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Fuel pressure problems continuing

 
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dan(at)syz.com
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PostPosted: Wed Feb 15, 2017 10:16 pm    Post subject: Fuel pressure problems continuing Reply with quote

Hi everyone.

My RV-10 has 9 hours of flight time so far with a factory new Lycoming IO-540 D4A5. On the last flight I did a couple of weeks ago, I tried a full rich climb to 10000', but ended up with my fuel pressure dropping down to around 11 psi when I got to 8700', at which point I turned on the boost pump and got the pressure back up to around 20 psi. Back down at a lower altitude, I tried turning off the boost pump, and the engine pump maintained pressure at 22 psi. But something was clearly not right, and since I was pretty much at the right time to do an oil and oil filter change anyway, thought it was a good time to do the equivalent of an "annual" to make sure everything was still looking good, and try and sort out the problem.

Looking at my logs from past flights, typically my fuel pressure with the engine pump has been about 20-23 psi at 6000', 21 gph (full rich). I have typically seen higher pressures on the ground (27-29 psi) at lower RPMs and fuel flows.

My fuel system is pretty much stock Van's, except I have an Andair fuel selector with an extension so it's lower in the tunnel, and I have gascolators in the wing roots (here in Canada, we require them). The fuel pump, filter, and fuel flow transducer (red cube) are all in the tunnel, as per plans.

I thought I'd start with looking at the gascolator screens, since they're the easiest to get to, and being the first filters from the tanks to the engine, likely to collect the most. And I thought I hit the jackpot - both screens had some sort of lint buildup in them. I thought I'd kept the tanks clean while constructing, but guess it wasn't as good as I thought. So I cleaned them up figuring I found the problem (assuming that since they caught debris, likely there wasn't anything blocking things further down the line), changed the oil and filter, put things back together and figured I'd go flying.

But in doing my runup, after a few minutes when the engine was getting warm and running at relatively low RPMs, I turned the boost pump on for a few seconds and then back off. When I turned it off, I saw the pressure dropping slowly. It got down to around 13 psi. I tried advancing the throttle a bit to see if maybe the engine was running too slow to provide enough fuel pressure, and the engine promptly died. No sputtering or anything - just quit, like I pulled the mixture to ICO and that's what I wanted it to do. It's not done that before.

Obviously, at this point I wasn't going flying until I could figure out what was wrong. I managed to restart the engine (after a bit of effort - the engine had warmed up a bit, and I'm still sorting out the best way to do that). I couldn't get it to die again in the runup, so I decided to do a few high speed taxis/aborted takeoffs (fortunately, I'm at a quiet airport). Normally when I take off, I use the boost pump - this time I purposely left it off to see how the engine pump handled things on its own. I did three, and every time when I brought the engine up to full power and started a takeoff roll, the fuel pressure dropped significantly. Twice down to 12 or 13 psi. When I throttled back, the pressure came back up again. And this was tested on both left and right tanks - both behaved the same.

So back to the drawing board, assuming there is still a blockage or restriction somewhere. I re-removed the gascolator screens again in case there was further contamination, but they look clean.. as do the fuel strainers that go into the tanks. Tomorrow, the plan is to get into the tunnel to check the fuel filter there (not looking forward to that - I'm going to have to remove a bunch of stuff to get into there, and had thought that since the gascolator screens had caught debris, the one in the tunnel likely will be fine). Nevertheless, it looks like I'm going to have to inspect the whole fuel system with a fine toothed comb to try and find if there's a blockage or something. I don't think there are any kinks in the lines - they were all bent with a tube bender and looked OK when I installed them, but I'll be taking another close look of course.

While I have everything apart, if anyone has any ideas of what to check, I'd greatly appreciate them - I want to make sure I'm as thorough as possible, and don't want to close things up only to realize later I should have checked something else out while I was in there.

