesels, 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.

Safety Tips

diesel combustion diagram
This diagram shows the diesel combustion process in your engine. engine diagram

Follow these instructions carefully. Read and be sure you understand them before you begin. Remember that these are general instructions. 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. 
  • 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.
  • Never work on a vehicle that is only supported by a jack. Use jack stands to support the vehicle while you work. Work on a solid, level surface. Never jack a car up on dirt or grass.