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So I’ve been chasing a problem with my truck not starting without being plugged in in low temps to temps up to 80 degrees. I’ve replaced my gp relay, and am going to replace the glow plugs within the next week. I am wondering what else y’all would suggest to check? I’ve heard everything from needs injectors to low compression. The truck does white smoke if I crank on it but will not start unless it’s plugged in.
First thought is glow plugs. You can do a quick check with a test light by just removing the wiring plug from the valve cover gasket. Connect the test light lead to the negative post ant touch the outer most pins, (G-I-C-I-G) the one marked G. If the test light lights up the plug is most likely good, no light the plug is bad. When replacing use Motorcraft/Beru plugs only, as aftermarket ones just don't last, and can swell up, when they burn out, causing near impossible removal without removing the head.
I think @Hit Man X can start his in 50 degree weather without GP's and he's got lousy compression. Beru/Motorcraft GP's only unless you want to pull the heads/ engine later when they swell up and crack off.
Anyway, mid 50s for me requires extra cranking with no glow plugs or block heater. It will do it but quite grouchy. I can start it on one battery and no preglow into upper 60s. Never tried less.
My money is on you have a junk starter. Verify you have Group 65 batts rated at 850CCA. Clean all your battery cable ends at the block too, there is hidden corrosion. A member named Knottyrope made an FYI thread to clean them all. Once clean cover with bearing grease to seal.
Starter wise, order up one for a 2003 Super Duty from Autozone or your preferred dealer. Most online purchases save about 15-20% and carry a lifetime warranty. You will be amazed how much quicker it spins.