Expert advice: are faulty glow plugs the reason your car won’t start?

Glow plugs

When is a flat battery not a flat battery? When it’s a problem with glow plugs. OK, that’s a bit simplistic but frequently people breakdown with what they think is a battery problem, only for it to turn out to be the glow plugs that are at fault.

The reason is that diesel engines use these heaters to help them start. In warmer weather they’re not as necessary. However, when the weather gets cold, glow plugs are vital. That’s why in November 2017, the number of call outs Green Flag attended for glow plug problems rose by 112 per cent. Read on to find out what the problem is and how you can diagnose it.

What are glow plugs?

Diesel engines rely on the heat of compression rather than spark plugs to ignite fuel in the cylinder. But in low temperatures, the cold engine block absorbs any compression heat which makes starting difficult. The glow plug provides the answer, ensuring it’s an essential part of the modern diesel engine.

Although you’ll probably never see one in action as they’re buried deep in the bowels of the engine, glow plugs look like a pen. And their tip glows, rather like a car’s cigarette lighter, hence the name. This glowing end provides extra heat in each cylinder so the fuel ignites more easily and the engine starts up. Once the engine is running, the glow plugs go back to being idle.

Glow plugs

How do you know glow plugs are working?

When you turn the diesel car’s ignition on, the glow plug symbol (below) illuminates on the dash board. After fewer than a handful of seconds this goes out, indicating that the glow plugs have done their work heating the cylinders and the engine is ready to start. On more modern diesel cars that have a starting button rather than a key, the car’s computer knows to delay starting until the glow plugs have done their thing.

When that orange squiggle goes out, the glow plugs have done their job

What happens when glow plugs fail?

You will know your glow plugs are wearing out because the orange light on the dash will shine for longer before extinguishing. And the car might struggle to start. In colder weather the symptoms might eventually appear like a flat battery. That’s because you will be cranking the engine in vain as you try to start it but you’ll be draining the battery of its charge. In the end, the battery will cry enough. But that’s the effect of the faulty glow plugs making starting hard, rather than the cause of the problem.

As most cars have four-cylinder engines, that means four glow plugs. If one glow plug fails, the engine may still start but it might run roughly initially. That’s because it will take a bit of time for the cylinder without the functioning glow plug to get up to temperature and work properly. If that’s happening it’s telling you that the glow plugs need replacing.

How long do glow plugs last?

This is a difficult question to answer but we’re looking at tens of thousands of miles at the least. The glow plug is only used during the start-up phase. That means those in a car doing a lot of short journeys will be used more and have a shorter life span in terms of miles than those in cars that do high mileages between start-ups.

Scott Wilson is vehicle and customer data insight manager for Green Flag

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