it mit be a bad silenod see if you have power comeing out of it, they are none to go bad and they are expensive , you mite have with one bad glow plug but not all of them. the 7.3 liter is a cold blooded pig the injectors work of oil press so the colder it get the thiker the oil for winter time mite want to try diffrent weight oil

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Been helping a friend with her '99 7.3L cold start problem. she had to plug the block heater in when it was as warm as 30 degrees just to get it started. Note this is a vehicle with 190K miles on it and was a little neglected. Problems so far: corroded and loose battery terminals, batteries old and weak, 4 glow plugs burned out, GPR has burnt terminal and 1 volt drop across terminals, pulled injectors and tested found all 8 injectors had flow and leakage problems. Also checking into potential intake heater problem? Its surprising this vehicle even started with the block heater plugged in.

So far replaced both batteries, all 8 glow plugs, rebuilt all 8 injectors, replaced OEM GPR with Stancor contactor #586-902 from Mouser electronics (note there is a little history on this OEM relay(ref the Power Stroke 7.3L web site).

Starts 100% better now and still looking into the problem with intake heater?

A gross check of the glow plugs can be made by clamping a DC current probe around the output lead of the GPR and a DC volt meter on the output relay terminal. If the relay is working and all GP's are good you will measure 180-200 amps on the current probe with a DC terminal voltage on the volt meter of at least 11.8 Volts(assuming good batteries).

Hope this helps.