The engine (and presumably fuel pump) were new from Lycoming, though they sat for about 1.5 years with preservative oil. How likely is it that the fuel pump itself might be the problem? The boost pump manages to bring my pressure back up to where it's supposed to be, but considering its design is different, it might be more tolerant to a fuel system's quirks.

Thanks for any help or advice!

Dan
---
Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213


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glastar(at)gmx.net
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PostPosted: Thu Feb 16, 2017 1:10 am    Post subject: Fuel pressure problems continuing Reply with quote

Dan,

just a wild guess, but please check as well all your fuel connection for
either a loose connection or a flare which might be compromised (you
probably should see some signs of Avgas there).

We had 10 years ago an accident with an RV-9 where the engine quit,
reason were incorrect flares and over torquing of fuel connections.
A partly blocked injector on #1 might have been a factor.

Good luck in finding the reason.

Cheers Werner


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Kelly McMullen



Joined: 16 Apr 2008
Posts: 1062
Location: Sun Lakes AZ

PostPosted: Thu Feb 16, 2017 5:58 am    Post subject: Fuel pressure problems continuing Reply with quote

I'm not sure I have any good ideas of where to look, nor why some RV-10s seem to have similar problems and others don't.

I have pretty much the same setup you do, except no gascolators, and my fuel flow sending unit is between the servo and the flow distributor. I have not detected any issues. I just looked at my last flight. Fuel pressure dipped to 18.6 as full throttle was applied for takeoff without boost pump. Immediately came back up to over 23, and for almost all of the flight was between 25 and 26. That includes a climb to density altitude of 9500.

My takeoff fuel flow is about 22.7, with field elevation of 1240. I wish it were another gph higher, as my EGT on some cylinders goes over 1300 on initial climb.

I'd say your engine quit because the pressure was too low for the servo to deliver the required fuel. It is supposed to have 15 psi minimum.

How hard would it be to take the gascolator out of the system for a test, just on one tank, to see if it makes a difference? The other item I would consider testing for pressure drop is the fuel flow sending unit. IIRC it does have a measurable drop, which is one reason why it is preferable to have it positioned after the mechanical fuel pump, either between the pump and servo, or the servo and flow divider.

I normally would suggest verifying the fuel pressure, but your symptoms match your reported pressures.

My other aircraft is a 200 hp Mooney  with exactly the same mechanical pump and RSA5 fuel servo. It was certified without any filter between the tanks and the fuel servo inlet filter. It has the same Pro-sealed wet wings, no gascolator, just a screen in the fuel selector body. Fuel pressure is very similar to what I see on my RV-10.
-sent from the I-droid implanted in my forearm

On Wed, Feb 15, 2017 at 11:15 PM, Dan Charrois <dan(at)syz.com (dan(at)syz.com)> wrote:
Quote:
--> RV10-List message posted by: Dan Charrois <dan(at)syz.com (dan(at)syz.com)>

Hi everyone.

My RV-10 has 9 hours of flight time so far with a factory new Lycoming IO-540 D4A5.  On the last flight I did a couple of weeks ago, I tried a full rich climb to 10000', but ended up with my fuel pressure dropping down to around 11 psi when I got to 8700', at which point I turned on the boost pump and got the pressure back up to around 20 psi.  Back down at a lower altitude, I tried turning off the boost pump, and the engine pump maintained pressure at 22 psi.  But something was clearly not right, and since I was pretty much at the right time to do an oil and oil filter change anyway, thought it was a good time to do the equivalent of an "annual" to make sure everything was still looking good, and try and sort out the problem.

Looking at my logs from past flights, typically my fuel pressure with the engine pump has been about 20-23 psi at 6000', 21 gph (full rich).  I have typically seen higher pressures on the ground (27-29 psi) at lower RPMs and fuel flows.

My fuel system is pretty much stock Van's, except I have an Andair fuel selector with an extension so it's lower in the tunnel, and I have gascolators in the wing roots (here in Canada, we require them).  The fuel pump, filter, and fuel flow transducer (red cube) are all in the tunnel, as per plans.

