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purge valve bypass
Bob.
Just thought...but if you had purge valve bypassing slightly... when you turn on electric pump on is it possible it increased flow to raise pressure even with purge valve bypassing? Again just thought I had. |
Bob, I looked at the data (nice data service, BTW) and it did not appear that the low pressure occurred on the last ground run where you were testing various combinations. It looked like it was never lower than 23 when the boost pump was off and the rpm was even up to near 2700. If this is correct, then it indicates (to me) that the mechanical pump is functioning properly even at high fuel flows. And, that the issue of dropping pressure is due to higher suction from angle of attack and lower absolute ambient pressure from altitude resulting in fuel vaporizing due to the low pressure to the point that the mech pump can not keep up.
So, unless the "community" as some absolute suction pressure target for a clean system to check against. The only option seems to be to clean and check screens and filters, and verify there is no tiny leak that is allowing air into the system. It is still possible a tiny leak could be there but has not reduced the pumping effectiveness enough on the ground to cause pressure drop. Clearly this situation needs to be addressed as it will only occur more easily during hot fuel conditions. I will add more if my brain lets me. Curiosity, where did you place your red cube in the system? |
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just a thought--
years ago in automotive, we would get low, or very little fuel pressure from some 350 chevrolet engines that had a return line at the pump. Blocking off the return, solved all of those issues.
Granted, we are dealing with a different situation, it's similar to what Bob is experiencing. I know that he has inspected all the hoses from the firewall aft to the tanks, and found no issues. I know that he has looked at every component, its location, and how it all interacts with each other. This one is puzzling. |
I just received this from Lycoming. At the moment, they are following the legal technicality of the warranty term. Which means that the fuel pump, while it may be DOA, the repair is on me. I'm still attempting to convince them that it should be a warranty repair.
--------- Bob, Thank you for the information. It appears that the engine driven fuel pump needs replaced. The part number of the fuel pump is 62B26931. I downloaded this issue in our system, the reference number is 2567. Please order the part through an authorized Lycoming Distributor. http://www.lycoming.com/Lycoming/PRODUCTS/DistributorLocator.aspx Thank you, |
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Bob,
What's the chance of sending the pump to the manufacturer for evaluation? Bill |
Same here
Your numbers look just like mine!
After all this it's probably too late to convince you that nothing is wrong with your fuel pump. |
I will be very surprised if its a mechanical fuel pump issue.
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We'll see what the real root cause is over the next few days. I'll be checking the filter screens as well as replacing the fuel pump.
Lycoming has authorized a warranty repair. bob |
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