Definition
A glow plug is an electrical heating element in the combustion chamber or intake of a diesel engine that raises air temperature to help the engine start when cold, since compression alone may not produce enough heat in low temperatures.
Compression ignition depends on compressed air being hot enough to ignite fuel. In a cold engine, in cold weather, or in an engine whose compression has declined with wear, it may not be — and a glow plug or intake heater supplies the difference.
Not every engine has them
Many marine diesels do not, and design varies: individual in-cylinder plugs, or a single intake heater grid. Know which your engine has before diagnosing a hard start.
How the circuit fails
Individual plugs fail one at a time, so the engine gets progressively harder to start cold while running normally once warm — a symptom owners often blame on fuel. The relay, the timer and the wiring fail too, and on a salty boat the connections are a common culprit.
Where it matters most
Colder water and colder mornings — see Hobart — and on older engines with reduced compression, where the margin is smallest. Never use starting aids in an engine with glow plugs unless the manufacturer specifically permits it.