 
But in doing my runup, after a few minutes when the engine was getting warm and running at relatively low RPMs, I turned the boost pump on for a few seconds and then back off.  When I turned it off, I saw the pressure dropping slowly.  It got down to around 13 psi.  I tried advancing the throttle a bit to see if maybe the engine was running too slow to provide enough fuel pressure, and the engine promptly died.  No sputtering or anything - just quit, like I pulled the mixture to ICO and that's what I wanted it to do.  It's not done that before.


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Kelly McMullen
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Kelly McMullen



Joined: 16 Apr 2008
Posts: 1062
Location: Sun Lakes AZ

PostPosted: Thu Feb 16, 2017 6:48 am    Post subject: Fuel pressure problems continuing Reply with quote

I went back and checked. Lycoming says minimum fuel pressure required is 14 psi.

-sent from the I-droid implanted in my forearm

On Thu, Feb 16, 2017 at 6:57 AM, Kelly McMullen <apilot2(at)gmail.com (apilot2(at)gmail.com)> wrote:
Quote:
I'm not sure I have any good ideas of where to look, nor why some RV-10s seem to have similar problems and others don't.

I have pretty much the same setup you do, except no gascolators, and my fuel flow sending unit is between the servo and the flow distributor. I have not detected any issues. I just looked at my last flight. Fuel pressure dipped to 18.6 as full throttle was applied for takeoff without boost pump. Immediately came back up to over 23, and for almost all of the flight was between 25 and 26. That includes a climb to density altitude of 9500.

My takeoff fuel flow is about 22.7, with field elevation of 1240. I wish it were another gph higher, as my EGT on some cylinders goes over 1300 on initial climb.

I'd say your engine quit because the pressure was too low for the servo to deliver the required fuel. It is supposed to have 15 psi minimum.

How hard would it be to take the gascolator out of the system for a test, just on one tank, to see if it makes a difference? The other item I would consider testing for pressure drop is the fuel flow sending unit. IIRC it does have a measurable drop, which is one reason why it is preferable to have it positioned after the mechanical fuel pump, either between the pump and servo, or the servo and flow divider.

I normally would suggest verifying the fuel pressure, but your symptoms match your reported pressures.

My other aircraft is a 200 hp Mooney  with exactly the same mechanical pump and RSA5 fuel servo. It was certified without any filter between the tanks and the fuel servo inlet filter. It has the same Pro-sealed wet wings, no gascolator, just a screen in the fuel selector body. Fuel pressure is very similar to what I see on my RV-10.
-sent from the I-droid implanted in my forearm

On Wed, Feb 15, 2017 at 11:15 PM, Dan Charrois <dan(at)syz.com (dan(at)syz.com)> wrote:
Quote:
--> RV10-List message posted by: Dan Charrois <dan(at)syz.com (dan(at)syz.com)>

Hi everyone.

My RV-10 has 9 hours of flight time so far with a factory new Lycoming IO-540 D4A5.  On the last flight I did a couple of weeks ago, I tried a full rich climb to 10000', but ended up with my fuel pressure dropping down to around 11 psi when I got to 8700', at which point I turned on the boost pump and got the pressure back up to around 20 psi.  Back down at a lower altitude, I tried turning off the boost pump, and the engine pump maintained pressure at 22 psi.  But something was clearly not right, and since I was pretty much at the right time to do an oil and oil filter change anyway, thought it was a good time to do the equivalent of an "annual" to make sure everything was still looking good, and try and sort out the problem.

Looking at my logs from past flights, typically my fuel pressure with the engine pump has been about 20-23 psi at 6000', 21 gph (full rich).  I have typically seen higher pressures on the ground (27-29 psi) at lower RPMs and fuel flows.

My fuel system is pretty much stock Van's, except I have an Andair fuel selector with an extension so it's lower in the tunnel, and I have gascolators in the wing roots (here in Canada, we require them).  The fuel pump, filter, and fuel flow transducer (red cube) are all in the tunnel, as per plans.

