John Deere 6620 combine fuel problem resolved

Thanks for everyone jumping in to help me solve the mystery as why my John Deere 6620 would only run 30 seconds or so. After I had went through everything from the tank to the injection pump and found no problems I started over again. The hand pump/plunger on the combine was leaking out the top when pumping so I replaced that. No cure. I did put a new style hand pump on. Then I was in the process of gravity feeding diesel fuel to the transfer pump on the side of the injection pump and found that I hadn't bought a long enough rubber hose to hook it up properly. So, I just threw on a new electric fuel pump and the engine works just fine. Seems like the hand pump always leaves a little air in the filters at the top. Now with the new electric pump the transfer pump seems to push the air space right out and can fill the filters entirely up. My old electric pump operated, but it must not have been building the pressure/gpm necessary. It would bubble fuel out of the pipe to the transfer pump about 3/8" to 1/2" high. But, by the time you figure what the engine is using and what the injection pump is returning to the tank the old pump just couldn't keep up I guess. Thanks again.

Yep.

My suggestion of "teeing" into the feed line at the transfer pump on the IP and monitoring lift pump pressure would have "outed" the weak lift pump in all of about five minutes.

Thanks Bob. I didn't have all of the fittings to accomplish that. I probably had some gauges that would have worked. You had it pretty much pegged. Thanks again.