912 ULS fuel pressure

[Dave C]

I have a question for anyone who flies with a 912 ULS and a mechanical fuel pump. How much fuel pressure do you see in cruise flight with the mechanical fuel pump? What is the maximum that you see?

I have been having issues recently with high fuel pressure during cruise flight. I have an Evektor Sportstar and recently have seen typical fuel pressure of around 6.9 psi in cruise flight. On one recent flight the pressure reached as high as 8.2 psi for a minute or two. On the next flight it had an excursion to 7.9 psi. The Dynon EMS has the fuel pressure redline at 7.2 psi

We connected a mechanical gauge up and on the ground verified that the EMS was accurate. In this case the fuel pressure was doublechecked from zero to 5 psi which was the highest it went during our ground testing. Fuel pressure from the electric fuel pump alone with the engine not running is 2.5 psi. With the engine running the electric fuel pump has no effect on fuel pressure.

We swapped the mechanical fuel pump out with an old "AC" pump that was replaced a year and a half ago. During test running and a flight with this pump fuel pressure was steady at about 4 psi the whole time. I have been made aware that by design this pump runs at a lower pressure. I am not sure typically how much lower that is.

I have identified three suspects that might cause the problem. All three sound unlikely to me but here they are:

1. The fuel pump went bad causing higher pressure operation
It seems unlikely that a spring in the pump would get more rigid with time or the pump would gradually increase its output

2. The fuel return is overly restricted.
This seems unlikely but it is probably the next thing that I'll look at. From my understanding the spring in the fuel pump sets how high the pressure can get and the primary role of the return is to prevent vapor lock. It is not clear that the return with its restrictor orifice could have a significant effect on fuel pressure. It would be interesting to know what the pump will achieve pushing against a complete blockage.

3. The fuel pressure transducer is bad
The seems unlikely because it passed our accuracy test but maybe it is inconsistent or diverges from reality in the untested territory above 5 psi.

[drseti]

There is a fourth possibility, Dave. The newest style mechanical fuel pumps have a third port for a drain line. It is intended for a hose to carry fuel away from hot engine components in the event that the pump's rubber diaphragm leaks. Normally, the hose is routed along a motor mount leg, back to the firewall and then down. The placement of that hose is critical, as its end mustn't be subject to positive or negative air pressure. Was the hose recently moved, rerouted, bent, or crimped? Did you maybe do something to it during your recent 5-year rubber replacement? You can try just replacing the drain hose - make sure it ends inside the cowling by six inches or so, not sticking down into the slipstream.

BTW, because of the pump's rubber diaphragm, it has a mandatory replacement interval of 5 years. Also, you say you replaced the AC pump a year and a half ago. There was a mandatory SB to replace it with the new style pump about a decade ago - your mechanic should have caught that during one of the many intervening condition inspections! This is a case where, though "mandatory SB" is not regulatory, compliance is a really good idea.

[Dave C]

Yes, a fourth factor. I did some math on this to get a ballpark estimate of how big this factor could be. If the tube was sticking out into the airstream and facing forward like a pitot tube the stagnation pressure at 100 kts (and std atmosphere) would be about 0.25 psi above ambient. This is not insignificant but it is small compared to the 1-3 psi higher than normal pressure I am seeing.

I do wonder what pressure is normal for the 912 ULS, the "newer" fuel pump, and a fuel system like in the Sportstar. Paul, what are you normally seeing?

Does anyone have a old ("newer" style) fuel pump that they replaced after 5 years that they would be willing to let me try out? I would love to be able to swap pumps to see if that makes the difference. If it works I would then gladly buy a new one (in case anyone is concerned)

[drseti]

I do wonder what pressure is normal for the 912 ULS, the "newer" fuel pump, and a fuel system like in the Sportstar. Paul, what are you normally seeing?

Less than 6 PSI.

Does anyone have a old ("newer" style) fuel pump that they replaced after 5 years that they would be willing to let me try out?

I think I have one in my hangar. Unfortunately, I'll be out of town for a few says.

[3Dreaming]

I have two removed for rubber replacement in the last month or so, and another one coming off soon.

[Dave C]

I have two removed for rubber replacement in the last month or so, and another one coming off soon.

I would gladly pay you for your trouble if you are willing to send me one. I am in Chapel Hill, NC

-Dave

[drseti]

I'm glad Tom's able to help you, Dave. I'm still in PHL, probably for a few more days.

[Dave C]

I do wonder what pressure is normal for the 912 ULS, the "newer" fuel pump, and a fuel system like in the Sportstar. Paul, what are you normally seeing?

Less than 6 PSI.

Does anyone have a old ("newer" style) fuel pump that they replaced after 5 years that they would be willing to let me try out?

I think I have one in my hangar. Unfortunately, I'll be out of town for a few says.

Thanks Paul. I realized I didn't really answer your question about the vent tube. It may have moved. I have to ask my mechanic about that. It was running straight back and over the top of the cylinders and I think terminating right after that.

[3Dreaming]

Thanks Paul. I realized I didn't really answer your question about the vent tube. It may have moved. I have to ask my mechanic about that. It was running straight back and over the top of the cylinders and I think terminating right after that.
[/quote]

From the HMM,

■ CAUTION: Drainage line have to be routed into a ram-air and
vacuum free zone, according to the requirements and
release of BRP-Powertrain. The drainage line must not
be routed into the slipstream. Ram pressure or vacuum
impair the fuel pressure.
- The lines have to be routed so that in case of damage the surplus
of fuel/oil is drained off suitably.
- Route the lines in a continous decline.
- The lines have to protected against any kind of blockage e.g. by
formation of ice.

[Dave C]

It definitely wasn't at a continuous decline. The were some horizontal sections and likely some small upslopes. I'll keep the slope in mind.

Thanks for the tip!

-Dave

[Dave C]

We installed the 5 year old fuel pump from Tom on Saturday and tested it out. Fuel pressure was a virtually constant 3.5 psi. It looks like it was the fuel pump after all. I am not sure what the physical mechanism is for the pump increasing pressure over time but it may not have been by much. The pump may have been operating on the very high side the whole time and I didn't notice because it was "in the green". It makes me wish I had data logging on the EMS so I could look back at the last couple of years of data.

[Dave C]

My Evektor Sportstar has no fuel pressure regulator. It relies on the pump and it has a small orifice in the return line to allow the pressure to build. Is this a typical fuel system arrangement? Is it popular to have a regulator to bring the pressure down to a known amount?

[3Dreaming]

The return lines primary purpose is to allow vapot to escape from the top of the fuel system on top of the engine.

I have never seen a fuel pressure regulator on a 912ULS installation.