Car Glow Plug Guide

glowplug (alternatively spelled as glow plug or glow-plug) is a heating device used for starting diesel engines. In cold weather, high speed diesel engines can be difficult to start as the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition (which relies on that heat). Pre-chambered engines utilize small electric heaters (glowplugs) inside the pre-chambers. Direct-injected engines have these glow plugs in their combustion chamber. The glow plug is a pencil-shaped piece of metal which has a heating element at the tip. This heating element, when electrified, heats due to its electrical resistance and starts to emit light in the visible spectrum, hence the term glowplug. The visual effect is same like the heating element in a toaster. The fuel injector spray pattern then impinges directly upon the hot tip of the glow plug during the process of injection of fuel at top dead centre. This ignites the fuel even if the engine is insufficiently hot for normal operation, which reduces the cranking time to start the engine.

Method of operation


“Wait-to-Start” light (glowplug indicator light, ISO 7000-0457) in a diesel car.

In older generation diesel-engine vehicles, unlike in a gasoline-engine vehicle, for a ‘cold start’ the operator did not simply just turn the key to the “start” position and have the engine immediately start. Instead of it, the operator activated the glow plug(s) for a time first. Early diesels utilized the ‘Thermostat’ type glow plug in the inlet manifold. These take about 20 seconds to achieve working temperature and the vehicle operator had to manually time (or guess) when the 20 seconds had elapsed. With in-cylinder glow plugs, technological improvements have a warning light on the dash to indicate how long the preheating should last. The preheating phase was also made to be automatically activated whenever the operator turned the key to the “on” position for a long duration; the glow plug relay switches on the glow plugs and a light (see picture at right) on the instrument cluster illuminates. This process is known as “pre-heating” or “glowing”. Many modern diesels automatically activate their glow plugs whenever the operator unlocks the vehicle or opens the door to the car, thus simplifying the process and shortening the time the operator has to spend waiting before the engine will start. According to Bosch: “Older engines with Thermostat manifold plugs used a glow period of up to 20 seconds whereas more modern engines utilize around a 6 to 8 second heat period and provide afterglow at a reduced voltage.”


Before making any attempt to start a vehicle engine, the vehicle’s parking brake should be firmly applied and gear selection should be in Neutral (or Park). Any engine stop control needs to be returned to the run position and in cold weather, any ‘excess fuel’ starting aid needs to be set or used – such as Ki-Gass.

With in-cylinder glow plugs, when a pre-set time has elapsed, the glow plug relay turns off the “wait-to-start” light. A pre-heating cycle mostly lasts for 2 to 5 seconds. The operator then proceeds to turn the key for the “start” position. The relay switches off the glow plugs only after the engine is running (or, in older cars, at the same time the “wait to start” light goes out). In some of the cars, in order to maintain compliance with emissions regulations, the glow plugs might be operated immediately after engine start, or during periods of extended idle where engine temperature has decreased, as combustion efficiency is greatly decreased when the engine is below operating temperature.

With a Thermostart plug in the inlet manifold, within a few seconds the diesel vapours start filling the inlet manifold. As the plug continues to heat up, it opens a valve allowing diesel from a special reservoir mounted directly above the Thermostart into the Thermostart plug. This fresh diesel is also vaporised and is added to that in the inlet manifold. At 20 seconds, provided if the air is available, the diesel near the plug ignites and as the engine is cranked, the ignited diesel is drawn into the combustion chambers to which more diesel is added only after the compression stroke. This additional diesel ignites immediately, starting the engine with ease. Vehicles fitted with Thermostart glow plugs do not mostly activate via the normal ignition switch. A button elsewhere is given (along with, in some cases, the button to activate the starting motor). Where a Thermostart is activated through the same switch as the ignition, it is mostly activated by turning the switch one ‘notch’ counter-clockwise. After the 20 second period has elapsed, cranking is achieved through turning the ignition switch a further notch counter-clockwise. Once the engine has fired and is running, the ignition switch is released which allows it to spring back to the off position. The operator should then turn on the ignition switch generally one notch clockwise

Warm engine start

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

With a Thermostart, it is entirely up to the operator to choose whether to activate the glow plug or not. In the case of ignition key controlled Thermostart, turning the key two notches to the right starts the unheated cranking.


A glow plug helps to resemble a short metal pencil. A heating filament is fitted into its tip. Glow plug filaments needs to be made of certain materials, such as platinum and iridium that resist oxidation and high temperature.

Model engines

Glow plugs in model engines are different from those in full-size diesel engines. Full-size engines only utilize the glow plug for starting. Model engines utilize a glow plug as an integral part of the ignition system because of the catalytic effect of the platinum wire, on the methanol-base fuel they are designed to run on.

Model engine glow plugs are also utilized as re-usable igniters in theatrical pyrotechnics and the special effects industry to remotely ignite pyrotechnic devices which use flash and smoke composition powders.

Symptoms of Bad or Failing Glow Plugs

Common signs in diesel vehicles consist of engine misfires, issues starting in the cold, and more smoke coming out of the exhaust.

Glow plugs are an engine management component that are found on vehicles equipped with diesel engines. Their aim is to preheat, and help warm up the engine’s cylinders so that diesel combustion can occur more easily. They play a crucial part in warming the vehicle’s cylinders during cold starts, where starting the engine is most difficult. Glow plugs use an electrode which helps to warm up and glow orange when current is applied. When the glow plugs have an issue, they can usually result in problems with the driveability of the vehicle. Usually bad or failing glow plugs will have a few symptoms that can alert the driver of a potential issue.

