Guide to Defender Glow Plug Renewal / Removal

Jan 18th, 2011 | By | Category: Defender How To Guides

What are Glow Plugs ?

A glowplug is a heating device used to aid starting diesel engines they are also known as heater plugs.

Diesel engines, unlike petrol engines, do not use spark plugs to induce combustion. Instead, they rely solely on compression to raise the temperature of the air to a point where the diesel will combust spontaneously when introduced to the hot high pressure air. The high pressure and spray pattern of the diesel ensures a controlled, complete burn. The piston rises, compressing the air in the cylinder; this causes the air’s temperature to rise. By the time the piston reaches the top of its travel path, the temperature in the cylinder is very high. The fuel mist is then sprayed into the cylinder; it instantly combusts, forcing the piston downwards, thus generating power. The pressure required to heat the air to that temperature, however, necessitates the use of a large and very strong engine block. The problem posed is that in cold weather, if the engine has not been running (as is the case when the car is left to sit overnight), that large engine block becomes very cold; when one then attempts to start the engine, the cold engine block acts as a heat sink, quickly dissipating the heat generated by the pistons compressing air. The engine is then unable to start, because it can not generate and maintain enough heat for the fuel to ignite. For that reason indirect injected diesel engines are manufactured with glow-plugs in each prechamber.

“Wait-to-Start” light (glowplug indicator light) in a diesel car.

In an older generation diesel-engine car, unlike in a petrol-engine vehicle, the operator did not simply turn the key to the “start” position and have the engine immediately start. Instead, the operator turned the key to the “on” position; the glowplug relay switches the glowplugs on, and a light (see picture above) on the instrument cluster illuminates. This process is called “pre-heating” or “glowing”.

If the car had been running very recently, or if the ambient temperature was hot, the “wait to start” light might not come on; in this case, the operator may proceed to turn the key to the “start” position and start the engine without having to wait.

A glowplug is a pencil-shaped piece of metal with a heating element at the tip; that heating element, when electrified, heats due to electrical resistance and begins to emit light in the visible spectrum, hence the term “glow” plug; the effect is very similar to that of a toaster. The heat generated by the glowplugs is directed into the cylinders, and serves to warm the engine block immediately surrounding the cylinders. This aids in reducing the amount of thermal diffusion which will occur when the engine attempts to start.

When internal sensors detect that the core of the engine block has reached a certain designated temperature, or when a certain amount of time elapses, the glowplug relay switches off the “wait-to-start” light. A pre-heating cycle usually lasts for 2 to 5 seconds. The operator then proceeds to turn the key to the “start” position, as in a petrol engine. The glowplug relay switches off the glowplugs after the engine is running (or, in older cars, at the same time the “wait to start” light goes out). In some newer cars, glow plugs continue to operate for up to 180 seconds after engine start to keep the engine within emissions regulations, as combustion efficiency is greatly reduced when the engine is very cold.

A glowplug resembles a short metal pencil. The heating filament is fitted into its tip. Glowplug filaments must be made of certain materials, such as platinum and iridium, that are resistant to both oxidation and high levels of heat.

Large diesel engines, such as those used in heavy construction equipment and locomotives, do not need glow plugs. Their cylinders are large enough that the air in the middle of the cylinder is not in contact with the cold walls of the cylinder, and thus retains enough heat to allow ignition.

Symptoms of worn or faulty Glowplugs

So what are the symptoms of worn or faulty Glowplugs?

Hard Cold Starting

Several diesel mechanics concur that cold engine hard starting is a classic symptom of bad glow plugs. This can be especially pronounced on a cold morning. A bad glow plug will not heat the combustion chamber, and hard cold starting is the result.

Rough Running

When an engine with bad glow plugs finally does start, it runs very rough until it heats up. Misfiring occurs because the glow plugs did not heat the combustion chambers sufficiently. As a result, the fuel does not burn well until the engine reaches operating temperature to completely burn the fuel.

