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  #1  
Old 06-29-2021, 06:02 AM
BruceMe's Avatar
BruceMe BruceMe is offline
 
Join Date: Feb 2005
Location: Shawnee, Kansas
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Default Fuel pressure fluctuations (25-12psi) in cruise

This has only happened a small number of times, but I wanted to post here to see if anyone can help.

Setup: RV-4, IO-320 160hp, Bendix RS-4, all fuel lines fire-sleaved, 100LL only.

Condition: cruise flight, 7gph economy cruise, 1.5hrs cross country, 55F at altitude

Observation: The fuel pressure started to fluctuate in cruise, jumping from 25psi to 12psi with a frequency of about one cycle every two seconds. If I turn the pump on, it goes solid again at ~25psi. Turn it off, it started again after a few minutes. Switching tanks didn't change anything.

My engine-driven mechanical diaphragm pump is about 12 years old now... maybe it's getting weak or hairline crack?

Thoughts/opinions/similar experiences welcome.
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  #2  
Old 06-29-2021, 06:38 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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while I and many others sometimes see fluctuations in climb (especially top part of climb) or similar conditions related to heat, I believe that fluctuations in normal cruise, that appear out of the blue, are most likely to be the pump. Symptoms don't match up well for sender issues, though are possible (especially unlikely with a transducer - Kavlico) This assumes you didn't get near empty on a tank.

12 years makes a not so new pump. Remember that these work mostly with rubber parts and rubber dries out and cracks with age. If it also has quite a few hours, I would replace it. A fuel pump is definately one of those parts that wears with use AND age.

Larry
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Last edited by lr172 : 06-29-2021 at 06:47 AM.
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  #3  
Old 06-29-2021, 06:52 AM
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BruceMe BruceMe is offline
 
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Great input Larry and I think we're coming to the same conclusion, replace the pump.

Quote:
Originally Posted by lr172 View Post
Symptoms don't match up well for sender issues, though are possible (especially unlikely with a transducer - Kavlico)
Garmin G3X pressure transducer, not sure on make/model.

Quote:
Originally Posted by lr172 View Post
This assumes you didn't get near empty on a tank.
not empty

Quote:
Originally Posted by lr172 View Post
A fuel pump is definately one of those parts that wears with use AND age.
600hrs, normal usage

-Bruce
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  #4  
Old 06-30-2021, 01:27 AM
ppakotze ppakotze is offline
 
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Location: Cape Town
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Default Fuel fluctuations

Our IO-360 showed fuel fluctuations all over the place. It was the VDO resistive fuel sender that was failing. We replaced it with the latest Kavlico/Dynon fuel sender as our EFIS could accept either.

Afterwards I cut open the VDO sender and you could see the spot where the resistive bar inside the sender was worn out due to the pressure sitting there constantly during flight.

Regards
P.
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  #5  
Old 06-30-2021, 02:06 AM
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PaulvS PaulvS is offline
 
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Quote:
Originally Posted by ppakotze View Post
Our IO-360 showed fuel fluctuations all over the place. It was the VDO resistive fuel sender that was failing. We replaced it with the latest Kavlico/Dynon fuel sender as our EFIS could accept either.

Afterwards I cut open the VDO sender and you could see the spot where the resistive bar inside the sender was worn out due to the pressure sitting there constantly during flight.

Regards
P.
The VDO pressure sender for my oil system failed in the same way.
It would be less costly and much easier for the OP to first check the sender, in case it is a VDO sender that has failed in this way. It is possible that the elevated fuel pressure from the boost pump is pushing the sender wiper arm outside of the wear point.

One way to check a sender is to unscrew it from the engine, but keep the electrical connections (including a ground), attach it to a compressed air source and then vary the air pressure from 0 psi up to 25 psi and check the corresponding reading on the cockpit gauge or EMS.
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Last edited by PaulvS : 06-30-2021 at 02:12 AM. Reason: info on checking sender
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  #6  
Old 06-30-2021, 06:58 AM
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AlexPeterson AlexPeterson is offline
 
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There is a lot of evidence in multiple threads in this forum to suggest not running an engine driven fuel pump for 12 years. I'm not sure what the correct interval should be, but 6 to 8 years might be a good time to pro-actively change them. DanH had some great photos showing cracked diaphragms at not very many years; perhaps he will chime in here.

Your problem could very well be something separate from the pump.
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  #7  
Old 06-30-2021, 08:09 AM
BoydBirchler BoydBirchler is offline
 
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Have owned over 20 airplanes over nearly 50 years,8000+ hours experience, and have never had to replace a fuel pump!
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  #8  
Old 06-30-2021, 09:20 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by BoydBirchler View Post
Have owned over 20 airplanes over nearly 50 years,8000+ hours experience, and have never had to replace a fuel pump!
Are you implying from this that pumps don't fail from age? Your sample set is pretty small. Even if you had a 50 year old airplane with 8000 hours, it doesn't mean that no one elses pump will fail before that time. Plain and simple, natural rubber breaks down with age and most of the work done in our pumps is via rubber parts.

Even if the OP's pump is working properly, preventative replacement at 12 years is not a bad idea IMHO.

Larry
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Last edited by lr172 : 06-30-2021 at 09:26 AM.
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  #9  
Old 06-30-2021, 09:42 AM
scsmith scsmith is offline
 
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My fuel pump showed similar behavior after 11 years and I replaced it. My pressure would fluctuate between 25 and 18 on a 4-second cycle.

While I was at it, I put a cooling shroud on the pump.

Yes the VDO sensors went wonky. I would bet most of them are out of service by now.
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  #10  
Old 06-30-2021, 11:26 AM
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Danny King Danny King is offline
 
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Simular experience. I0360, G3X, fuel pressure fluctuating at cruise power from 24psi down to 19psi and back. Around 10 years of operation. Replaced fuel pump. No real change. Replaced transducer. Fluctuations decreased but not completely eliminated.

Nothing on the engine side of the firewall except #6 steel bulkhead fitting and fire sleeved fuel hose between firewall fitting and fuel pump. My guess is that it is not your fuel pump. It is probably the fuel pressure sender. With all the reports of this problem on VAF, some fluctuation might be considered normal.
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