916 fuel pressure problem

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

Rotax Wizard wrote:

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

 I ran a test with the fuel circuit split directly after the regulator and directing it into a container.  No drop in pressures - so the return line was eliminated.  

Are you referring to the check valves at the pumps or the one bypassing the filter?  I can't see how any of those could increase pressure?

Jeff Blakeslee wrote:

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. 

I will check with the Dealer to see if they have a spare regulator on Monday. Unfortunately, there's no dealers in Ireland.

I first noticed the problem when the engine was running (~2500 rpm) and the pressure was about 1psi higher than the equivalent statis test.  I put this down to the pumps getting ~14v of Alternator power instead of 12.5v from just the battery.  Not sure if this is valid though.