Low Fuel Pressure Aux Pump Only

Bob,

Does the aux pump sound similar to the main pump when running? If it’s a voltage problem, usually the affected pump will sound slower and often the sound is not even and steady.  I had a similar problem with one pump that turned out to be a failing connection at one of the round multi-pin connectors on the bottom of the Rotax Engine Fusebox. 

Yes, they sound different from each other, but they always have from the start, and now have 120 hours on it.  This issue just cropped up during my last flight.  Since the main pump pushes fuel through the bypass valve on the aux pump, vs. the aux pump “sucking” the fuel through the bypass on the main pump, I rationalized that as the reason for them sounding differently.  But I could certainly be wrong in my assessment. 

Has this condition persisted since the first event or was it transitory?  In hot conditions I have found that the aux pump is more likely to vaporize the fuel slightly at the inlet, causing a lower fuel pressure.  I believe that is because the aux pump must draw the fuel through the bypass valve. I had that happen once when using winter mogas on a warm day after the plane had been sitting in the sun for a few hours. 

Assuming the problem is not a transitory event, then my first step would be to swap the electrical connectors at the pumps and see if the problem follows the wiring harness or stays with the aux pump.  If it follows the harness, then you are looking for an electrical problem.  If not, then you are likely looking at a problem with the pump assembly.  

Yes, Jeff. If you read my #1 possible cause it’s a bad electrical connection and my proposed troubleshooting method.  And the issue is persistent - does not improve.  And I’m running 50% 100LL to offset the winter blend high RVP.  Looking for other thoughts. 

Sorry Bob, by the time we got to the second round of comments I had forgotten about the details of your original post.  If you have completed your step 1 without a positive result, then I agree 2, 3, or 4 seem like the only other possible causes.  Everything else in the fuel system is shared by the two pumps.  If it were me I would remove the fuel pump assembly.  

The question is, what to do with it next. You could definitely troubleshoot the components, but there are no part numbers from Rotax for the individual pumps or check valves.  I believe the check valves are from Andair and you could get them direct, but this may be an instance where it’s better to send the complete assembly to a Rotax service center for testing and repair.  With the issues they have had with the pumps I would not be surprised if they helped you out with it.