Random Low Fuel Pressure Indication.

[skippydiesel]

16 hours ago, Red said:

Skippy you have misunderstood, we are talking about the outlet valve in the mechanical pump failing open.

I'm gonna leave this, busy today maybe Nev can explain

 

P.S. its highly unlikely that is your problem anyway.

I know exactly what you are saying - read my long winded explanation  again.😈

[onetrack]

1 hour ago, facthunter said:

Filters are a problem in many fuel systems. Nev

Despite Skippys regular praise of Hengst inline fuel filters, I have found from long experience (60 years of fixing mechanical things), that those small inline plastic housing filters containing cellulose-treated paper as the filtering medium, are a prime source of fuel flow restriction and blockage. Even a partially-plugged inline filter increases the load on a fuel pump and shortens its life.

Remember, these little filters are the cheapest filter you can purchase, and it's hard to find specifications on them as regards construction materials, because they are cheap, and the lowest level of filtering you can acquire. A fine mesh screen filter is a preferred choice for primary inline fuel filtering.

 

https://www.industrialspec.com/about-us/blog/detail/screen-versus-paper-small-engine-fuel-filters/

[skippydiesel]

51 minutes ago, onetrack said:

Despite Skippys regular praise of Hengst inline fuel filters, I have found from long experience (60 years of fixing mechanical things), that those small inline plastic housing filters containing cellulose-treated paper as the filtering medium, are a prime source of fuel flow restriction and blockage. Even a partially-plugged inline filter increases the load on a fuel pump and shortens its life.

Remember, these little filters are the cheapest filter you can purchase, and it's hard to find specifications on them as regards construction materials, because they are cheap, and the lowest level of filtering you can acquire. A fine mesh screen filter is a preferred choice for primary inline fuel filtering.

 

https://www.industrialspec.com/about-us/blog/detail/screen-versus-paper-small-engine-fuel-filters/

Dear oh dear!  do your research -

 

First - The fuel filter topic has been exhaustively debated, elsewhere on this  Forum - this "thread" is about intermittent fuel pressure drop & what may be the cause.  

 

The Hengst H 102 WK / H 103 WK  (& similar Baldwin ) filters, I have recommended,  do not have pleated paper type filter medium - they are ALL gauze/mesh and are used by a number of factory supplied aircraft.

 

A very quick read of the article you have referenced,  makes a few good points (mostly "motherhood type)  but also has factual errors  eg Stating that a "Paper"  has necessarily a lower flow rate and captures final particles, than a gauze & visa versa, is plain BS.

 

The gauze filters I have recommended, are not the cheapest, or the smallest, have all been available for many years, used in many applications (stood the test of time) however I do agree finding specifications can be difficult to find. Try writing to the manufacturer.😈

[BrendAn]

On 21/04/2025 at 5:26 PM, 3rd harmonic said:

Looks like my memory wasn't exactly accurate, found on the RAAus accident/occurrence page:

OCC0871 28/12/2016

OCCURRENCE DETAILS SUBMITTED TO RAAUS: DEFECT: Failure of the outlet valve in the fuel pump caused a loss of power on take-off. Take-off aborted and aircraft landed on remaining runway. Part of the valve is pressed into the body of the pump. The part pressed into the body came out after - the outlet valve remained open which caused a drop in the fuel pressure. Note: the standby electric pump was 'On' however also lost pressure through this failed valve. OUTCOME: This aircraft is an LSA and as such this defect was reported to the manufacturer. Technical Manager reviewed the aircraft logbooks which was provided by the maintainer. The aircraft logbook showed that it has high hours and appears the fuel pump has failed due to fair wear and tear.

does not make sense. the electric pump doesn't care about an open valve on the Mech pump. it would if the valve somehow jammed closed, don't think that would be possible though.

Edited yesterday at 05:14 AM by BrendAn

[BrendAn]

in think thruster88s theory is most likely. why don't you replace the mechanical pump. if thats not the problem you will have a spare.

change filter first obviously.

