2012 prostar mf13 intermittent low power/topspeed

I know it's junk but it's what we got and it runs great sometimes.

History
New turbos a few years ago or less then 10k miles

First issue started as having to park Regen
They ran the bill up but never fixed it. It still has a code for emissions.

Next issue was if the truck sat more then a day it would crank and then stall and it was out of fuel. Have to crank for 3 minutes to get it to start.

Then the main issue. Throttle response was intermittent sluggish.

Took it to the dealer and they said it needed a new injection pump due to low fuel pressure. Huge quote of like $7k

I drove it one last time and it had no problems getting to the location

Hooked to a trailer and halfway through a hour ride I couldn't get over 55mph. Then 35mph. Throttle took a few seconds to catch up. It took two lights to get out of a turning lane.

So we parked the truck for a year. They use it as a yard truck.


So last week I decided to run some tests to make sure it is the injection pump. Drove it bobtail to Florida from Illinois. If I stay under 78mph is runs strong. If I hit 80mph it will get weak and struggle to hold 75mph.
Shut it off for lunch and it's strong again.

We replaced the filters. Ran a fuel line to the pump from the tank fill to rule out sucking air since it used to bleed down. I figure if it bleeds down it could suck air also and that would reduce fuel pressure.

No change.

It runs strong until I hit 80 and slow down. If I don't hit 80 it will be strong for hours(never tested long term)

My last test is checking the fuel pressure regulator. Maybe it is stuck? Was thinking about cleaning it and testing.

Before we spend big money we are trying to exhaust all possibilities.