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engine miss when aux fuel pump turned off

  • Peter Mahoney
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1 year 4 months ago #13457 by Peter Mahoney
Peter Mahoney created the topic: engine miss when aux fuel pump turned off
recently on my 912is which is 18mths old I get a very slight engine miss when I turn of my aux fuel pump in flight. only lasts a second. does not happen when I turn off the main pump. if I have the emergency switch on it still does it. it does not do it again if I turn it back on after that unless I fly for a while. any ideas

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1 year 4 months ago #13460 by Rob Seaton
Rob Seaton replied the topic: engine miss when aux fuel pump turned off
Does the fuel pressure drop?

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1 year 4 months ago #13461 by Peter Mahoney
Peter Mahoney replied the topic: engine miss when aux fuel pump turned off
Only 1 bar same as the other one does for a brief second. I put it on computer and noticed a slight drop in fuel flow from main pump.
Only the first time I do it. After that all normal.

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  • Roger Lee
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1 year 4 months ago #13462 by Roger Lee
Roger Lee replied the topic: engine miss when aux fuel pump turned off
If it drops 1 bar (14.5 psi) for a second that would be enough to make it miss. This engine can't tolerate lowered fuel pressure like the carb version. Once pressure drops too much the engine will quit. On a test stand at Kodiak I pulled the fuel pump circuit breaker. The engine quit immediately.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 year 4 months ago #13463 by Peter Mahoney
Peter Mahoney replied the topic: engine miss when aux fuel pump turned off
They both drop the same but only misses on the aux pump and only the first time.
Any ideas.

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1 year 4 months ago #13467 by richard Boslaugh
richard Boslaugh replied the topic: engine miss when aux fuel pump turned off
perhaps one of the pump bypass check valves is getting sticky

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1 year 4 months ago #13469 by Roger Lee
Roger Lee replied the topic: engine miss when aux fuel pump turned off
If the pressure is a little low on one side of the fuel delivery when you switch then it will miss. Maybe the pressure is not equal on both sides for some reason?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 year 4 months ago #13470 by Rob Seaton
Rob Seaton replied the topic: engine miss when aux fuel pump turned off
The regulator takes a second to adjust to the slightly different pressure?
I notice the pressure is always different between the two pumps and the aux pump does have a different/longer route through the plumbing of the pump pack.

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8 months 1 week ago #15501 by Glenn Martin
Glenn Martin replied the topic: engine miss when aux fuel pump turned off
Hi Peter,
did you ever find a reason and or a solution to this problem ?

I have had this happen a couple of times recently also...most disconcerting !

Thanks

Glenn

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8 months 1 week ago #15502 by Peter Mahoney
Peter Mahoney replied the topic: engine miss when aux fuel pump turned off
Glenn I have not found a solution. I changed the fuel pumps and same thing happened. a mate who has the same engine has the same problem. my LAME says it is common and not a concern as the engine runs fine. I live with it and once I turn of aux in flight I have no problems.
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7 months 3 weeks ago #15604 by james porter
james porter replied the topic: engine miss when aux fuel pump turned off
I have experienced this and at least in my case, I think it may be related to vaporization of the fuel. Occasions when the condition is experienced or not experienced is temperature (and fuel) related. When doing an engine run up for the first flight of the day- normal. All operations during cool or cold weather- normal.
After a long cool down period in cruise flight- normal.
I have only encountered this condition when 1. using unleaded fuel with ethanol. 2. during warm or hot ambient conditions. 3. during or after prolonged climb to cruising altitudes. 4. with somewhat restricted fuel flows.(aircraft manufacturer plumbed fuel hoses from tanks to fuel valve with 7mm hose (slightly less than that called for by Rotax).
Using "Both" on fuel selector provides increased fuel flow since I used 5/16: ID hose from the valve to the engine.
If the engine compartment is "hot" and the fuel valve is on either Left or Right the engine will react to deactivation of the AUX fuel pump. If I have the fuel valve on "Both" (the fuel flow is increased) the engine will not react to the deactivation of the AUX fuel pump.
Since I very rarely use 100LL my observations may only reflect variances.
Using the above experiences I have been able to completely avoid engine disruption when managing the fuel pump.
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7 months 3 weeks ago - 7 months 3 weeks ago #15607 by Bill Hertzel
Bill Hertzel replied the topic: engine miss when aux fuel pump turned off
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?
Last Edit: 7 months 3 weeks ago by Bill Hertzel.

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2 months 3 weeks ago #16727 by james porter
james porter replied the topic: engine miss when aux fuel pump turned off
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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2 months 2 weeks ago #16849 by Paul Hamilton
Paul Hamilton replied the topic: engine miss when aux fuel pump turned off
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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