Just read your post, I have little experience with the older machine, seems to me that if you have a good gravity flow of fuel from the tank than its more than likely a transfer pump issue. The check valve you refer to is probably a pressure regulator. If the spring and seat look good I would suspect a transfer pump. I'll check a manual and see how the fuel is delivered and regulated to the service assembly.
Use the search function in the upper right of your screen!
In the past 9 months there was an extensive series of posts on rebuilding the D2/D4 fuel transfer pumps. Plus there were a couple of posts of almost the exact fuel flow issue it sounds like you guys are having.
word of caution, don't rebuild the transfer pump without putting a filter between the tank and pump. rusty fuel tanks r the biggest cause of tranfer pumps failure. good luck.
I have the transfer pump off and I can .006 between the gears and .005 between the pump drive gear and the housing. The other gear I can only get .003 in between gear and housing. Is this excessive? What I did find when taking the pump off was the black hard plastic plunger that goes to the spring. I guess this is a relief valve of some sort? So with this gone is why no pressure at all and maybe if the pump is wore is the reason for low pressure. Should I get another pump?
If your fuel pressure goes into the green when the engine is running, you're in good shape. The engine will start if there is any pressure feeding fuel into the injection pumps. It is pretty common to find the button on the end of the spring is hard and not sealing properly against the cast iron block. The block can be slightly eroded as well. This can be filed flat if care is taken or lapped on fine sand paper laid on a glass pane or even surfaced if you want to go to the trouble.
An in-line strainer or filter is not a bad idea even if you haven't rebuilt the transfer pump. 👍
Most of the time fuel pump check valve is not working properly, it is a very simple thing that does a lot in the fuel system, holds a set pressure, bypasses fuel as filters get plugged keeps the pump from dead heading against itself and causing excessive wear, So that little spring and puck is pretty important in there! The fuel gauge comes after the filters, so the gauge shows dirty filters or check valve misfunction, or bad pump!! I think i got it all!! good luck!
0.004 " clearance between transfer pump gears and housing is considered worn out.
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