Dense White Smoke

Dense white smoke when the engine is cold is sign of bad glow plugs. By the engine having incomplete burning, raw fuel is allowed in the exhaust. The raw fuel may ignite inside the exhaust system, which shows up as dense smoke coming out of the tailpipe. When the engine reaches operating temperature, the smoke goes away, since the fuel is now being completely burned.

Combination of Symptoms

With bad glow plugs, all the symptoms may occur at the same time. First, the engine is very hard to start. When it finally does start, it runs rough and dense white smoke is seen from the exhaust pipe. Both rough running and white smoke disappear when the engine warms up.

How to remove those Glowplugs

First thing first, this is a fairly easy job to do so your not going to find anything too taxing here but you may find that this guide eases the process and helps you visualize the job.

Now to make access easy it will be asier to remove the pipes and the cyclone breather.

Next remove the nut from the feed/plug that is nearest to the bulkhead.

Now continue to remove the nuts from the remaining feeds/plugs until they are all off and remove the harness.

Next you will need a long/deep 10mm socket to remove the plugs.

Photo’s of the old glowplugs versus new ones.

Once the glowplug has been removed its just a case a putting the new one in. Just apply some copper grease around the plug threads and refit the plug remembering to use the correct torque settings (e.g 300tdi = 20Nm for the new plugs & 2Nm for the nuts).

Replace the wiring harness, replace the cyclone breather and put the hoses back on. Thats it really, Simples.

Testing Glowplugs

If you do suspect a faulty glow plug then you can try this method to confirm it.

Place the suspect plug in a vice (carefully as not to crush or damage it!) then attach a wire to the body of the plug and to the negative terminal from a spare battery. Then attach a wire to the top threads of the plug and connect to the positive terminal of the battery. The tip should glow red about 5 or 6 seconds, if it does not then the glowplug is indeed faulty.

Do not touch the now heated glowplug as it is extremely hot (Hotter than the centre of the sun, maybe) , leave it alone and allow it to cool before handling

[review pros=”Better Starting, Better Running” cons=”None really” score=25]

Tags: , ,

5 Comments to “Guide to Defender Glow Plug Renewal / Removal”

  1. Davey of Cornwall says:

    That bit is easy but where is the relay? My ’96 Disco still has appalling wiring. It used to suffer from flat batteries (yes it has two of them fitted by previous presumably to store more charge). I’ve replaced the grossly undersized botched wiring with welding cable and this helped but then the alternator lug was getting hot. Replaced the corroded crimp with a new lug crimped and soldered but then the alternator went belly-up. Fitted a 2YO spare bought online and got out Hall Effect multi-meter. Headlights on main beam, alternator output 65 amps! Dip beam alternator O/P now down to 45A. Lights out alternator output down to 39A. This seemed a lot as the batteries had been charged overnight. Checking the glow plug wiring with the engine running showed that 22 amps were being consumed. Three glow plugs must have burnt-out and one must be on all the time that the engine is switched on. I guess that the relay contacts have welded themselves together but where is it? Its not where the H manual says it is.

  2. Nigel Dawes says:


    I was wondering if I could pick your brains on the replacement of glow plugs on a 300tdi Land Rover Discovery.

    It is in regards to the Torque Settings.

    Basically you say 20Nm
    Haynes Manual say 23Nm
    and The manufacture of the Glow Plugs in this case Beru say 15Nm

    I have done mine up to 15 for now, as thought it be best to have them to loose rather than too tight, also the nuts I have done up finger tight and then just a little pinch up, as my torque wrenches do not go as low at 2Nm is this ok to.

    Oh and my Landy is still a pain to start, so I am thinking glow plug replay is next to change.


  3. Nigel says:

    Thanks for this, gave me the confidence to do the job this morning – 3.30am, couldn’t sleep!

  4. Philip says:

    I have just purchased a set of glowplugs for my 1989 Defender 2.5 Turbo Diesel. The problem is, the length beyond the sealing cone is 25mm compared to that of the existing type (NGK-Y-907R) which is 20mm long.
    I dont want to put the longer ones in in case of interference with the pistons.
    Any advice would be welcome please.

  5. David says:

    Great article clear pictures’ and instructions with nothing assumed

Leave a Comment