odd dips in fuel pressure, pump problem?

Hi Rusty,

I take your point that you have inspected the fuel tank screen, however I feel your logic regarding partial/occasional blockage is flawed - A partial/intermittent block would show as a random reduction in fuel supply (low pressure)  - this may be more pronounced as fuel levels drop and "sloshing" is more prounaunced.

If there was a piece of stuff eg tank sealant / large flake of paint / organic matter/ dead insects / fabric / etc would you have seen them ?- why not drain your tank(s), by removing the test valve, and filtering your fuel.  I did this recently, in preparation to remove my aircrafts wings and was truly amazed at the amount & variety of stuff that came out and I religiously filter all fuel into my tanks.

Your second paragraph - if you following Rotax recommended 5 year rubber replacement policy and you are using good quality hoses the likelihood of the hose degrading & causing your problem, is unlikely.

Last paragraph - 4 point manifold? Mine is a 5 point home made job. Sure you could put a T piece in the return line and as long as you are sampling fuel pressure befor the return line restrictor you will be reading fuel pressure in the delivery system but its not a great set up.

As I pointed out earlier - having your return fuel going to the gascolator, is not optimal and may be contributing to your problems - how hard would it be to return to a fuel tank?

Hi Sean,

The tank is clean.  I did drain it, and visually inspect the inside of the tank.  I also made a new fuel cart with a pump and big 10 micron spin on filter to make sure I don't introduce any debris.  I only fly locally, so I don't fuel from any other source.

Running a new return line back to the tank isn't worth the effort unless I have to take the front of the plane apart for some other reason.  I'm sure that will happen at some point, and hopefully it will involve fuel injection. 

I keep going back to why this started when nothing was changed.  The only remaining answer is the fuel.  I'm loath to use 100LL, even for a single tank, but it might eliminate the fuel question.  I'll probably take another look at the fuel lines, and any source of heat they may touch.  They aren't anywhere near exhaust, but coolant hoses are still 200 degrees, so they should be avoided as well.  

Fortunately, this is more of a mystery than a danger.  It doesn't drop below 3 psi, and the electric pump 100% resolves the issue if needed. 

Rusty

Certainly a mystery Rusty.

Depending on cowl configuration (air flow, etc) I am a bit of a sceptic when it comes to inflight fuel heating. On saying this I recognise Australian fuel types vary little, if at all, between summer & winter, with the possible exceptions being our "hill" country (otherwise known as The Australian Alps ha! & Tasmania in the far south) and my lack of 912iS knowledge.

My last 912ULS aircraft had very little fuel hose and no oil hose insulation, nor was the exhaust system "lagged". I only ever had fuel vaporisation issues on very hot days, on hot engine start, after flight/shutdown, never once airborne.

It sees to me that your fuel pressure fluctuations are unlikely to be temperature related (did you mention winter flying?) 

Couple more thoughts;

The fuel return line is not essential - have you tried shutting it off?

I assume you have a flow restricting jet in your return line. Rotax recommend  Mikuni #35 or similar. On my set up this delivers an average of a bit over 7L/Hr, back to the tank - you could try a smaller jet.

The fluctuations have only been happening for the past 5-6 weeks, so no idea about winter.  Of course at the rate I'm going, it will be winter before I figure it out.

I've thought about capping off the return as a test.  I flew the first 50 hrs or so with no return line, and only added it to eliminate some over pressure issues during descent and taxi.  The restrictor is home made, 0.015" which is essentially .35mm. 

Rusty

Russell

the Mikuni jet, number 35 idle jet, is not the size of the bore.  It relates to the flow amount.  The length of the bore plays a part in this.  The jet is not expensive so i would start there.  you can simply remove the return line as a test however that exposes you to perhaps overfill of the carb bowl with the newer (higher pressure) Corona fuel pumps as you have already experienced.  It also opens the door to vapor lock when the fuel gets hot.  

Good luck with the testing. 

Cheers