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I quote my Helm manual on Description and Operation:
"The glow plug system is electronically controlled by the PCM. If the temperature is below 55°C (131°F) the PCM will energize the glow plugs immediately after the key is placed in the ON position. Then, depending on the readings from the engine oil temperature (EOT) sensor and the barometric pressure (BARO) sensor, the PCM determines how long the glow plugs will be on. "
If the glow plugs aren't getting max current then they won't heat the cylinder as effectively therefore reducing burn efficiency.
Nut
Not to be rude PS- but if this is what you are going by, in no way can I gather from just this paragraph that this is, or can happen, during driving conditions. If driving conditions mean the first couple of minutes after start up then yes. If you are going down the highway at 65 and the glow plugs activate than no I can't agree with that either.
If I have a bad GPR and it's cold enough to worry about it, than NO I certainly would not be @ 65 MPH in less than 2 minutes. That's just TRUCK ABUSE!! Let her sit there and warm up.