Petrol: a spark at exactly the right moment
In a petrol engine, the fuel and air mixture won’t ignite by itself. It needs a spark – at a precisely timed moment, on every combustion cycle of every cylinder. The spark plug provides it: a discharge of up to several tens of thousands of volts jumps between its electrodes.
A spark plug works the whole time the engine is running. That’s why it wears steadily: the electrodes burn away, the gap between them grows and the spark gets weaker. The car manufacturer specifies how many kilometres it should be replaced after.
Diesel: it ignites by itself, the plug just helps
A diesel has no spark. The air compressed in the cylinder gets so hot that the injected diesel ignites on its own.
The trouble starts when it’s cold. A cold engine draws heat out of the compressed air, so it doesn’t reach the temperature needed for ignition. That’s when the glow plugs come in – small heaters screwed into the cylinder head that glow red-hot within seconds and warm up the combustion chamber.
In many newer diesels the plugs keep heating for a short while after starting, so the engine runs more smoothly and smokes less. Once a diesel is warm, it no longer needs its glow plugs.
That’s why, in a diesel with a key, it’s worth turning it to the ignition position in winter and waiting for the glow plug light (the yellow coil) to go out. Only then turn the engine over.
The key differences
| Spark plug (petrol) | Glow plug (diesel) | |
|---|---|---|
| Job | ignites the mixture with a spark | heats the combustion chamber for a cold start |
| When it works | the whole time the engine runs | before starting and briefly after |
| How it wears | the electrodes burn away | the heater burns out, the plug seizes in the head |
| Signs of wear | jerking, misfires, higher fuel consumption | hard cold starts, smoke and rough running after starting |
| When to replace | at the interval given by the manufacturer | when they stop working – ideally as a set, before winter |
| Risk when replacing | usually low | a seized plug can snap |
Why replacing glow plugs can be risky
A glow plug is long and thin, sits deep in the cylinder head and spends years working at high temperatures in carbon build-up. Often nobody touches it as long as it works. Over time it seizes in its thread so firmly that it can break when you try to unscrew it.
A snapped plug can’t simply be unscrewed. What’s left has to be drilled out and extracted with special tools, and sometimes the cylinder head has to come off. That’s what happened with a Vectra B 2.0 DTI in our workshop – removing the snapped plug would have cost more than the car was worth.
That’s why glow plugs are unscrewed with the engine warm, using penetrating oil, slowly and with feel. If a plug won’t budge, it’s better to stop than to snap it.
Signs of worn plugs
Spark plugs (petrol)
- the engine jerks or runs roughly, especially when accelerating,
- the engine light flashes – this can mean misfires,
- the car accelerates less eagerly and uses more fuel,
- the engine is harder to start.
Glow plugs (diesel)
- the engine turns over for a long time before it starts from cold,
- after a cold start it shakes and puffs white smoke,
- the glow plug light flashes or the engine light comes on,
- the diagnostic tester shows a glow plug circuit fault.
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What about LPG?
An LPG system puts extra strain on spark plugs. Gas needs a stronger spark, so the plugs wear faster than on petrol alone. On a car running on LPG it’s worth replacing them more often than the car manufacturer specifies.
What this means for you
- Spark plugs and glow plugs can’t be swapped – they’re completely different parts.
- On a petrol car, replace the spark plugs at the interval in the handbook, and more often with LPG.
- On a diesel, check the glow plugs before winter, replace them as a set and don’t put it off for years.
The plugs are just one item. See what else is worth checking in a winter diesel service.