1. Engine misfires or decrease in power and acceleration

Engine misfires are one of the first symptoms of a problem with the vehicle’s glow plugs. If the glow plugs malfunction, they will not provide the additional heat that helps in diesel combustion, which might cause the engine to experience misfires. The misfires might cause a loss in power, acceleration, and even fuel efficiency.

2. Hard starting

Another symptom of an issue with the vehicle’s glow plugs will be hard starting. Unlike gasoline engines, which utilize a spark to ignite the fuel mixture, diesel engines rely solely on cylinder pressures to ignite the diesel fuel mixture. If the glow plugs fail, the engine needs to overcome additional pressure in order to ignite the mixture, which might result in hard starting.

3. Black smoke from the exhaust

Another symptom of an issue with the glow plugs is black smoke coming from the exhaust. Faulty glow plugs might disturb the sensitive diesel combustion process, which may cause the engine to produce black smoke from the tailpipe. Black smoke can also be a result of wide variety of other issues, so having the engine properly diagnosed is highly recommended.

How to Replace the Glow Plugs in Your Car

Car glow plugs work hard in regulating temperatures to help diesel engines run. Glow plugs usually wear out and need to be replaced.

Glow plugs are used in diesel engines to aid heat the fuel in preparation for the combustion chamber when the engine is cold. They are powered with the help of all 12 volts available from the battery, and are located on top of the cylinders. They are used frequently, and the duration of their use is determined through the weather conditions where you live. Cars in colder areas are will be needing new glow plugs much more frequently, while glow plugs in warmer areas can outlast 100,000 miles. With continuous utilization and extreme temperature variances, glow plugs are a hard working part. Rough starts or misfires while starting, smoking when starting, and trouble starting in cold conditions are just basic signs of a failing glow plug. They tend to be relatively cheap and are quite easy to replace with the right tools.

Part 1 of 1: Replacing your glow plugs

Materials Required

  • Deep socket set and ratchet
  • Pliers
  • Replacement glow plugs
  • Socket set
  • Valve cover gaskets (if equipped and having a desired of replacement)

Step 1: Disconnect the battery cable. Find and disconnect the black negative battery cable. Any time you work on an electrical system on your vehicle, disconnect the battery cable first. Glow plugs are routed to have the full 12 volts which are available from the battery.

  • Note: Often when you are replacing parts such as glow plugs, there is a certain amount of digging that might be required in order to access the part. This varies greatly upon make, model, and year. Keep track of anything and everything you are removing or set aside when accessing the glow plugs on your vehicle. It is good practice to bag and label your screws as well as your bolts.

Step 2: Remove the valve cover (if applicable). If the glow plugs are found under the valve covers, you will have to remove them first. If not, just continue to Step 3. For removing the valve cover, unscrew all the retaining screws around the perimeter of the cover. Keep an eye out for the valve cover gaskets which are between the engine and the covers.

  • Note: This is an excellent time for replacing the valve cover gaskets on your vehicle.

Step 3: Locate the glow plugs. The glow plugs have to be screwed into the cylinder head. Locate the top sticking out and the wire that delivers their 12 volt power supply.

Step 4: Disconnect the power supply to the glow plugs. On top there will be a nut, bolt, plastic terminal, or other type of connection device for removing the wire from the glow plug quickly. Remove the terminal and set the wires aside. Newer models will be needing a terminal similar to spark plugs. These can be easily pulled straight off with pliers help. Just but be careful not to break the terminal.

Step 5: Remove the glow plug. Use a socket and ratchet for removing the plug. Put the ratchet over the plug and turn to the left (counter clockwise). This might require a deep socket. For most of the part, these should come out clean. You will get to know through the length and shape of the plug how damaged it is (see picture below).

  • Note: Glow plugs can break down inside of their compartment only. If you are removing a broken plug, ensure to remove all broken pieces from the engine.

Step 6: Prepare to install the new glow plug. Now is the time for taking a few precautions and possibly improve the parts associated with the glow plugs. Clean all the area around the glow plug hole’s opening. If the supply terminal is dirty, just use a wire brush or appropriate tool to clean it for a solid connection.

Step 7: Install the new glow plugs. Hand tighten them into their position. Use your socket and ratchet for snugging them into place. Be careful you don’t over tight them. It will result in damage to the plug, preventing them from working correctly, or possibly breaking the plug in its place.

Step 8: Attach the power supply. Reattach the wires that help to supply the glow plugs with power. These should screw or clip into place in exactly the same manner you removed them in step 4.

Step 9: Replace the valve covers. Replace the valve covers or anything which is removed in the process of accessing the glow plugs. If you removed the valve covers, now is the time for replacing the gasket between the cover and the valve block. Insert all of the screws which are removed with the valve box after you have swapped out the gasket. Tighten them with the help of your hand and then use your ratchet to tighten them down.

  • Note: Some manufacturers need the valve screws to be torqued to a specific pressure. Look to your vehicle’s manufacturer for this information if you are dealing with valve cover removal.

Step 10: Connect the negative battery cable. Reattach the negative battery cable into the battery. This restores power to the vehicle and the glow plugs can be tested by a scan tool or multimeter if desired.

Video of How to replace a glow plug

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