Change Car Glow Plugs
Glow plugs allow a diesel engine to start easily from cold by raising the temperature in the combustion chamber. When they fail, the engine is hard to start, fuel consumption rises and a warning light can appear on the dashboard.
What is the role of a glow plug?
A diesel engine runs on compression ignition: the air compressed in the combustion chamber reaches 700 to 900 °C, igniting the injected fuel. In cold weather the glow plug compensates by heating up to 1,000 to 1,250 °C in a few seconds.
Service life and symptoms
Average service life is around 120,000 km. Watch for: the glow-plug warning light coming on, difficult cold starting, black smoke from the exhaust, loss of power when cold.
What you need to know for the replacement
Access is often awkward
- The plugs are in the cylinder head, but often hidden by the air filter housing or the high-pressure fuel lines
- On some engines the high-pressure lines have to be removed and replaced
- On others, the engine has to be eased forward slightly to reach the plugs
Essential precautions
- Slacken the plug by a few turns, blow out with compressed air, then unscrew fully
- Never refit a plug that has been dropped on the floor
- On refitting, apply high-temperature anti-seize compound to the threads
- Tighten to the recommended torque
Cost of replacement
- A glow plug costs £10 to £22 from a motor factor
- Change them all at once (4 on a four-cylinder engine)
- Average garage price for a set: around £140 excluding labour
Practical tips
Difficulty 2/5
A simple job in principle, but glow plugs can seize in the cylinder head over time. Always use a releasing fluid and never force them: a plug snapped off in the head turns a simple job into a major one.
Tightening torque
Always observe the specified tightening torque (generally 8 to 15 Nm). Over-tightening crushes the seal and makes future removal impossible.
Replace the full set
Even if only one plug has failed, replace them all at the same time. The remaining plugs are probably near the end of their life and the cost per plug is low.
Price
Allow £8 to £25 per plug depending on the type (standard or with a built-in sensor). A full set comes to £30 to £100.