 
But in doing my runup, after a few minutes when the engine was getting warm and running at relatively low RPMs, I turned the boost pump on for a few seconds and then back off.  When I turned it off, I saw the pressure dropping slowly.  It got down to around 13 psi.  I tried advancing the throttle a bit to see if maybe the engine was running too slow to provide enough fuel pressure, and the engine promptly died.  No sputtering or anything - just quit, like I pulled the mixture to ICO and that's what I wanted it to do.  It's not done that before.




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kearney



Joined: 20 Sep 2008
Posts: 485

PostPosted: Thu Feb 16, 2017 7:27 am    Post subject: Re: Fuel pressure problems continuing Reply with quote

Hi Dan

I would go back to first principles.

The first thing I would do would be to break the fuel line after the mechanical fuel pump and measure the fuel flow with just the electric pump. I did this using a 25l jerry and timed how long it would take to fill. If the fuel flow is too low, that would suggest the problem is between the tank and the pump. I think I was getting a free flow of 50-60 GPH.

If the flow is ok, I would also check the fuel pressure transducer and see if it is reading correctly. Given that the fuel pressure line has an inline reducer, I would also see if there is any small debris blocking the reducer.

Did you use the USHER gascolator. I did use these and found that they were not a problem.

As far as cable throws are concerned, there should be a bit of "bounce" in the cables where the cables hit the stops BEFORE the cable is all the way in at the control panel (say about 1/8"). This ensures complete travel.

Cheers

Les


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dan(at)syz.com
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PostPosted: Thu Feb 16, 2017 12:56 pm    Post subject: Fuel pressure problems continuing Reply with quote

Quote:
On 2017-Feb-16, at 6:57 AM, Kelly McMullen <apilot2(at)gmail.com> wrote:

I'm not sure I have any good ideas of where to look, nor why some RV-10s seem to have similar problems and others don't.
I have pretty much the same setup you do, except no gascolators, and my fuel flow sending unit is between the servo and the flow distributor. I have not detected any issues. I just looked at my last flight. Fuel pressure dipped to 18.6 as full throttle was applied for takeoff without boost pump. Immediately came back up to over 23, and for almost all of the flight was between 25 and 26. That includes a climb to density altitude of 9500.
My takeoff fuel flow is about 22.7, with field elevation of 1240. I wish it were another gph higher, as my EGT on some cylinders goes over 1300 on initial climb.
I'd say your engine quit because the pressure was too low for the servo to deliver the required fuel. It is supposed to have 15 psi minimum.
How hard would it be to take the gascolator out of the system for a test, just on one tank, to see if it makes a difference?

Not terribly hard - I could pull the gascolator in one of the wings and make a little "patch" fuel line between the tank and the line currently connected to the output of the gascolator. If I can't find the problem elsewhere, that's a great idea of something to try.

Quote:
The other item I would consider testing for pressure drop is the fuel flow sending unit. IIRC it does have a measurable drop, which is one reason why it is preferable to have it positioned after the mechanical fuel pump, either between the pump and servo, or the servo and flow divider.

I wish I would have put it there in the first place - even if it's not the primary source of the problems I'm having, it's notoriously inaccurate when the boost pump is running. I've been avoiding moving it if the only problem is inaccurate readings of fuel flow when the boost pump is running, but if it's a contributing factor to the fuel pressure problems, I may have to bite the bullet.

Quote:
I normally would suggest verifying the fuel pressure, but your symptoms match your reported pressures.

Yes... when I was flying and it got down to 11 psi momentarily and the engine kept running, I thought it's possible the pressure sender was at fault. However, yesterday during my runup when the engine quit when it got to around 13 psi, that pretty much exonerated the pressure sender to be at fault.

Thanks for your suggestions!

Dan
---
Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213


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dan(at)syz.com
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PostPosted: Thu Feb 16, 2017 1:22 pm    Post subject: Fuel pressure problems continuing Reply with quote

Quote:
Hi Dan

I would go back to first principles.

