Fuel pressure test with new fuel pump

by Stephan Cassel in General Tech Discussion 10 years ago

Dear Rotax Owners,

My fuel pressure has always been close to the lower boundary in cruise. About 2.5 PSI.
With the new pump installed it shows 2.0 PSI in cruise. I have a return line with restrictor.
On idle it shows about 4 PSI.

I mounted a pressure test meter directly on the fuel hose and turned the electric pump on.

Interesting readings:
The high quality sender together with gauge shows 3 PSI
The pressure test meter (analogue) shows 4 PSI

I suspect that the so called high quality sender is to blame.

Now to my questions:
1. Is 4 PSI normal/acceptable value when engine is off and electric pump is activated ?
2. Can I assume that the sender fault i more or less linear, i.e that I have 2 + 1 = 3 PSI in cruise?
3. Is 3 PSI a bit low and for that reason should I install more restriction in the return line?

I understand that too high pressure will increase the risk for vapor lock.

Thanks for any comment on this subject

Best regards
Stephan Cassel
Europa Aircraft, Rotax 912s (500 hours)

Re: Fuel pressure test with new fuel pump

by Roger Lee » 10 years ago

Roger Lee

125 5.12k 4.53k

Hi Stephan,

4 psi is normal and with your testing you're certainly on the right path. Typically an electric fuel pump should add around .5 psi to the mechanical pump when running together. Sometimes these numbers can be off because too many senders aren't very accurate as you have found out and we have senders that read in tenths of a psi. From what I have seen out in the field 4-6 psi fuel pressure seem to be the norm with the new pump. My point on this is that if you had a pressure sender that read up to 150 psi and we used 50-60 psi like our oil system we would never really notice a 2-5 psi gauge inaccuracy. Many times the senders are to blame, but many people want to blame the pump. I commend you only testing the actual fuel pressure. I would replace the pressure sender. After that point and the pressure is still low then you could down size the restricting orifice. It is possible you may not have the right size in there to start with and worth double checking.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Fuel pressure test with new fuel pump

by Stephan Cassel » 10 years ago

Stephan Cassel

5 7 0

Hi Roger,

Thanks for quick reply. Yes, good idea to replace the sender. If I get 3 PSI in cruise and the
pressure tester gives about the same for different throttle settings (on ground) I will leave the restriction as
it is. If lower then 3 PSI I will down size the restriction. Due the risk of vapor locking in hot days it feels better
to be on the low side of the green arc.

Thanks again for help.

Regards
Stephan Cassel
LN-STE Norway

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Re: Fuel pressure test with new fuel pump

by James Ott » 10 years ago

James Ott

22 322 358

I have had a similar problem since the engine was new (now at 60 hours & it has the latest fuel pump) with low fuel pressure (1.8 to 2.1 psi) during full throttle climbout. But in cruise it seems to read OK at about 4-5 psi. Because the cruise reading seems about right I have concluded that the sender is OK, so I have not tested it against another pressure gauge. I am wondering if I need to install a smaller bypass restrictor, but the one I have now is 0.014" per Rotax recommendation. I hesitate to go any smaller for fear of potential plugging.

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Re: Fuel pressure test with new fuel pump

by James Ott » 10 years ago

James Ott

22 322 358

Roger, I am interested in your opinion on my previous post.

Jim

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Re: Fuel pressure test with new fuel pump

by Roger Lee » 10 years ago

Roger Lee

125 5.12k 4.53k

It is common on pressure switches that 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 (#35 Mikuni) restrictor in the
fitting (no restrictor will cause low pressure) You should check for
any fuel line restrictions on the supply side, kinks, restrictive fittings
or the like. The fuel filters should be replaced on a regular basis
depending on the type of filter used.

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. I would remove the fuel pressure sender and purge the fuel line up to the sender fitting to make sure all air is removed. Air will cause fluctuations in readings at different altitudes.

Put a mechanical gauge inline to test the pressure once a purge has been done on the fuel pressure sender line.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Fuel pressure test with new fuel pump

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