I have 98 k3500 6.5 248k miles, recently replaced fuel tank , sending unit, sock. Supply and return lines, lift pump, and fuel filter. Truck ran perfect before the repairs, but now. After several attempts to purge air and prime the fuel system I can not keep truck running. To save the batteries and starter I jump the fuel pump relay to prime the truck. The truck starts and runs for a few minutes, no misses or surges while it runs. Then dies like no fuel. So I check the filter and the housing is always empty, vapor lock I would say. My question is if I'm jumping the pump relay it bypasses the ops, correct? If so, then I'm still pulling air, correct? Or can the ops still kill power to my lift pump even though I'm jumping the relay? I welcome any feedback as I need to know if I'm looking in the right area. Thank you.

 

The OPS is only secondary power for the LP on 96+ trucks-the ECM powers the LP through the relay normally.

The LP should have a prime cycle when the glows operate and then run constantly when the engine runs from the relay or the OPS-power cant be shut down by the OPS unless it is bad-AC Delco only!

Jumping the relay should make the LP run no matter what.

You might have a plugged vent in the filler cap causing excess vacuum in the tank,or you could have a big leak between the fuel level in the tank and the LP.

Did you make sure the O-rings on the fuel lines were good when you changed the LP?
What brand LP are you using?
What brand sending unit did you use?many aftermarket sending units are being shipped with socks that are too fine a mesh for diesel-i tell folks to reuse the GM sock or remove the sock completely and install an inline screen or filter pre LP.

 

Thank you for the advice, I have not been able to work on the truck as recent snow has in my area has kept me busy and snowed in the truck. Shade tree mechanic, it was a mid 40's weekend in feb. when I started this project. I thought no sweat replacing the fuel tank. Well Ive learned better now.

The tank and sending unit are spectra I believe, they were what was immediately available the day I dropped the tank. Fuel lines are completey new from tank to lp and return union. I did not replace the sock has racer 55 stated but will as soon as I can while checking all o rings once more. I did find a thread on another forum about five years old of another truck owner experiencing the exact same problems. It was also reccommended to replace the sock, but did not solve problem, but I will do it because I do remember the replacement one looking inferior to the stock one. Hopefully this will solve it. I will update as soon as completed. Also the lp was replaced with the highly reccommended year 93 version Ac delco. This appears to have done nothing as the old one was performing just as well, but since I didnt know the age it didnt really hurt to try.

My first diesel was a 1982 6.2l 4 door camper special then I upgraded to a 95' 6.5td half ton now its a 98' 6.5td k3500, I love these trucks all of them were plus 200k when I bought them and all ran dependable with minor maintainance. Its hard to believe this fuel tank thing has beating my head, but I'm sure I can get worked out with all the helpfull advice from this forum's members. So thanks once more.

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also removed filler cap very early in the process, no change either way,