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You're talking about a different thing. Glow plugs inside the engine heat the combustion changer before the engine is cracked to first warm up the combistion chanber enough for the diesel to burn on startup. Some models leave them on for a few seconds to help combustion while the engine is cold, which is fine.
This is completely different from a glowplug used to heat up the coolant, which might stay on for a couple of minutes.
The intake ones are yet a different system. I remember them in very ancient direct injection Japanese systems. They're not as efficient, but a direct injection engine can start unaided at much lower temperatures. I can't recall any modern engine with them.
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