engine miss when aux fuel pump turned off

by Peter Mahoney in 912iS Technical Questions 8 years ago

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Re: engine miss when aux fuel pump turned off

by Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.96k

The fuel flow path is the identical length for each pump. No difference!

What is different, is that the MAIN(First) Pump PUSHES Fuel through the Aux Bypass Valve,
And the AUX(Second) Pump PULLS Fuel through the Main Pump Bypass Valve.
These valves produce a 1psi pressure drop.

When shutting down the Main Pump, the flow in the bypass line needs to get moving and the valve needs to open.
The bypass line can heat-up to the under-cowling temp since there is no flow in the line when both pumps are running.
This can produce transient pressure drop on the low-pressure side of the pump producing a vapor bubble in the warm fuel causing the pump to lose its prime and the engine to stumble.
Once the fuel gets moving, it cools the line. Problem resolves!

When shutting down the AUX Pump the diversion path is on the High-Pressure side of the pump and does not cause an issue.

Note that this is the Opposite of your symptoms.

Neither pump is more functional than the other. They are identical Twins!
The Labeling of the two pumps is completely arbitrary. Main/Aux, One/Two, Left/Right, A/B, it doesn't matter, Pick one!
The electrical connector to the fuel pumps are identical other than the labels and can be arbitrarily swapped.
If your AUX Switch is actually operating the Main(First) Pump you will "Normally" never know the difference. Or care!

Try swapping the power leads on the two pumps and see if the issue switches sides.
If so, you will know that the issue lies Within the Fuel Pump Housing and is a mechanical and not an electrical problem.
If the symptom goes away, one check valve may be restricting slightly more than the other.

How hot does YOUR pump housing get in operation?

PaperThermometers?

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Re: engine miss when aux fuel pump turned off

by james porter » 7 years ago

james porter

14 78 70

A few weeks ago I experienced a complete engine failure shortly after takeoff. The OAT was about 100 F and I had just completed an enroute refueling with non ethanol MOGAS. After a very long taxi both fuel pumps operated normally at run up and maintained about 42 psi. Shortly after takeoff fuel pressure went to about 4 psi (with both pumps on) and the engine quit.
The landing was uneventful and I had to be towed back to the hangar to spend the night and depart the next morning in cooler temperatures.
The attendants told me that the pilot of another Rotax powered aircraft had aborted several takeoff attempts the day before and had to spend the night as well.
After installing a temperature probe on the fuel lines at the firewall I found that temperatures in that area (roughly the center of the fuel system) easily reached in excess of 65 C (OAT 90 F) in climb.
It should be noted that I have a 2" blast tube directed at the fuel system about 6" from the temperature probe.
Obviously the under cowl temperatures are too high and the design was not efficient.
I modified the cowling with an air dam and wrapped the exhaust pipes with thermal wrap. This reduced the climb temperatures at least 11 C. This also reduced my oil and coolant temperatures in both climb and cruise.
The only downside is that my oil temperatures do not reach 212 F during climb even on hot days.
I have paper temperature indicators on each of the rectifiers. One has previously recorded 71 C.
I assumed that the aircraft manufacturer had designed and tested the cowling and that it provided sufficient cooling. The U.S. representative had an engineer evaluate an overheating problem and as a result an air dam was added at the bottom of the cowling. The manufacturer then redesigned the cowling and added an air inlet (which my blast tube is attached to) but didn't add the air dam.
Rather than rely on the manufacturer a temperature probe could be added to verify that the temperatures are safe for the use of certain fuels and to warn of temperatures exceeding safe limits. This is a very inexpensive piece of equipment.

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Re: engine miss when aux fuel pump turned off

by Paul Hamilton » 7 years ago

Paul Hamilton

30 145 298

I have been having a similar problem. Preflight fuel pump checks at low OAT ambient temperature 60 degrees F and first flight of the day independent fuel pumps works fine NO hesitation. Hot ambient temperature OAT 80 AND hot engine after flight and pump check before next flight:
Both pumps on runs fine, Main pump off runs fine, both pumps on runs fine, aux pump off hesitation/missing and low fuel pressure. The hesitation/missing severity is directly related to how hot it is outside and how hot my engine compartment. Very hot outside and hot engine compartment it has died, not just missed. I can immediately restart it on both, or either and it runs fine. This is consistently repeatable.

I have been living with this because it is only the aux pump out where the missing happens and it always restarts.

Note the density altitude is high, flying out of 4700 MSL airport on hot days and I am running 10% alcohol 91 MOGAS.

Hopefully this will add to the knowledge base for this issue.

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Re: engine miss when aux fuel pump turned off

by Rodney Gamble » 6 years ago

Rodney Gamble

0 2 0

I had a drop in RPM in the circuit when switching off the Aux pump at a safe height after take off.
Result after LAME investigation = Rotax 912iS fuel filter canister needed changing after only 70 flying Hrs time (About 100 total engine Hrs since the last filter change) using 100% Aust MoGas.

LAME cut open the filter & it was stained. All MoGas was drained & replaced with AvGas & a new filter. The drained MoGas fuel appeared to be very clean.

With the new filter fitted to date after just a few Hrs I have had no further problems, main fuel pump pressure now 43.7 to 43.9 PSI in cruise on main pump.

Before the filter change the main pump was indicating 44.3 PSI so not a great difference in the pressure. I unfortunately did not note the pressure of the main pump at the time of the drop in RPM, as my reaction was to quickly switch on the Aux pump as per training

As a note I source my fuel from newly built busy servos. I always use a Mr Funnel & refuel myself at my property. I am now trialing the next 50 Flying Hrs using 100% AvGas only. I will report back on the condition of the filter after the trial.

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Re: engine miss when aux fuel pump turned off

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 7.96k

I am going to guess that you are referring to the filter AFTER the pumps.
Did you have a look at the filter BEFORE the pumps?

Ethanol in MOGAS = E-10 or E-0 ???

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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engine miss when aux fuel pump turned off

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