[skippydiesel]

18 minutes ago, BrendAn said:

in think thruster88s theory is most likely. why don't you replace the mechanical pump. if thats not the problem you will have a spare.

change filter first obviously.

Thanks BrendAn,

 

I try hard to fight the, all to human, urge to go for the quick fix, that usually involves many dollars. Consequently, replacing the mechanical pump ($600?) is the last thing I will do SUBJECT a future symptom which steers me directly down this path.

 

My feeling is that, the random nature of this fault, that has not resulted in any apparent fuel flow interruption, despite gauges saying that is what is happening, is likly electrical/sensor/connection.

 

Part of my fuel supply inspection/ review will be the check the only filter in the system, a gascolator - personally I think they are a hangover from the past that have little to offer, over in line gauze filters. If the gascolator had not been supplied with the aircraft, I would not have purchased one. The gascolator filter area is small compared with available in line filters, fiddly to service and I at least always end up with fuel all over my hands, require periodic seal replacement, rely on a compression fit to seal (always suspect) are heavy & costly.

 

I am about to start a 50 hr service - will focus on fuel reticulation system and electrical connections.

 

If all else fails (hopefully not the noise up front) I will replace the mechanical pump.😈

[Thruster88]

The fact that two independent sensors, pressure and flow, show a change at the same time and the change goes away with operation of the boost pump suggests that the problem may be real. How old is the pump in calendar from first use? They are supposed to be changed at 5 years so not a total waste of money to do it now.

 

One thing you could do is remove the inlet fitting and inspect the internal filter screen, see the post above by Underwood.

[Red]

1 hour ago, BrendAn said:

does not make sense. the electric pump doesn't care about an open valve on the Mech pump. it would if the valve somehow jammed closed, don't think that would be possible though.

But you (and apparently everyone else here) is dismissing the fact that the mechanical pump was also still operating in this scenario but with an open outlet valve.

I'm baffled that apparently nobody else (except the blokes who conducted the investigation) can see how that can reduce pressure.

 

 

[BrendAn]

25 minutes ago, skippydiesel said:

Thanks BrendAn,

 

I try hard to fight the, all to human, urge to go for the quick fix, that usually involves many dollars. Consequently, replacing the mechanical pump ($600?) is the last thing I will do SUBJECT a future symptom which steers me directly down this path.

 

My feeling is that, the random nature of this fault, that has not resulted in any apparent fuel flow interruption, despite gauges saying that is what is happening, is likly electrical/sensor/connection.

 

Part of my fuel supply inspection/ review will be the check the only filter in the system, a gascolator - personally I think they are a hangover from the past that have little to offer, over in line gauze filters. If the gascolator had not been supplied with the aircraft, I would not have purchased one. The gascolator filter area is small compared with available in line filters, fiddly to service and I at least always end up with fuel all over my hands, require periodic seal replacement, rely on a compression fit to seal (always suspect) are heavy & costly.

 

I am about to start a 50 hr service - will focus on fuel reticulation system and electrical connections.

 

If all else fails (hopefully not the noise up front) I will replace the mechanical pump.😈

$600 for a Rotax pump. Better tell Wal he is selling them too cheap..his are half that

[Blueadventures]

4 minutes ago, BrendAn said:

$600 for a Rotax pump. Better tell Wal he is selling them too cheap..his are half that

Wal about $450-

[skippydiesel]

18 minutes ago, Thruster88 said:

The fact that two independent sensors, pressure and flow, show a change at the same time and the change goes away with operation of the boost pump suggests that the problem may be real. How old is the pump in calendar from first use? They are supposed to be changed at 5 years so not a total waste of money to do it now.

 

Pump/engine has been in operation about 18 months - Hobbs 150 hrs 

 

One thing you could do is remove the inlet fitting and inspect the internal filter screen, see the post above by Underwood.

 

Yes - on my to do list.

Thanks for your interest/input😈

[BrendAn]

5 minutes ago, Blueadventures said:

Wal about $450-

Mate got one off him the other week a lot less than that. Must have second hand. Looked like new though.

 

Anyway I would rather spend a few hundred bucks to be sure .

