2003 cummins having problems with starting cold weather 35 degrees here leave block heater plugged in all night still no start next morning have replaced relays for grid heaters and also ambeant air temperature sensor cleaned battery connections ground tested heaters do get hot they only stay on for few second then on again few seconds later. also second factory lift pump on fuel filter housing runs for about 30 seconds what do yall think

 

I'll bet you have a fuel system issue of some sort. If you have your trucks block heater plugged in you would not need your grid heater to work at all. Your 100% sure your heater is working? Often times your engine itself will be warm enough to not call for the grid heater to turn on.

 

35* is getting kinda cool for #2 fuel. Are you on a winter fuel or at least a winter blend?

 

I don't think we have that kinda fuel here

 

See if you can't find some #1ULSD. If you can't, mix in some kerosene, or a touch of unleaded.....

 

I disconnected the grid heaters on my '03 and ran without for several years. Temps down to the high teens with no starting issues. Your 15 year old sticks may be due to rebuilding or replacement. You can send them out for testing and determine what you want to do with them. When the time came for mine, I went with F1 and had them tested and rebuilt. No complaints with F1.
Back to morning brew . . .

 

Thanks FMB

 

I doubt it is the fuel gelling at 35 degrees-never had a problem around here @25F with #2.
Good luck finding your problem.