I know this topic gets beaten to death, but here's a twist.

I've been having fluctuation fuel pressure from normal 16psi idle down to 9psi idle but ONLY if I have Power Service white bottle in the tank. If I don't have any additive I have solid pressure. Now it will drop to about 6psi WOT on my Auto Meter guage with no isolator the pump is going bad I know but why the problems with PS? While PS is in the tank I can drop FP way below 5psi I've tested it on several tanks with and without PS and every time I have PS in the pressure will drop but then return to normal on a WOT run. Not once has the pressure dropped flickered or done anything out of the normal on a untreated tank of diesel.

Ideas before I throw $$$ at this problem? I know don't add PS but that isn't the answer I'm looking for.

TIA.

 

Maybe the blades in the Carter lift pump get loosened up when slicker fuel is being used and can't move around quite as much with untreated diesel? Soulds highly speculative though.
Or could be that big piece of gunk in the gauge line that slick fuel can get around and pulsate, but regular gets stuck behind
-P