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High fuel pressure warning

 
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recapen(at)earthlink.net
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PostPosted: Mon Mar 13, 2017 6:41 am    Post subject: High fuel pressure warning Reply with quote

While flying my RV6A yesterday, I got an intermittent High Pressure warning from my AFS3400EE. The trigger is set to 60 PSI. The boost pump was not on. Fuel flow remained normal and engine ran properly. Aborted the training session, returned to homeplate, and checked for leaks or other evidence of actual overpressure...with no success!

This is an IO360B1F6 with AFP injection and the FI mechanical pump.

Can the mechanical pump generate that high of pressure with normal fuel flow?
It was below freezing at altitude but I've never heard of avgas (30% 100LL and 60% ethanol free car gas) getting thicker at low temperatures.

My current thinking is that I may have something in the Fuel Pressure Sender that either causes it to act like a pressure accumulator or falsely signal high pressure.

Any thoughts would be appreciated!


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BARRY CHECK 6



Joined: 15 Mar 2011
Posts: 584

PostPosted: Sat Mar 18, 2017 11:26 am    Post subject: High fuel pressure warning Reply with quote

Ralph:

There are a few issues that can cause this type of result.
1 - Restricted fuel line.
2 - Bad transducer.
3 - Improper setting in the program. Yes I read what you said just
REMOVE the old setting and reset it, don't just LQQK at it, re-set it.
4 - Clogged filters.
5 - Clogged / Restricted Injectors. Remove, Clean in Hoppies #9 and
reinstall. OH! Do you have GAMI Injectors? If so make sure you put
them back in the same cylinder.
6 - FUZZ floating around in the fuel distribution system. I mention
this because I had a strange problem that would come and go in a
second. It took a LONG time to find the problem since I cleaned the
system at least 4 times... A STINKING PIECE OF FUZZ!!! It would ONLY
come out and block an injector for a few seconds and then disappear.
I don't know where it was hiding but it showed up and caused all sorts
of problems.
7 - Question - Problem: Do you see the high pressure when you shut
off the BOOST PUMP?
8 - Question - Problem: Do you have a RETURN line going back to the fuel tank?

Story: I had the SAME high pressure reading on a friends plane
(RV7A)... Every-time he turned on the BOOST PUMP the warning would
sound and light would come on. As a precaution and since we were
working on the Conditional Inspection we changed the Inline Fuel
Filters coming out of each wing. When I did I found BOTH aluminum
fuel lines to be crimped down. I cut off the crimped ends and
installed a longer rubber tube to the inline fuel filters. Problem
Solved.

Barry

On Mon, Mar 13, 2017 at 10:40 AM, Ralph E. Capen <recapen(at)earthlink.net> wrote:
Quote:


While flying my RV6A yesterday, I got an intermittent High Pressure warning from my AFS3400EE. The trigger is set to 60 PSI. The boost pump was not on. Fuel flow remained normal and engine ran properly. Aborted the training session, returned to homeplate, and checked for leaks or other evidence of actual overpressure...with no success!

This is an IO360B1F6 with AFP injection and the FI mechanical pump.

Can the mechanical pump generate that high of pressure with normal fuel flow?
It was below freezing at altitude but I've never heard of avgas (30% 100LL and 60% ethanol free car gas) getting thicker at low temperatures.

My current thinking is that I may have something in the Fuel Pressure Sender that either causes it to act like a pressure accumulator or falsely signal high pressure.

Any thoughts would be appreciated!



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recapen(at)earthlink.net
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PostPosted: Sat Mar 18, 2017 1:24 pm    Post subject: High fuel pressure warning Reply with quote

Barry,

Good stuff to check...

I have already done 2 (replaced transducer): I did 3, 4, and 5
7 - with or without boost pump - I get the warning
8 - I have a return line - but it is only used during purge operation (I have AFP injection)

Looking (at) #6 seriously now!

Fleew again today - no issues after resetting (per #3) - it started with 46 PSI before engine start....causing me to reset it....

Really looking (at) #6!

Thanks,
Ralph

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Bubblehead



Joined: 26 Oct 2007
Posts: 48
Location: N. Richland Hills, TX

PostPosted: Sun Mar 19, 2017 11:11 am    Post subject: Re: High fuel pressure warning Reply with quote

BARRY CHECK 6 wrote:
Ralph:

There are a few issues that can cause this type of result.
1 - Restricted fuel line.

4 - Clogged filters.

