03 duramax won't start

Can't get my 03 duramax to start. Last time it ran was middle of Jan., went to start it in May it ran about 10 sec. & shut down. I pump up primer pump until it gets hard & wait a few seconds & it goes soft. If I pump up the pump & have someone start it, it will run until I stop pumping. I replaced the fuel lines last summer & fuel filter last summer & ran fine until I let it set over the winter. I replaced the fuel lines on the tank still can't get it to run on it's own. Could you tell me where to go next. If it helps I live in northeast Ohio & the engine heater was plugged in all winter.

Welcome to the Forums!

Are you "just" pumping the primer? First, it should be bled, by removing the (OEM plastic) screw on top of the filter assy. Pump until fuel is present, close the bleeder screw (not too tight, it's plastic), then pump again until it's hard. It is normal for it to not remain hard after a few seconds/minutes. If it starts/stalls again, check again for air. If has air again, either the filter element isn't sealed to the assy (very common if the canister is the plastic model), you have a leak between the tank and filter assy, or the filter assy itself is leaking. I recommend replacing the filter element, in any case. At least R/R it to ensure it is sealed properly. Once the filter is removed, plug the center hole (thumb, finger, etc.) and pour the fuel into a clear jar. Let it set for a while and see if something settles out. Any contamination/debris will plug the filter, causing it to exploit any weakness in the lines, normal or not.