Have 2006 Cayenne. Very confusing fuel pressure issue.

I have that typical problem where the engine dies after a few seconds. Already determined that it is related to when the Left fuel pump is engaged - fuel pressure at the rail goes to ZERO. I know the typical thing to suggest is that this pump is failing.

I have already manually tested the flow from both pumps to be strong.

So first diagnostic was to test pressure at the rail.
I manually apply 12volt to the RIGHT pump and note the pressure at the rail
I manually apply 12volt to the LEFT pump and note that it is ZERO at the rail

But -- I have actually tested at the output of the fuel filter.
I manually apply 12volt to the RIGHT pump and note the pressure at exit of filter
I manually apply 12volt to the LEFT pump and note that it is also the SAME pressure as the RIGHT at exit of filter

Noting the flow out of filter should have no bearing on which pump is running.

What gives? Thoughts?

 

This is the operating method for the fuel pumps

 

Thanks, good details to know.
But this is not my problem.
When the Left pump runs there is no pressure to the fuel rail -- but there IS pressure out of the filter that is supposed to feed the fuel rail.

The issue is same as others that say have to remove fuse #14 -- but the pump runs fine and both create pressure.

 

Turns out that when someone replaced the fuel pumps, there was one of the fuel hoses in the tank hooked up incorrectly.
Mixed a suction line with a pressure line.

Mark this solved or just delete this thread.