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Low fuel pressure problems Rotax 912 uls


DrZoos
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Hi guys well we ran fuel flow tests with plane tipped up got 68l hr on left tank and 144 l hr on right tank. Cant see why the diff, but both are plenty. Will look into that later.

 

Elec pump only goes to about 0.22 prior to startup

 

Im getting low fuel pressure down to 0.11 on touch and goes and take off with both pumps running ...fuel pressure drops slowly on climb out, at about 300feet the alarm set at 0.15 goes off. It then falls as low as 0.13 sometimes as low as 0.11. . It will then start climbing back up and at about 900 feet the alarm goes back off. pitch definitely exacerbates the issue At idle getting 0.3 on either tank. Straight and level at high rpm is about 0.25-0.27 doesnt matter which tank im on its about the same.

 

When you throw it into a climb fuel pressure drops

 

Gascolator is clean. Filter done...

 

Bert flood sugested cutting pump vent tube shorter so it wasnt hanging out in slipstream. Did that, no change...

 

Need some ideas please. Its a new plane under warranty with 31 hrs

 

 

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It shouldnt be we checked it and its a april 2013 delivery... But i will double check

 

I will ad return line to the list of possibilities...

 

What do ya reckon about a low reading sensor. With both pumps on at idle 0.3 seems low, maybe its reading low all the time??

 

But even if it is a sensor should the pressure be dropping by 65% at times eg climbing out.

 

 

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Just found this.... Interesting,

 

Quote

 

I've just had a case of fluctuating fuel pressure also on a ULS. It would constantly change anywhere from 0.15-0.45 bar.

 

This engine had a fuel flow sender fitted behind the splitter to the return line but before the splitter to the two carbs.

 

The problem source relates to the carbs opening and closing the fuel inlet valves at the float bowls. Whenever one opened the pressure would drop as there was no reservoir to supply the fuel - everything had to first go through the flow sender which had a 2mm restrictor jet.

 

The very simple solution was to fit a small automotive fuel filter as reservoir between the carbs and the flow sender. This holds ample fuel to supply the instantaneous fuel demands from the carbs and also made the flow sender far more accurate. The fuel filter retains a small air pocket and this is under pressure and will expand to move fuel to the carbs and at the same time dampen the uneven flow in the flow sender.

 

After this the fuel pressure is rock steady at 0.4 bar.

 

Even if you don't have a flow sender here, a small filter behind the pump does no harm. The pump does not supply constant pressure - the pressure and fuel supply is pulsed. Having a reservoir with a small air pocket does wonders here to even out the pressure.

 

I ended up with a small fuel filter before and one after the fuel flow sender. Now it is perfect and the sender produces exactly the number of pulses per liter that the manual says it should.

 

End Quote

 

 

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Hi guys well we ran fuel flow tests with plane tipped up got 68l hr on left tank and 144 l hr on right tank. Cant see why the diff, but both are plenty. Will look into that later.Elec pump only goes to about 0.22 prior to startup

 

Im getting low fuel pressure down to 0.11 on touch and goes and take off with both pumps running ...fuel pressure drops slowly on climb out, at about 300feet the alarm set at 0.15 goes off. It then falls as low as 0.13 sometimes as low as 0.11. . It will then start climbing back up and at about 900 feet the alarm goes back off. pitch definitely exacerbates the issue At idle getting 0.3 on either tank. Straight and level at high rpm is about 0.25-0.27 doesnt matter which tank im on its about the same.

 

When you throw it into a climb fuel pressure drops

 

Gascolator is clean. Filter done...

 

Bert flood sugested cutting pump vent tube shorter so it wasnt hanging out in slipstream. Did that, no change...

 

Need some ideas please. Its a new plane under warranty with 31 hrs

Check that there is a .35mm restrictor in the return line to the tank.

 

Fuel pressure limits are Max 0.4 bar & Min 0.15 bar and normally run around 0.3 bar on a 912uls.

 

 

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Guest Maj Millard

Mmmm... sounds like you are getting real complicated there Dr Zoos, and on fuel systems especially, you need simplicity not complication. Keep it simple theory applies. One reason I don't like fuel flow senders in my main fuel supply line. Adjusting your fuel flow/ pressure by inserting fuel filters is also fraught with risk...The Rotax 912 installation manual gives clear and proven diagrams for fuel system set-ups..............Maj....

 

 

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Interesting problem you have there, Dr. Zoos. I had a fuel flow issue which nearly prevented my getting to Austfly. The problem would only surface after I had been on level cruise for about ten minutes. Bubbles would appear in the fuel line and the engine would run in morse code. After trying everything, the cause turned out to be due to my having moved the fuel vents about 40mm from on top of, to just underneath the wingtips. The different airflow at that point reduced tank pressure enough to almost stop fuel flow. After adding tiny metal "hoods" to deflect the airflow over the vents, the engine didn't miss a beat.

