Do the glow plugs do anything after the car is started and the relay is off? I washed my engine today and accidentally knocked off the vacuum line on the IP that shuts down the engine, turned the key to shut the engine off but nothing turned the key back on and WOW the engine became almost silent, thinking maybe my glow plugs are bad?

Nope. They are only on between when you turn the key to when you use the starter.

If I understand the post, you basically had a running engine and with it running turned the key off and back on again which would initiate a glow cycle on a running engine. On a cold engine, having the glow plugs heat while the engine is running will help complete combustion and reduce "clatter" which is probably what you thought was causing your engine to run more quietly. That is why the later engines feature "afterglow" which allows the plugs to continue to heat after the engine starts and cut down on emissions. So, yeah, they actually will do some good, at least on a cold engine after it is running but typically they are only "needed" to start the car. Even on the afterglow engines the GPs shut off after a minute or so. So contrary to your thinking there was something wrong, the fact that it quieted down proved the GPs are working fine.

I hope this answers your question.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

I'll agree with nhdoc, the key position should determine if the GP's engage, after a turn off and on.
I will assume you did this on a cold engine, also with nhdoc, normal. Try with a hot engine, better yet, reconnect the line.

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