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Originally Posted by sailingdog
Effectively, they act as a spark plug until the diesel engine has heated up enough to provide its own ignition source.  |
Sorry, SD, I don't think that's quite right - The glow plugs "preheat" the combustions chamber and/or a pre-combustion chamber to facilitate the compressive firing of the fuel. No fuel is injected or "bled into" the engine during the preheat time, and I've never held the glow plugs on during cranking to start, so the spark plug analogy doesn't really apply. I doubt there's actually enough heat there to fire the fuel anyway.
There are "quick acting" and "slow acting" glow plugs available for various engines, and the required preheat time varies with each type. For the slower ones I'd agree with a max count of 20 for a cold start. In milder weather you will likely need less time.
Should you find yourself "needing" excessive preheat time, check the condition and function of the glow plugs. If they check out it's quite likely that your compression is beginning to suffer, making starting difficult.