Just what exactly is a glow plug and what does it do? A glow plug is a heater and is so named because it glows red hot when activated. Diesel engines are compression ignition engines and rely on hot air (under extreme pressure) to combust the fuel. A cold engine with cold air temperatures creates difficult combustion conditions resulting in either no engine start or a very difficult start with smoky exhaust. The addition of glow plugs to heat the cold combustion chamber allows the diesel engine to start quickly and smokelessly. Glow plugs are connected to a glow plug controller which is either switched on by a temperature switch or the engine computer at the time that the extra heat is required. Glow plugs usually cycle on and off after the engine has started for up to two minutes to ensure good combustion.
How do you know if your glow plugs are not functioning properly? There are a few ways based on the vintage of your diesel. On older diesels, around 2000 and earlier models, you will notice long crank over or no start when cold. You might also notice that the engine runs rough on start-up and smokes excessively for a short period of time. On newer computer controlled diesels you’ll rarely ever experience these poor engine start and running issues. This is because, after the slightest glow plug defect is detected, your check engine lamp will come on. With it a stored trouble code indicating which glow plug may be faulty will be logged in the computer.
Almost every automotive diesel (car or light truck) uses glow plugs however there are some exceptions. The Cummins diesel as found in Dodge trucks has no glow plugs but instead uses a grid heater in the air intake. This warms the air as it is sucked into the engine. It only switches on during very cold weather.
How often should you change your glow plugs? Unlike spark plugs, there is no recommended maintenance replacement interval for glow plugs. They are normally changed when either cold start issues present themselves or your check engine lamp comes on with a glow plug code (and after diagnosis it is determined that the glow plug is faulty).
Cranking over a diesel engine takes a lot of energy and your starter motor works very hard. All diesel vehicles have either two batteries or one humongous battery to provide the power for the starter. Starters, due to their size are very expensive, so a quick easy starting engine is much easier on your starter. With the quick start that glow plugs provide your starter lasts much longer, saving you money.