How Many Glow Plugs Are In A Diesel Engine?

The glow plugs in a diesel are like the spark plugs in a gasoline engine, but they function differently. The only similarities between glow plugs and spark plugs are the numbers. A diesel engine will have as many glow plugs as it has cylinders, which is the same as the spark plugs in a gasoline car. A four-cylinder diesel engine will have four glow plugs, while six glow plugs are in a six-cylinder diesel.

However, glow plugs and spark plugs have different tasks. In a gas-powered car, the spark plugs will consistently ignite as long as the engine runs. On the other hand, the glow plugs in a diesel engine will only "glow" or ignite in colder temperatures, such as when starting a diesel during chilly winter mornings.

Unlike spark plugs that are necessary for combustion in a gas-powered vehicle, the glow plugs in diesel engines will go to work when the machine requires more heat in the combustion chamber. Diesel engines don't need spark plugs for combustion. It relies on high compression of superheated air and injecting diesel fuel at the right moment to produce power.

But when a diesel engine is cold, it needs glow plugs to raise the temperature in the cylinder walls and prevent the superheated and compressed air from losing some of the heat necessary for combustion.

The average service life of a diesel glow plug is around 100,000 miles, but your mileage may vary (literally). The glow plugs could deteriorate and fail sooner on some trucks that do regular heavy-duty work in colder temperatures. Regardless, the most common symptoms of a failing glow plug include hard starting (in cold or hot climates) and thick smoke from the exhaust pipe after cold starts.

Not all diesel engines have glow plugs. Some larger diesels in commercial trucks or haulers don't need glow plugs, but it all depends on the make, model, and type of vehicle. However, most small diesel engines require glow plugs or a heater source to assist in cold starts.

The other telltale symptoms of bad diesel glow plugs include rough idling, misfiring, and lousy fuel economy, the same signs you'll notice when a spark plug goes haywire. Fortunately, glow plugs are not that expensive, and a handy DIYer can replace them easily using basic hand tools. For reference, eight glow plugs for a Ford 6.0-liter Power Stroke diesel V8 cost around $55 per set.