6 - FUZZ floating around in the fuel distribution system.
Story: I had the SAME high pressure reading on a friends plane
(RV7A)... Every-time he turned on the BOOST PUMP the warning would
sound and light would come on. As a precaution and since we were
working on the Conditional Inspection we changed the Inline Fuel
Filters coming out of each wing. When I did I found BOTH aluminum
fuel lines to be crimped down. I cut off the crimped ends and
installed a longer rubber tube to the inline fuel filters. Problem
Solved.

Barry



I deleted some text to focus on 3 or 4 items.

Barry, I am not sure I understand cause and effect on the aluminum lines being crimped down. Those are presumably on the suction side of the engine driven pump. How does that cause higher pressure on the discharge side of the pump?

Clogged filters or crimped lines between the tank and pump could make the pump have to suck harder and maybe cavitate which would cause reduced pressure on the discharge side of the pump. I can see how obstructions downstream of the pump could cause high pressure but not an obstruction on the tank side.

Also, how were you able to isolate the problem to the fuzz in the distribution side?

I have a lot of experience operating pumps of all kinds including similar positive displacement pumps. I also have a ME degree and took the requisite fluids classes. An obstruction downstream of the pump is essentially a "resistor" and the system due to the servo is a constant flow (current in electrical terms) system. To overcome a restriction the pump has to put out higher pressure just as voltage might have to be increased in an electrical circuit to keep current the same.

I think a restriction between the pump and the tank does not affect the output pressure unless the restriction is so severe the pump cannot draw enough fuel. If we measured pressure on the inlet side of the pump we would see lower pressure which is needed to flow the same amount of fuel. But unless the pump is starved for fuel, the outlet pressure will be the same as always. If the pump does get starved outlet pressure will drop, just as it would if we ran a tank dry.

John


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mel(at)becknet.com
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PostPosted: Sun Mar 19, 2017 12:22 pm    Post subject: High fuel pressure warning Reply with quote

Some high-wing, high-performance, single-engine GA aircraft have a fuel system that features fuel injection rather than a carburetor. This system combines gravity flow with the use of one or two auxiliary fuel pumps positioned before the fuel filter or screen. The Teledyne-Continental system is an example:

-mel beckman

On Mar 19, 2017, at 12:12 PM, Bubblehead <jdalmansr(at)gmail.com> wrote:

[quote]


BARRY CHECK 6 wrote:
> Ralph:
> There are a few issues that can cause this type of result.
> 1 - Restricted fuel line.
> 4 - Clogged filters.
> 6 - FUZZ floating around in the fuel distribution system.
> Story: I had the SAME high pressure reading on a friends plane
> (RV7A)... Every-time he turned on the BOOST PUMP the warning would
> sound and light would come on. As a precaution and since we were
> working on the Conditional Inspection we changed the Inline Fuel
> Filters coming out of each wing. When I did I found BOTH aluminum
> fuel lines to be crimped down. I cut off the crimped ends and
> installed a longer rubber tube to the inline fuel filters. Problem
> Solved.
> Barry


I deleted some text to focus on 3 or 4 items.

Barry, I am not sure I understand cause and effect on the aluminum lines being crimped down. Those are presumably on the suction side of the engine driven pump. How does that cause higher pressure on the discharge side of the pump?

Clogged filters or crimped lines between the tank and pump could make the pump have to suck harder and maybe cavitate which would cause reduced pressure on the discharge side of the pump. I can see how obstructions downstream of the pump could cause high pressure but not an obstruction on the tank side


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Bubblehead



Joined: 26 Oct 2007
Posts: 48
Location: N. Richland Hills, TX

PostPosted: Mon Mar 20, 2017 12:56 pm    Post subject: Re: High fuel pressure warning Reply with quote

Mel, you are correct. Cessna Centurions are an example. I think the original poster though was talking about a fuel injected RV-6 and Barry Check 6 was talking about a fuel injected RV-7A with fuel filters in the wing roots. The two pumps (electric boost and engine driven) draw fuel through the wing filters and push it through the finger strainer on the fuel servo.

Actually in the RVs both carburated and fuel injected versions have an electric plus engine driven fuel pump. In the carburated version the output pressure of the pumps is much lower than the fuel injected versions.

I have the same setup on my IO-360 powered RV-8.


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BARRY CHECK 6



Joined: 15 Mar 2011
Posts: 584

PostPosted: Tue Mar 21, 2017 7:28 am    Post subject: High fuel pressure warning Reply with quote

Hi John & Gaggle:

My response is within the body of your email.