 

 

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Guest Maj Millard
Interesting problem you have there, Dr. Zoos. I had a fuel flow issue which nearly prevented my getting to Austfly. The problem would only surface after I had been on level cruise for about ten minutes. Bubbles would appear in the fuel line and the engine would run in morse code. After trying everything, the cause turned out to be due to my having moved the fuel vents about 40mm from on top of, to just underneath the wingtips. The different airflow at that point reduced tank pressure enough to almost stop fuel flow. After adding tiny metal "hoods" to deflect the airflow over the vents, the engine didn't miss a beat.

Yes I 'm thinking Dr Zoos s fuel problems could be vent related also.....................Maj.....063_coffee.gif.b574a6f834090bf3f27c51bb81b045cf.gif

 

 

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Im getting low fuel pressure down to 0.11 on touch and goes and take off with both pumps running ...fuel pressure drops slowly on climb out, at about 300feet the alarm set at 0.15 goes off. It then falls as low as 0.13 sometimes as low as 0.11. . It will then start climbing back up and at about 900 feet the alarm goes back off. pitch definitely exacerbates the issue At idle getting 0.3 on either tank. Straight and level at high rpm is about 0.25-0.27 doesnt matter which tank im on its about the same.

When you throw it into a climb fuel pressure drops

Hi DrZ

 

Any chance that you can do a full power run up on the ground and see what happens to the pressure? This will give an indication of what is happening in the fuel system before getting aerodynamics involved. Other suggestion is to borrow a mechanical fuel pressure meter and just check it against the one fitted in your aircraft. Is the pressure gauge mechanical or electronic such as in a Dynon?

 

The fuel pressure dropping on climb is reasonably normal so long as it is stable but if is not then it could indicate a fault with a leaking valve in the mechanical pump.

 

The part that is worrying is the vastly different flows from each tank while on the ground and it would pay to check every connection to see if there are any partial blockages. You have probably checked already but see if the fuel vents are not blocked and do the free flow tests from each tank with the caps removed

 

 

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Guest Maj Millard

A ground check with a mechanical fuel pressure Inserted into the supply line could cover a couple of bases. Additionally where is the current fuel pressure taken from, the Rotax fuel distribution block or elsewhere ?. It is delivery pressure to the carbs so should be just before delivery to the carbs. If on the supplied outlet on the block make sure fuel pressure hose to gauge is on the correct outlet. Also check for entrapped air in the fuel gauge line , or a faulty gauge.

 

Check fuel pump S/no against Rotax service bulletin removing some types from service. The removed ones are AC or Bing and have a flat section on the top of the fuel pump circumference. Pierburg or the newer type are ok................Maj..........014_spot_on.gif.1f3bdf64e5eb969e67a583c9d350cd1f.gif

 

 

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Check the rear of your fuel pressure gauge.

 

The Savannah gauge has a rubber bung in the rear with a small nib to be cut off.

 

If left as is, the pressure reading drops with altitude gain. (The pressure gauge is relative to the outside pressure).

 

Not sure if this is your problem, but I had this problem in the savannah.

 

Cheers

 

 

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Hi guys thanks for your input. Im not getting any "symptoms" form the engine.

 

On the ground i havent gone over about 4500 rpm, but as soon as you raise the revs the pressure starts dropping. With both pumps on on the ground and going to 4500 rpm it drops from say 0.3 to 0.25 or so.

 

The pressure guage comes of the block. From the rear of the block , it then goes a few inches to the guage.

 

The number was checked and not applicable but i will re check that.

 

There appears to be no correlation between altitude and the problem, its rpm and pitch related. But more so rpm. And more so rpm demand related.

 

 

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"pressure" is resistance to flow. When your engine is at low rpm and not using fuel, pressure will be high as the carbs are not using all the fuel the pump can supply.

 

As rpm increases and therefore fuel usage, the pressure is basically converted to flow and measured pressure is less.

 

Ofcourse rpm of the engine/pump plays a role too.

 

If you turn a garden hose on very slowly and put your finger over the end to block flow, you willfeel full water pressure but relase your finger and the water will flow and pressure will be reduced.

 

 

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Yeh ta, i knew that about folw / pressure, but the issue is how much its dropping. I expect some drop, but not such a significant drop.

 

According to rotax the electric fuel pump should have a max pressure of 0.3 which mine seems to get about 0.22 on its own pre start up. So maybe the sender is reading low. But even if it is, should the fuel pressure drop so significantly.... ??

