The glow plug relay has its own temperature sender to tell it how long the preglow cycle should be. The preglow does not affect the temperature reading of the sender; only actual cylinder head temperature does this. You can get multiple preglow cycles of the same duration by turning the key on and off. I am running 80/20 used motor oil/gasoline ($0.03 per mile), and I need to preglow twice at least, when cooled overnight. When my 617 is hot, the preglow cycle is very short, less than 1 second, or non-existant. I use a high pressure electric fuel pump to give me 14 psi at the injector pump, so that I get first-turn ignition when the precombustion chambers are hot, and the engine is cold. The OEM lift pump takes many revolutions to build adequate charge pressure. Excessive cranking ruins the battery and starter, as well as the flywheel gear teeth. These are expensive fixes. My 617 has been recently overhauled, but it will NOT fire when cold without extensive cranking unless preglowed and primed. Preglowing also reduces exhaust smoke on startup, since proper combustion requires that the precombustion chambers be very hot. If you want to run diesel (or other stuff) you have to have the patience to follow procedures. Otherwise, stick to gassers.