916 fuel pressure problem

The accurate analogue pressure gauge arrived and I connected into the AN6 fitting in place of the N1EU70D.

I ran one and then both Rotax pumps and got the same pressures as above within a1 PSI (46 and 48 psi).  I believe this means the sensor is reading correctly and I do have pressure slightly above the specs.   

As far as I can tell, the only thing that is left is the pressure regulator or am I missing something?

The rotax recommended location of the primary fuel pressure sender is at the inlet side of the pressure regulator on the fuel rail. Rotax offers a fitting that replaces the banjo bolt at the pressure regulator inlet, which allows the connection of a sender. See the illustrated parts catalog, section 73-10-00 page 7. Going by your diagram, your sender is before the fuel rail. It makes me wonder is there would a be slight pressure drop across the fuel rail so that the pressure would be lower at the rotax recommendation measurement location. Seems like a long shot, but It might be worth getting the fitting to temporarily connect your manual gauge or sender at that location to see if there is a difference.  

My 912iS has the sender just before the fine filter, and I get a consistent 42-43 PSI using the same sender you are. I have the older style pumps that are mounted in a single duplex enclosure. I believe you have the newer, larger pumps that are delivered as individual units that you plumb in series. Your problem makes me wonder if, in some installations, the new pumps can overcome the regulator. 

I've attached a picture to be sure we're looking at the same IPC.  There seems to be 2 slightly different layouts shown for the pressure regulator at top of page in box and then at the bottom of page and I think you are talking about part 34?

My config matches the bottom of the page and doesn't look like it has room for part 34.  Maybe you have to swap out the whole regulator housing.

AFAIK All Slings have the sensor located the same as mine and as you say - the pressure should be the same (unless there's a restriction somewhere in the engine fuel rail).

Yes I was talking about part 34, and it was a long shot anyway and probably would yield the same result.  My first reaction to your problem was the same as yours, to make sure there was no restriction in the return lime so the regulator can do its job. At least you have now verified that the electronics are accurate and the return line is clear.

I would think the fine filter offers some resistance to flow, so make sure you really did install the (temporary) bypass check valve in reverse so that it is forcing the fuel through the fine filter.  Also, are you using the Rotax fine filter or another type?

And then of course you could try a new regulator, but I think it’s likely that will turn out to be a waste of money. I’ve got a couple old regulators left over from 5-year rubber changes, maybe you could find a shop locally that would have a used one around you could use as a test.

I know a few years back Rotax issued a service bulletin that raised the max allowable pressure by a few PSI, up to 46, which is what it is listed as now in the manuals. This was based on reports from aircraft manufacturers that they could not get the fuel system to meet the original spec. So it sounds like being on the high side is somewhat common.  I don’t have any experience with these new style (larger) pumps, can Sling offer any advice? Maybe they are seeing similar results with other builds.

I’m also curious if the fuel pressure would fall a bit with the engine running. You would not think there would be enough additional fuel flow to make a difference, but sometimes these things are counterintuitive. On my 912iS Bristell, the fuel pressure does not change with the engine running or not. 

Hi All

The newer pumps that Rotax recommends were to cover the very high volume that the high output engines have for fuel supply.  The smaller pumps are just not enough flow at full power.  The pressures however are the same, the regulator is the same.  Perhaps there is an issue with your return line or restriction with the check valves. (?) 

Cheers