Are glow plugs supposed to stay on after the engine has started to help with the firing of the first few revolutions?

2 Answers 2


Actually with modern engines, they can and do come on after startup. I think the reasons are spelled out fairly well in this article on Engine Builder Magazine:

The government wanted to reduce the amount of diesel hydrocarbons and nitrogen oxide levels being emitted from diesel engines. Now, this is where the importance of the glow plugs comes in. Not only do the glow plugs cycle to aid the engine during cold weather start up, but also after the engine is cranked – they will continue to cycle to reduce the amount of start-up emissions. This helps reduce the smoke that most diesel engines produce when being cranked on cold mornings.

To reiterate, they come on after engine startup to reduce emissions and smoke. The combustion chamber will not immediately come up to temperature after startup. It takes a while for things to get heated, like the heads, cylinder walls, and most importantly, the cooling system. Once these things are warmed up, emissions are where they should be and the glow plugs don't need to come on anymore.


No. Glow plugs are used only during colder temps. Once the motor starts the combustion process has begun and no longer requires additional heat.

Like fusion once it starts it ignites itself!🍻

  • Don't quote me, but as far as I know that is wrong. Glow plugs stay on a little longer (seconds, probably) after the car has started and the plug light went off to make the engine work smoother and not die. Can anyone confirm? Commented Jan 20, 2016 at 9:38
  • Maybe seconds... But not very long... They should go off...
    – Dee
    Commented Jan 20, 2016 at 19:54

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