Have you taken a fuel sample into clean glass jar and let it sit for inspection of contamination?
When you opened the drain checking pressure (via it flows out instead of sucking air method) — did it studder while you had valve open? No effect of flow during studder? If you are not 100% positive on this, do it again while checking for sample. You might need to fill a jerry can from the drain waiting for it to studder and see if there is any change in flow. Any change could be a pressure related issue.
Truly a pressure gauge is best to see.
@WarWagon had the dreaded fuel line flapper right at the ip inlet that caused him tremendous hard to find issues. I have seen it multiple times over the years. That why I am so strong about a T in metal fittings right at the ip inlet. I’ll see if I can find pics he sent me incase he doesn’t post link to his here for you to see example. This can happen anywhere in the system, but th only place to read it is at the ip.
Remember, fuel pressure gauge on the dash will
Pay for itself when liftpump dies and you are wearing out the ip so much faster and dont know it. Don’t need fuel line in the cab- electric gauge and a sensor only at the ip- 1 wire to bring in the cab.
Another possibility is fuel you are buying. They changed to winter blend near you? If so and problem When away, then return as they go back?
Turning on A/C requires raise in rpm at idle for compressor load. Ds4 uses pcm to do this iirc. Db2 has a solenoid. Anyways- if you turn on the a/c toax recycle mode (fan speed doesn’t matter) it should hold rpm higher and steady. See if that affects it.
