912iS fuel manifold check valve flow direction

I believe I found the answer here. If that post is correct, then my check valve thinking was wrong. Its actual purpose is to allow unfiltered fuel flow to the engine IF the fine filter gets clogged. So, I currently have the check valve backwards (allowing flow back to the manifold), but it needs to allow fuel to flow from the manifold to the "tee" (see drawing).

I am curious though, how would one know that the fine filter is clogged and bypassed? Will that be reflected on the G3X is some sort of over pressure anomaly?

Steve,

You answered your own question correctly! The way the system is plumed, the 17 PSI is not gauge pressure, it’s is the differential pressure between the inlet and outlet of the fuel filter. The pressure after the filter is regulated by the engine fuel pressure regulator, let’s call that 43 PSI nominal. The fuel pressure on the inlet side of the filter will be at least as high, but will go up as the filter gets loaded, whereas the post filter pressure will remain constant. If the pressure difference pre and post filter exceeds 17 PSI the check valve will let unfiltered fuel flow (towards the T) to the engine fuel rails.  

The location of the sender port on the manifold will show you the pre-filter pressure. While this does not necessarily show the precise pressure at the fuel rails, it’s typically very close and also lets you know when the filter is getting dirty. A clean filter will show you a pre-filter pressure very close to the post-filter pressure, about 43 PSI. So when you start seeing higher pressures (>45 PSI) it’s time to change the filter. This gives you plenty of bandwidth to change the filter long before the bypass would activate. So to answer your question, if you are seeing a pressure 17 PSI higher than with a clean filter, you can assume unfiltered fuel is flowing. That would be circa 60 PSI displayed.  

Also, fuel pressure on the iS engine is differential pressure referenced to engine air box pressure, not gauge pressure. So you should use a differential type fuel pressure sender with an air tube connected to an airbox port as its reference. That way the sender is seeing the same reference pressure as the engine’s fuel pressure regulator.