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Sudden fuel pressure loss

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Sudden fuel pressure loss

Postby Steevo » Sat Apr 20, 2019 7:59 pm

Just thought I would post this here just in case someone has seen it before.

Was using my J160 on Friday and did about 2 hours of flying. All good. Had a spot of lunch and started up the Jab. I was presented with a load of low fuel pressure warnings on my engine monitor. My fuel pressure is usually between about 3.2 and 3.8psi and now it was showing just over 1psi.

I switched on the electric pump and fuel pressure went back up to about 3.7psi. Switched off electric pump and fuel pressure immediately drops to just over 1psi.

Aborted the flight.

My first thought was that the mechanical pump had packed up. This was a bit strange as it was a brand new pump only fitted a few months ago and done only about 6 hours.

Anyway, today I removed the pump and took it home. I opened it up and I can not find anything wrong with it. Everything inside was, tight, complete and in excellent condition. No holes in the diaphragm, spring was all good and cannot see any leaks.

The pump is obviously still under warranty, but before I have a horrendous battle with Skycraft and probably ground the aircraft for ages, does anyone have any ideas as to what else it could be. No doubt I will have to end up buying another pump which is over £300.

I have kind of ruled out the rest of the fuel system as the electric pump does exactly as expected. The only changes that have been made recently are the fuel filter being changed and right before the flight, I topped up the oil.

When I took the pump off, I thought I would check the pushrod that drives the pump from the camshaft. The pushrod is in excellent condition and moves smoothly in and out when you turn the prop. But it doesn’t move that much, just a few millimetres which I suspect is correct.

So before I spend a load of money or/and have a battle with Skycraft, does anyone have any other ideas as to what it could be.
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Re: Sudden fuel pressure loss

Postby PKnight » Sun Apr 21, 2019 9:48 am

I had a similar experience a few months ago. First question to determine is whether or not the fuel pressure is actually low or whether there is a sensor error.
After changing the fuel filter I found that I had a small (tiny) bit of grot in fuel flow sensor. The effect of this was to reduce fuel flow and hence pressure but the system returned to normal if I switched the electric pump on presumably because the extra pressure from the pump was sufficient to overcome any resistance from the bit of grot.
In my configuration the fuel flow goes Electric Pump - Filter - Flow Sensor - Mechanical pump. Yours may be different.
Anyway check the efficiency of the fuel flow as per manual, and see if the sensor is reading correctly. Also check the engine runs normally (high speed taxi is a good check).
Hope this helps
Peter
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Re: Sudden fuel pressure loss

Postby PKnight » Mon Apr 22, 2019 8:31 am

P.S.
The other difference was that the engine and sensors would be hot.
Hot day, long flight, good heat soak off all the bits while you had lunch...
Might be a vapour lock if using something other than Avgas/UL91
See if all is well when you next cold start.
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Re: Sudden fuel pressure loss

Postby chrislizp » Mon Apr 22, 2019 6:31 pm

A friend of mine who flys a Tecnam with a Rotax engine which should be run only on Mogas has had a similar problem - his view is as follows.
"The vapour pressure and temperature characteristics of Mogas are that it has a much higher vapour pressure and will thus turn to vapour/gas at a much lower altitude than Avgas. As far as temperature is concerned it will also transition to vapour at a much lower temperature. Thus when the aircraft is shut down the under cowling temp rockets and for some time, say an hour, the fuel under the cowling can be in vapour form resulting in very low fuel pressure, or failure to start. Once the electric ‘boost’ pump is switched on ( before an attempt to start) the vapour is swept back to the tank via the return line (if fitted) and fresh cool fuel is then presented to the engine for a clean start. Pump should then be switched off during the start procedure to minimise risk of fire if over primed etc. On my Rotax 912, this low fuel pressure on hot start characteristic occurs throughout the UK summer."
He always flys with the electric boost pump on!
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Re: Sudden fuel pressure loss

Postby Steevo » Mon Apr 22, 2019 7:09 pm

Anything is a possibility at the moment.