The first thing I would do would be to break the fuel line after the mechanical fuel pump and measure the fuel flow with just the electric pump. I did this using a 25l jerry and timed how long it would take to fill. If the fuel flow is too low, that would suggest the problem is between the tank and the pump. I think I was getting a free flow of 50-60 GPH.

Hey Les. Yes, I've done a fuel flow test with the electrical pump right before my last runup where the engine quit, measuring the flow into a jerry can right before it goes into the throttle body. I tested the flow from both tanks, and in each case it was around 53 gph. So that's working OK (and the electric pump has successfully brought the pressures back into the 20+ range every time I've used it when the engine pump on its own was getting critically low).

Quote:

If the flow is ok, I would also check the fuel pressure transducer and see if it is reading correctly. Given that the fuel pressure line has an inline reducer, I would also see if there is any small debris blocking the reducer.

I'd thought it might be an instrumentation problem, until my engine quit unexpectedly during the runup when I saw the fuel pressure get down to around 13 psi on the engine pump. Unless the engine quit for some other reason coincidentally, it seems to me the low pressure indication is a real problem.

Quote:

Did you use the USHER gascolator. I did use these and found that they were not a problem.

Yes - two Usher gascolators, one in each wing root. And the low pressure indication seems independent on which tank I'm on.

Quote:

As far as cable throws are concerned, there should be a bit of "bounce" in the cables where the cables hit the stops BEFORE the cable is all the way in at the control panel (say about 1/8"). This ensures complete travel.

I'm not sure how much bounce I have in the cables, but I know when they hit the stops at the throttle body in either direction, they can grab and hold onto a piece of paper between the mechanism and the stop, so I think the travel is pretty much where it should be.

Over the next few days, the plan is to pull out the seats, centre console, throttle quadrant, etc. to get into the tunnel under the panel and pull out the fuel pump, filter, and flow transducer out of there to see if I can find anything not getting along.

I know in the past when people have tried replacing their mechanical fuel pumps to solve similar issues, it's usually not been the culprit. With that said, I have to say that with everything I've tried so far, it's starting to look more and more guilty. Particularly during my last runup - I was at around 23 psi on the engine pump, turned on the boost pump for a few seconds which brought it up to about 26, and then when I turned off the boost pump, the pressure dropped steadily down to 13 psi at which point the engine quit. I suppose another possibility might be the electric pump bypass one way valve sticking? At any rate, I'll hopefully learn something useful when I pull things out of the tunnel.

Thanks for the suggestions!

Dan
---
Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213


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Gordon Anderson



Joined: 16 Apr 2012
Posts: 35
Location: Switzerland

PostPosted: Fri Feb 17, 2017 11:45 am    Post subject: Re: Fuel pressure problems continuing Reply with quote

Dan,

The good fuel flow readings from your test with the electric pump are a nice health check for the boost pump but do not necessarily prove that system upstream of the mechanical pump is good.
The mechanical pump has to draw fuel through the gascolator, tank selector valve, filter, non-return valve, hoses and elbows. The extra hoses and NR-valve mean that the mechanical pump sees a lower inlet pressure than the boost pump at the best of times. That and differences in pump design could mean that the boost pump deals fine with the gascolators etc. and any contamination in the system, but the mechanical pump cannot. If the mechanical pump inlet pressure is already in the critical range at idle, then a very small reduction in inlet pressure due to higher flows at high engine power can have a large negative effect on outlet pressure.
First I would carefully check the system in the tunnel for any contamination. If that shows nothing, I would try to slightly pressurise the tank through the air vent (as you did to pressure test the tanks?) with the engine running and the boost pump off. If an increase in tank pressure by equivalent of a few inches of water suddenly resolves the fuel pressure problem, then you have to design some of the pressure drop out of your system, eg. relocate gascolators or find alternatives to any 90° elbows.
Best of luck with chasing the problem!

Gordon Anderson, Switzerland


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