I would guess that the plunger started sticking, if it could fail both in the "open" and "closed" state . . . did you investigate what happened at all, or just pulled it out and sent it back?

So far as drawing down the fuel supply pressure: as engine speed increases, case pressure in the VE increases. As case pressure increases, return fuel flow through the pump increases (through the fixed orifice in the return line banjo bolt). The point is, while the volumetric capacity of the lift pump increases with engine speed, the volume draw increases more, for a given fuel delivery to the injectors. The supply pump might supply more than enough fuel for wide open throttle at 1800 rpm, but not be able to keep up with WOT at 2900 rpm even though the pump strokes/revolution remain the same. . . Not to mention that the restriction of the supply lines and fuel filter become effectively greater at higher flow rates. Meaning that the 3200 rpm governor spring can play a part in this calculation as well.

It is also possible that your fuel filter, or fuel heater, or something else has a restriction, or intermittent restriction. Did you check everything between the lift pump and the filter when you replaced the pump?

It just seems strange that things seemed to be working before, and now they aren't. I suppose it could all be due to poor quality replacement parts, but maybe not.

On a 1989, where would one plumb in a lift pump pressure gauge? Aside from the obvious gauge and hose, what would be needed? Do you need any kind of a snubber on this lower pressure application? Thanks!!

From Geno's https://www.genosgarage.com/product/.../gauge-related I put mine after the fuel filter in place of the bleeder.

If you're running a mechanical gauge then you'll need an isolator to keep fuel out of the cab if the line develops a leak. Since the isolator costs more I would go with the electrical gauge.

me too.

Tells ya when the filter is being affected by clogging its media.

Or not, if you're like me and change the filter annually, regardless of pressure variances.

I'm just wasteful, I tell ya!

I got hold of some bad diesel and I had to change filters every 2 weeks or so several times. I put the gauge on the figure out what was going on. I't real embarrassing to launch off the light and then the headlights are trying to bury themselves in the dirt.

I took a closer look at the engine and there are two banjo bolts right above where the fuel filter screws on, the banjo bolt right above the center of the filter has a small plug maybe screwed into it. Can the smaller plug be removed and a gauge put there for after filtered pressure lift pressure? If so, what thread size is the plug?
Could someone tell me on the VE pump there are two wires going to it, one goes to the FSS and the other to the advance mechanism? If I hook voltmeter to the advance solenoid when it's 35 - 40 degrees out I get no voltage with key on. What controls the voltage to it?

Thanks!!!

See above. That's where the Geno's product goes. It gives you a 1/8 inch pipe thread to connect a gauge.