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Fuel pressure fluctuations solved

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PostPosted: Thu Jul 11, 2019 12:48 pm    Post subject: Fuel pressure fluctuations solved Reply with quote

Hi everyone.

I thought I'd provide an update on fuel pressure problems I was having and how they eventually were solved, hoping this might help others who may run into that problem themselves in the future.

Ever since my RV-10 was built, I had issues with the fuel pressure when on the engine pump (boost pump off). On the ground, it would produce 25-27 psi, but as I climbed higher, the fuel pressure dropped to around 20 psi. That wasn't the biggest concern though - when at higher altitudes (>9000' or so), periodically there would be a drop in pressure. It would usually last only 10 seconds or so (hard to tell exactly because my EFIS filters the pressure readings) but would often dip to 16 or 14 psi (the red line for the IO-540 D4A5). The higher I got, the more frequent it happened, and sometimes I'd see a dip down to 12 psi (once, 10 psi). The engine never stumbled, though I don't know how close it may have come to doing so, considering the drops in pressure never lasted for long. Nevertheless, it was something I was always watching closely and never trusting.

I tried all kinds of things to fix and isolate the problem (insulating scat tubes in the tunnel, checking all fittings for tightness and leakage, temporarily bypassing a wing root gascolator (I'm in Canada and they're required), and nothing fixed the issue. 2 years later at my last annual, I figured I was going to get this solved once and for all.

As many of you know, Van's suggests placement of the red cube fuel flow transducer in the tunnel, upstream of the engine pump. Turns out that was the issue (I know some of you had already come to that conclusion, so I'm adding another voice to the choir for that problem). In contacting Electronics International (makers of the FT-60 red cube), they said that absolutely their transducer shouldn't be located upstream of the pump in a "suction" configuration like this. Their suggestion was to place it between the engine pump and throttle body (many of you have placed the transducer between the throttle body and spider, but they said that's a better location for those with a fuel return line which I don't have). I don't know the rationale of choosing one location post-engine pump over the other, but that's their claim. In any case, having it on the positive pressure side of both pumps was their suggestion, so I moved it to a point midway in the line between the engine pump and throttle body.

The difference was night and day. I now see consistent and stable fuel pressure readings of 25 psi even at higher altitudes. I did a 6 hour sequence of flights awhile ago at altitudes varying between 13,000'-16,000' and never saw a single fuel pressure drop during that time. That was unheard of for me before. So I'm calling this particular issue, that had been dogging me since the beginning, fixed and resolved.

So my advice to anyone who hasn't yet done their fuel system - from my experience, definitely do NOT put the fuel cube where Vans tells you to. Instead, put it where the manufacturer of the fuel cube says - between the engine pump and throttle body if you don't have a fuel return line, and throttle body and spider if you do (perhaps either location is fine, but definitely you want to have the cube on the pressurized side of the fuel system, as presumably otherwise it can apparently cavitate and cause bubbles). Obviously, having the cube in close proximity to the hot engine means you also definitely need to wrap the cube and surrounding fuel lines in well-insulating firesleeve too. And remember to orient the cube with the wires pointed upwards, as well.

Hope this helps someone!

Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213

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