D8H Low Fuel Pressure

My service manual for 46A10725-up states a fuel pressure by pass valve setting of 20 psi. Did you check operation of the bypass valve?
Boost pressures is 35.0 +/- 3 in. hg. @ full load and speed. w/o muffler
31.0 +/- 3 in. hg. w/muffler.
Static A/F ratio control setting is 0.020"

I checked the relief valve in the pump, everything looked fine, gave the spring a little tug to try and stretch it and obtain a little higher pressure. Those things don't really stretch! The rubber gasket on the plunger looked fine and it didn't appear there was any crap in there holding the plunger up off the seat.

Is there another relief valve elsewhere I'm missing?

Just the one relief valve. Is the valve seat in good condition and seating tight?
I'd be looking for other restrictions in the fuel system, primary and secondary filter, fuel galleries, etc.

I would be thinking correctly? that this tractor has the compact injector pump fitted, with the low fuel pressure and lack of power I tend to go firstly to the little hand primer pump, that is the idiot proof 2 bolt job as this has a big effect on fuel pressure, it can be noted usually on trying to turn the Tractor each way with the engine dropping off power, I don't know if you have replaced this item but it is a simple thing to check. tctractors

Checked later service manual an your 32+/- psi fuel pressure is correct and a must at full load.
Static rack setting is 0.080".
Try separate pressurizing of the fuel system and check for leaks.

tctractors - I replaced the hand primer pump as the old one sounded like it was sucking air near the base once you stopped pumping. The new pump definitely moves he fuel but it never gets hard I can pump until I get a solid stream of fuel coming out of the bleeder valve, close it and pump another 15-20 pumps and it never gets hard. I wondered if maybe the hand primer has an internal pressure relief? I will say, if I run the hand pump while the engine is running the fuel pressure will jump to 25-30 psi but as soon as I stop pumping it's back down to 10-13 PSI.

Old Magnet - I took the entire filter housing down to its individual parts and blew air through every port hoping to dislodge something. NOTHING! I'm on my second set of secondary filters since i started trying to fix this. I tried running it out of a jerry can to eliminate anything in the bottom of the tank even though it inspects well visually.

Was thinking about that idea of trying to pressurize the fuel system up to the injection pump. I was thinking I could drill holes in some flat bar and blind off that 2 bolt flange going into the injection pump?

tctractors - I replaced the hand primer pump as the old one sounded like it was sucking air near the base once you stopped pumping. The new pump definitely moves he fuel but it never gets hard I can pump until I get a solid stream of fuel coming out of the bleeder valve, close it and pump another 15-20 pumps and it never gets hard. I wondered if maybe the hand primer has an internal pressure relief? I will say, if I run the hand pump while the engine is running the fuel pressure will jump to 25-30 psi but as soon as I stop pumping it's back down to 10-13 PSI.

Old Magnet - I took the entire filter housing down to its individual parts and blew air through every port hoping to dislodge something. NOTHING! I'm on my second set of secondary filters since i started trying to fix this. I tried running it out of a jerry can to eliminate anything in the bottom of the tank even though it inspects well visually.

Was thinking about that idea of trying to pressurize the fuel system up to the injection pump. I was thinking I could drill holes in some flat bar and blind off that 2 bolt flange going into the injection pump?

Thanks for the help guys, Just finished putting in the new transfer pump. 1.) the primer got hard after like 5 pumps which it hasn't been doing! Figured I was onto something! 2.) Started the engine and the fuel pressure shot right up to 30 PSI and it's actually making 45 PSI at High Idle. So I guess i wont buy any more Cat classic fuel pumps.

I never use classic parts if I can help it! Cat should be ashamed to sell such cheap quality parts! the old bs " oh we warranty it and will replace it" why the aggravation of installing and then removing the part again. What happened to "Only Caterpillar parts give Caterpillar performance"?