Poor fuel or lift pump problems?

[leaf40]

Can poor fuel result in bad OPS/lift pump symptoms?
I have a 1993 Sierra 2500, 6.5TD, NV4500, that has been acting up. The truck would run fine when cold, producing 650* EGT and about 5 lbs. of boost, RPM's at 2750 to 3000, with no heavy load on a level to slightly uphill test run of about 4 miles. After several tests, the same route will end up with the EGT's dropping to less than 350*, and the boost drops to 0, there is a steep hill at the end of the run.
1) I changed the fuel filter, no difference.
2) I replaced the OPS, no difference.
3) I replaced the lift pump and the truck passed the test.
4)I drove about fifty miles and the problem returned.
I drained the fuel from the tank and put in 15 gal. from a different supplier and am testing once again. I also wired an OPS bypass switch to the lift pump to determine if the pump is loosing power.
I should note that the old lift pump does work.

Thanks in advance,
leaf

[gmctd]

The fuel pick-up tube in the tank has a sock-type filter on it, which can plug, restricting flow.

The inj pump also has an internal screen-type filter, but let's not think about that, just yet.

And, the Fuel Shutoff Solenoid may be slowly failing, as it warms up.

If not -

The lift pump should pump 15 gallons per hour. If you do not want to pull the tank just yet, run a clear hose from the pump output back to the tank filler, jumper lift pump power lead to battery, and see if it pumps clear for an hour.
Set it up for continuous pump - stop and start may hide the problem.

Or, disconnect the fuel inlet line, back-blow it with 20psi shop air to clear it temporarily.

Disconnect outlet fuel line at pump and the main filter, and back-blow it, also.

If a blockage is the problem, it's probably from the tank to the pump - the 'vacuum' side.