Bit hard to get out and walk if engine stops. Unless you have air

Brakes like bugs bunny.,😁

Edited yesterday at 06:42 AM by BrendAn

[skippydiesel]

10 minutes ago, BrendAn said:

$600 for a Rotax pump. Better tell Wal he is selling them too cheap..his are half that

Pleased to hear it - I just Googled Rotax 912 ULS Mechanical Fuel Pump and found about $570 (converted from US),  I added $30 shipping😈

[BrendAn]

Hope you get it sorted soon. I don't think it's valorisation or orifice size.

 

I thought 912 vaporising was more of a heatsink thing when you shut down and fire it up again shortly after.

The return orifice is a standard size and fixed . A problem with that would have been noticed well before now.

 

[facthunter]

It could easily be fuel too hot  How well lagged are the fuel lines and how close to hot parts do they run?  Nev

[skippydiesel]

Thanks BrendAn - I think we are on a similar track.😈

[BrendAn]

24 minutes ago, facthunter said:

It could easily be fuel too hot  How well lagged are the fuel lines and how close to hot parts do they run?  Nev

Have you had that happen with a 912.  I would have thought if it was the way the lines are routed it would happen every Time he flew it not just this trip. It will be interesting to see what it was when he tracks the problem down anyway.

Edited yesterday at 07:36 AM by BrendAn

[facthunter]

What's that got to do with it?  It's a Motor with hot Pipes in a tight Cowl and Most have the Carbs located much Lower in the Cowl and heat rises?. Its the only  set up I know that considers it wise to return fuel to help cool what fuel remains. A pretty ordinary way of doing it In my opinion.. Fuel vapourisation has to be ruled out  to ensure safe flight. The pressure readings on this plane are NOT acceptable and we have been asked our Opinions. Nev

[BrendAn]

1 hour ago, facthunter said:

What's that got to do with it?  It's a Motor with hot Pipes in a tight Cowl and Most have the Carbs located much Lower in the Cowl and heat rises?. Its the only  set up I know that considers it wise to return fuel to help cool what fuel remains. A pretty ordinary way of doing it In my opinion.. Fuel vapourisation has to be ruled out  to ensure safe flight. The pressure readings on this plane are NOT acceptable and we have been asked our Opinions. Nev

 i only asked if you had seen the same thing on a 912 before.  i thought the fuel return was to push vapor out of the lines.

[skippydiesel]

13 hours ago, BrendAn said:

  i thought the fuel return was to push vapor out of the lines.

My understanding of the fuel return function is:

• To bleed off bleed off any vapour that forms in the fuel supply line before it gets to the carburettors
• Maintain a continuous circulation of  fuel, to assist in keeping the temperature below the fuel boiling point

For best effect the return line should be plumbed into a large fuel tank, not into the main fuel supply line (including gascolator)😈

[facthunter]

To answer the Question. No Not ever regarding  fuel Vapourising issues on an 80 or 100 hp Rotax. but vapourising is not an uncommon thing with small aircraft on Mogas.  Hot fuel can cause bubbles and float sinking (and consequent Flooding) even with some cars that had carburetters. Nev

[skippydiesel]

I have experienced fuel vaporisation with a Rotax 912 ULS powered aircraft.

Symptoms were nothing like what I have seen on my current aircraft.

 

Aircraft 1999, ATEC, Zephyr, Rotax 912 ULS - This engine was installed before the fuel return line concept became commonplace/Rotax recommended practise. It did have a return line, routed through a, relativly complicated adjustable, pressure relief valve.

 

Hot day - from memory, 40+C on the ground.

Landed/shut down engine, after aproximately 2 hr flight.

Short stay on ground

Difficulty starting/getting engine to run - suspected fuel vapourisation.

Got engine to run - post start checks all good

Taxi to run up bay

Extended duration high engine power checks,  4000 rpm (brakes on that aircraft would not hold at higher) to give time for vapour to dissipate - all good.

Commenced TO - engine lost power/rough running- aborted - engine returned to smooth running on backtracking.

Repeated high power checks

Tried again - all good😈