Some more info. The Jab is always run on Avgas. I had another look at the pump and thought I had found the problem. I remembered that when I took it apart, the spring shot off. I thought I had done it as I was taking it apart but looking at it, there is no way I could have made the spring come off. On closer examination, it looked as if the plastic stopper had not been twisted the full 90 degrees so had come free in the pump allowing the spring to uncompress a bit which does reduce pressure.

So happy with that knowledge, I reassembled everything and put it back on today. Started her up and voila, fuel pressure was running at around 3.5psi. While I was doing that, I did a fuel flow test which came up as just under 1.5L in 60 seconds. I also checked all fuel lines etc, all looked good.

So I decided to take her for a few circuits. First circuit no problem. Second circuit the engine coughed a couple of times during the climb, this was with the electric pump on as well and the pressure was holding steady at about 3.5psi.

Got on the ground and on the taxi back to the stand, I started getting fuel pressure warnings again. With the electric pump off, I was getting around 1psi again and with the electric pump on about 3.5psi.

I just cannot see anything obviously faulty with the pump. Not sure if the cough and the pressure problem are related. I can only explain the cough as if someone had throttled back about 100rpm for a brief half a second and then went full power again. The engine has coughed a few times now but only ever at full power in the climb. I can go full power straight and level with no problem and it has never done it at any other time.

I have checked all over the carb very closely and cannot find any evidence of debris or any contamination. I have also tried another fuel pressure sensor so it is not a sensor problem.
Jabiru J160
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Re: Sudden fuel pressure loss

Postby PKnight » Mon Apr 22, 2019 10:01 pm

The fact that there is a 'cough' in the climb implies that there is almost enough fuel getting to the engine but not quite sufficient to sustain full power.
the low pressure on taxi might be a different symptom of the same problem exacerbated by the fact that the engine pump will be less powerful at low revs .
So there must be a small restriction in the fuel flow somewhere.
Perhaps a the fuel filter has some debris on the upstream side or a slight kink in the fuel pipe.
Frustrating and hard to find but I think you are making progress by eliminating possibilities.
Good luck
Peter
P.S. I assume you do not have a fuel flow meter.
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Re: Sudden fuel pressure loss

Postby PKnight » Tue Apr 23, 2019 7:28 am

The latest edition of the engine manual gives the allowed fuel pressure at the carburettor as between 0.5 - 5.0 psi.
Therefore the pressure that you are getting while taxiing a hot engine is within the allowed range and not a problem.
It is only an issue if it indicates a more serious problem.
The 'cough' on climb out (2nd time) may be a reduction in fuel flow which is greater when the engine is hot or it may be nothing to do with anything.
Only remaining suggestion given that you have a perfectly adequate fuel flow is to ask where you measured the fuel flow. Most people measure it at the inlet to the mechanical pump as it is easy to get to.
Other option is to measure it at the end of the fuel hose which is removed from the carb (harder to get to and replace) That will eliminate any possibility of a restriction in the last remaining hose.
otherwise I am out of ideas unless you fancy swapping the fuel pump back
Peter
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Re: Sudden fuel pressure loss

Postby Steevo » Tue Apr 23, 2019 4:03 pm

I do have a fuel flow meter but on the J160 I have never got it to read anywhere near accurate. I have had this on other Jabs where turbulence in the fuel lines affects the reading. Have solved it on others by moving it but not got around to it on this one. At the moment it says my fuel flow in cruise is about 36 l/h on average.

Yes, the fuel pressure meets the specification of the manual (depending which manual you read as it has changed a few times). But the issue here is that it has run at a steady 3.5 since I have had it and now all of a sudden it has dropped to over half. That is in 18 months. Another fuel pressure sensor has been tried.

You are right, I measured the flow at the inlet to the pump. This is where I was told to measure it by Jab. They gave the reason that if the mechanical pump isn’t running then it will create a big restriction that will not really give me a true indication. They do, however, have a method of measuring the mechanical pump itself which involves an external gravity fed fuel tank to run the engine. It was a lot of hassle so have not tried it.

I bet this turns out to be something silly and simple.
Jabiru J160
PS Engineering PM3000
Funke THT800H OLED
Funke ATR833 OLED
Flight DataSystems GD-40 CO Monitoring
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