Fuel Pressure

I just had my second fuel pressure sending unit go bad. Before you ask, yes, I have it mounted on the fire wall. Does anyone have a good reliable model that works well on the Rotax?

What happens when they go bad?

When mine fail my fuel pressure drops very low. I installed another pressure gauge in the line and I still have good pressure, but my Dynon shows only 1.5 to 2.0 lbs. I replace the pressure sensor and the Dynon reads good again.

What pressure sender are you using? Location can make a difference. Is the pressure sender behind any exhaust pipe, muffler or exhaust outlet? If it is you can reduce the heat affect by wrapping the exhaust pipe. Heat will cause an untimely death. When mounting to the firewall use some type of vibration isolation if possible. When mounting the pressure sender on the firewall it is better when it is slightly higher than the balance tube for the carbs. I have been using the same fuel pressure sender for almost 6 years without any issues, but it is mounted on the firewall, high to drain fuel away from it when sitting and vibration isolated.

That said you can get a bad sender from time to time.

I'm using VDO sending units. I do have the units mounted high but the only thing I don't have is something to absorb the vibration. They are not mounted near any of the exhaust. When I use the mechanical gauge inline I am getting over 5 PSI. I'd like to a more reliable sending unit.

VDO senders have been used on the Rotax for a long time. That said they do go bad and mounting can be a major contributing factor.
There are other sending units on the market, but if you have the SLSA you would need an LOA from Vans. VDO's can be sensitive to vibration and that may be all that is killing yours.

https://store.dynonavionics.com/cgi-bin/commerce.cgi

The 15psi Kavlico is what you are looking for. But it will require a small wiring harness modification.

Dynon have moved away from VDO to Kavlico. You can get more info from the Dynon site if need be. It is an easy change out with a small wiring modification. As Michael points out it would be a good swap.

From another thread:
If you intend to buy the new version fuel pressure sender,
Dynon part number 101690-000
Carbureted Fuel/Fluid Pressure Sender-Kavlico, 1/8-27 NPT, 15 PSI , $85
... don't get it from Dynon - they charge a $20 fee plus shipping. In fact they recommend that you use one of their retailers. Aircraft Spruce is one of them, their part number 10-04820, DYNON KAVLICO CARB FUEL SENSR, $83 plus $2 shipping by mail.

My fuel pressure runs around 5 psi, at the top of the green and close to the yellow. I installed a switch to cut the electric fuel pump, mostly for not being bothered when I need the master on while in the hangar. A couple of time I cut the electric fuel pump in flight and was scared to see the fuel pressure drop to between 1 and 2 when operating with only the mechanical pump. I did not see any change in RPM (all this checked with savyyAnalysis) but it scared me enough to leave the electric pump on in flight. Note that with the engine off the pressure from the electric pump reads 3.7 psi, consistent with the mechanical pump of 1 to 2 for the total around 5. My question: is it normal to get such a low pressure from the mechanical pump alone? I don't remember seeing this behavior in the Sportcruiser in which we systematically stop the electric pump outside the take-off or landing phases.

I also have a switch for my electric pump. I actually sometimes have to switch it off in cruise because the pressure is running consistently in the yellow with both pumps working simultaneously and with a full tank. I can't recall the exact numbers but without the electric pump running I usually see the pressure in the mid green range when in cruise, 4-5 psi maybe.
I had experienced low fuel pressure early on only when in consistent WOT long climb outs due to a defective pump (my bad, not the manufacturer's). Back then the pressure would drop below 2psi and the engine had slightly less power/rpm but maintained the climb. The power loss was very minimal though, so you might really be able to feel it if you are not closely watching the engine parameters.

MacPara said:

I also have a switch for my electric pump. I actually sometimes have to switch it off in cruise because the pressure is running consistently in the yellow with both pumps working simultaneously and with a full tank. I can't recall the exact numbers but without the electric pump running I usually see the pressure in the mid green range when in cruise, 4-5 psi maybe.
I had experienced low fuel pressure early on only when in consistent WOT long climb outs due to a defective pump (my bad, not the manufacturer's). Back then the pressure would drop below 2psi and the engine had slightly less power/rpm but maintained the climb. The power loss was very minimal though, so you might really be able to feel it if you are not closely watching the engine parameters.

Click to expand...

It seems that I have the same symptom. What was bad with your fuel pump?

I had pressure tested the fuel supply and return line system during the build. Unfortunately I decided to use water as a gas substitute. After the test I vacated the system from the water the best I could and then it sat another year before the fuel system was finally filled with gas.
In the meanwhile the pump had internally rusted. It worked fine but restricted the flow too much on WOT which was noticeable in a slow but constant pressure drop (over a few minutes). Also the flow rate was very dependent on the tank fill level as if it needed some assistance from gravity to push the juice through.
I replaced the pump (nasty job with a flaperon autopilot servo installed), flushed the system and the issue was resolved. Next time I'd be testing for air tightness instead of fluid tightness of the fuel system ...
Did you do the 1 gallon drain test as part of the PAP? I had not (builder arrogance) and it would have indicated a flow problem had I done it. I did the test before and after swapping the pump to verify that the issue was resolved.

It is common on pressure switches to have air trapped prior to the
sender to get incorrect readings. The mechanical pump is not actually
operating at 1 or 2 psi.

You should check that the return line has the CORRECT restrictor in the fitting (no restrictor will cause low pressure) Also you should check for
any fuel line restrictions on the supply side, kinks, restrictive fittings
or the like. The fuel filters should be replaced or cleaned on a regular basis
depending on the type of filter used. At annual.

Given that the aux pump is inline (series) with the mechanical pump it is
normal to get a higher reading with both running. The key is that they
do not over or under-pressure the system.

In many such complaints in the past in other aircraft the most common
condition that causes this is air found between the sender and the pump.
The air will damp the readings and give a false number.
Air in the fuel pressure sending unit line will cause lower pressures. If you have these low readings I would remove the pressure sender and purge any and all air from that hose right up to the sender fitting. I would also put a mechanical gauge inline after the purge to check the pressure.

Thank you Roger, I will investigate this sender issue.