 

So this morning i did some stationary tests.

 

At 2400rpm during warm up with one pump it holds 0.27, with both it holds 0.3 or 0.29

 

Pre flight with both pumps on at full throttle it would drop from 0.3 to 0.25 and hold 0.25. With motor pump only it would drop to 0.23 This is the least drop ive seen ever. But it was short lived. (maybee heat is affecting the sender accuracey)

 

Post flight i did the same, test but got much lower readings. With both pumps on it was dropping to 0.16 and with just the rotax pump running it was dropping to 0.11

 

In flight with both running on touch n goes it was going down to 0.13 then creeping back up at full rpm during take off.

 

Engine Serial number is 6780390 so not affected according to ad which goes up to 6 778 295

 

Fuel pump part number is 893110 and sn 12.1191 which according to ad is unaffected

 

Attached are some photos showing plumbing and mounting positions. I note some people have had probs with senders mounted above the engine... The elc pump is mounted low on the firewall.

 

20131119_085444.jpg.a2b811d706077efa0577249a1c57b093.jpg

 

20131119_085454.jpg.91d7d008f9fb40c0cfda0c49c3984f7e.jpg

 

20131119_085509.jpg.ed8b1630696b2019617fd53d921ddc3c.jpg

 

20131119_085522.jpg.cc12116265ff37139b008e4bd56ba79e.jpg

 

20131119_085538.jpg.b767ab44ac0b452568f4f190a172be46.jpg

 

20131119_085550.jpg.1da2aba96c372ad2369b11e85cd405a4.jpg

 

 

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Niel i first wondered that myself, but we did flow tests from the carbie line and got 68L / min (maybe a kink or some issue) and 144L / min (rotax recommend 35L / min minimum) no kink there ont he right thats a huge fuel flow. Yet the problem happens on either tank and the numbers are close enough for identical on either tank.

 

What floods have now suggested via dealer is to reroute pump breather hose. into a bottle low on the firewall. Im not convinced thats the problem, but i dont have much choice.

 

I think a temp wrap of the sender in heat shield is worth a go first to see if it is a heat induced problem. So i will try that tomorrow weather permitting.

 

What about you guys have you ever heard of heat making the sender read wrong when hot only. I know lots of others have relocated the sender, but i think that was because the heat killed them.

 

 

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Think i found part of the problem / solution read this page 16

 

The pic i have attached is a screenshot

 

It says with the new fuel pump a low fuel pressure for upto five seconds is fine ???? WTF

 

Mine goes longer then five seconds generally maybe ten seconds .... Thoughts appreciated.

 

http://www.flyrotax.com/portaldata/5/dokus/d05681.pdf

 

lowpressure.png.df272fbbaa503daf835ce7525375827c.png

 

 

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Guest Maj Millard

Didn't realise you had the newer pump. Yes there can be pressure fluctuations with them, but they seem to do the job of moving fuel fine..............Maj......014_spot_on.gif.1f3bdf64e5eb969e67a583c9d350cd1f.gif

 

 

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Neil my plane is new so i had no idea i even needed to read the fuel pump ad, as mine was already the new pump that is supposed to work. My dealer had rang floods several times about the issue and floods where pointing us in all types of directions to do with breather hoses. And despite us explainign the exact symptoms mentioned in the ad, niether floods or the dealer cottoned on to it. You would think floods would. Im not even a L1 maintenance person and ive managed to solve this riddle despite recieving contrary advice from the supposed experts.

 

This did not fix the problem, it only identified part of the problem as my alarms are going off longer then 5 seconds.

 

But i did try something today which i think has identified the cause of the continual and longer then allowable alarms.

 

I wrapped the sender in heat shield , guess what . Instead of getting alarms every touch and go, i did six touch and goes and only got the alram once.

 

So i now feel i know whats causing it and how to fix it. The new fuel pump is causing the pressure drop. The heat is causing the sender to read wrong.

 

To anyone else having low fuel alarms for Rotax 912 or low fuel pressure problems for Rotax 912 (i put that way so search engines find it) firstly the new fuel pumps are allowed to have pressure drops see page 16 of the fuel pump ad put out in june 2013. Secondly the fuel pressure senders are affected by heat. I was told by many knowledgeable people that heat will cause a pressure sender to stop working but it will not cause the sender to work incorrectly. In fact two people yesterday who should know better told me it was impossible for heat to be causing the sender to read incorrectly. Wrong.

 

As soon as i wrapped the sender in heat wrap. Problem 90% fixed . I still had one alarm and it was 9 seconds. So im going to install extra heat shield and see if i can eliminate the problem all together.

 

Cheers to all for your input.

 

 

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