odd dips in fuel pressure, pump problem?

Hi Rusty

The pattern looks like vapor locking (vapor in the lines) 

Consider what the temperature is and try some high octane fuel.  ( I would suggest some AVGAS since it is a predictable level of the RVP of the fuel) Just my opinion. 

Cheers

Thank for the thought.  I only run 93 octane non-ethanol, and this has been going on for more than one fuel purchase. The only change since last summer is that I now have direct airflow to the pump, and more flow through the cowl than I ever did before.  I've monitored cowl temps as well, and they're much lower than last year.  That would make me not really suspect vapor lock. 

I went 77 hours with no fuel pressure issues, then this started about 14 hrs ago.  The engine never misses a beat, or runs rough, and you'd only know it happened if you were looking at the pressure.  The boost pump stops it, which does still sound like vapor lock, but I just don't believe it.  Mainly I'm just curious if this is a known failure mode of mechanical pumps.  

Rusty

Hi Rusty,

Does your 912UL engine have a fuel return line ?

I only ask because I have read, I think in this Forum, that the 80 hp engines often don't/didnt in the past,  have a fuel return.

I didn't initially have a return line, but added it after seeing some over pressure during ground operations.  Mine only returns to the gascolator though, not to the tank.  I made that change last summer, so nothing that would have caused this recent issue.  The engine has been really well behaved, so nothing has been changed.  That's the mystery.

Rusty

Randoms thoughts -

Returning to the gascolator - Could returning to such a small vessel be the problem in that fuel pump pulses may be magnified causing fluctuation in pressure readings?

Seems to me that your return should go back to a tank, via a Rotax recommend a #35 Mikuni jet. The restriction jet will tend to even out/dampen fuel pump pressure pulses on the delivery side. The tank will completely remove any pressure pulse influence on the fuel pump supply side.