When its warm out, I don't use the glow plugs for nearly as long as soon as the light goes out. 280Kmi and no starting smoke in warm weather. Whe its cold, I glow them for 10-15 more seconds to get rid of the smoke. From what I have seen, if you need to glow 10-15 EXTRA in warm weather then something is not right. here is no afterglow in your car. Glow plugs are shut off when the starter engages. If you listen carefully you should be able to hear the glow plug relay shut off if you hold the key long enough. On my car thats 15+ seconds after the light goes out.

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Brian Toscano