Quote:
Barry, I am not sure I understand cause and effect on the aluminum lines being crimped down. Those are presumably on the suction side of the engine driven pump. How does that cause higher pressure on the discharge side of the pump?

Clogged filters or crimped lines between the tank and pump could make the pump have to suck harder and maybe cavitate which would cause reduced pressure on the discharge side of the pump. I can see how obstructions downstream of the pump could cause high pressure but not an obstruction on the tank side.


Barry - John, I would AGREE with you. There was some unusual plumbing
which brought the plumbing between the seats - I do not recall what
the items were at that location, I have never seen that configuration
before, there is some form of check-valve.
After trying and looking at all the basic issues the inspection lead
me to the inline fuel filters. Should they make a difference and HIGH
fuel pressure? NO! But, they did!!! I can't explain it, I just went
step by step inspecting the fuel supply system. This was surely NOT
what should be. It just WAS. And after cutting off the crimped ends
and installing new filters, the high pressure was under control. Hey!
I do not LQQK a gift horse in the mouth. AND, I would surely like a
better explanation.

Quote:
Also, how were you able to isolate the problem to the fuzz in the distribution side?

Barry - This was fairly easy... I saw the fuzz! It was a small gray
flap about the size of 3 pin-heads. Where it was hiding I have know
idea, it was found on cylinder #3 fuel injection line. How it held on
AFTER I cleaned the system on three different occasions with AvGas I
don't know. It became evident when I cleaned it yet again with
Pressurized Brake Cleaner Fluid, spraying down the tube and into a
white cotton T shirt.

Quote:

I have a lot of experience operating pumps of all kinds including similar positive displacement pumps. I also have a ME degree and took the requisite fluids classes. An obstruction downstream of the pump is essentially a "resistor" and the system due to the servo is a constant flow (current in electrical terms) system. To overcome a restriction the pump has to put out higher pressure just as voltage might have to be increased in an electrical circuit to keep current the same.

Barry - ME TOO, John, I'm an EE & ME and Retired. As I said, I agree
with you. But! If it works, it works!!! All the LOGICAL
explanations did not work. I truly HATE doing Pull & Replace
Mechanics, I could not find the one item that may have caused the
problem. There might have been more than one reason why the pressure
went high, I don't know. My first thought was the Programming of the
monitor was incorrect, maybe it was, I did reprogram the settings, not
just LQQK at them. But, that does not make sense since the values
read correctly and power is always removed from the circuit every time
the plane is shut down. Sort of like when a computer hangs up... Do a
COLD ReBoot. It was very lucky that I did take these steps, as the
ends of the aluminum fuel lines were reduced from 3/8" Dia down to
maybe 3/32" Dia. The owner was VERY impressed how low his fuel flow
was - But was VERY questionable on why his leaning range was so very
small.

Side Note: Engineers are a VERY unusual breed (Yes we are). We all
went to the same classes we all graduated with the same degree but we
ALL will try to reinvent the wheel. After 40 years in the field I
learned one thing: They are not paying me (enough) to reinvent the
wheel and if it ain't broke don't fix it! LOL!!! LMAO!!!
Barry


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Bubblehead



Joined: 26 Oct 2007
Posts: 48
Location: N. Richland Hills, TX

PostPosted: Tue Mar 21, 2017 8:02 am    Post subject: Re: High fuel pressure warning Reply with quote

Barry, your response is one of the best ever! I agree with you on not looking a gift horse in the mouth. Sometimes things just don't make sense but the engineer in me (and former Navy Nuke too) just keeps trying to find cause and effect.

I have had the same fuzz problem with injectors, which is why I added filters in the wing roots. I am going to clean my injectors this weekend because I do see fuel pressure creeping up and I have not cleaned them in a couple of years. The Conditional Inspection is due in May and I'll clean the filters in the wing roots plus the finger strainer at that time.

Congrats on your EE, ME and retirement. I love electrical almost as much as mechanical engineering and could have easily come out a EE. Never had time to go back and get the EE but did get an MBA at age 56. I am 62 and still having a lot of fun in my career so no hurry to retire. Maybe at 66.5 but maybe not.

I really appreciate the fact that you are willing to say "I don't know why" and "I can't explain it." There are things we just don't have enough information to unlock but some people refuse to have enough humility to admit they might not have all the answers.

John


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