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  #11  
Old 07-21-2016, 06:30 AM
mahlon_r mahlon_r is offline
 
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The low pressure pump would not be able to produce 25 psi at low rpm... But to verify which pump is installed is an easy visual. The high pressure pump has a wave like ring washer around the bottom of the pump between the screw heads that hold the pump together and the pump body and the low pressure pump has no wave washer between the screws that hold it together and the pump.

If you are a member of the yahoo Lycoming group at the bottom of the files section there is a picture of the two pumps side by side.
https://groups.yahoo.com/neo/groups/lycoming/files

It does sound as though there may be a blockage or a leak causing the FP to deteriorate as demand increases.

Good Luck,
Mahlon

Quote:
Originally Posted by jrs14855 View Post
LW15473 Is the correct pump. The pictures in Spruce catalog show the part number stamped close to the mounting flange on the left side, the part number is shown vertical with the LW at the top. You can view this at aircraftspruce.com, search Lycoming fuel pump, click on the image to enlarge it.
Another remote possibility if the fuel pump was removed and reinstalled after the engine runs is the possibility that the actuator rod is not properly aligned and making contact with the arm of the fuel pump. Very unlikely but it has happened.
I have no experience with the purge valve system but an easy check would be to simply cap off the line to the purge valve and do a test run. An alternative is to disconnect the line between the purge valve and the tank, stick the line in a can and run the boost pump.
You could also run a line from a gas can direct to the engine driven pump for testing. Starting would require cranking the engine a bit with the mixture full rich to prime, then normal starting procedure.
Most of the above very unlikely but possible. Verify the pump part number first.
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  #12  
Old 07-21-2016, 08:44 AM
SuperCubDriver SuperCubDriver is offline
 
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Today I had another look at the pump. There is no part number on the pump but it has the wave like ring washer at the bottom. I prepared an aux tank which will eventually go in the rear baggage area today and I will connect the tank to the inlet of the engine driven pump and will see how the pressure develops. That way the pump doesn´t have to suck through the boost pump.
Mixture control is unresponsive til cutoff.

I´m interested what timing numbers are best for the IO-375, and yes I adjusted to 25 deg. On my -7 with an IO-375 this worked out ok.
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Last edited by SuperCubDriver : 07-21-2016 at 08:53 AM.
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  #13  
Old 07-22-2016, 12:46 PM
SuperCubDriver SuperCubDriver is offline
 
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Default Update - problem solved

I connected the aux tank to the engine driven pump inlet and the engine was running on full power with 25 psi and 17 GPH. Then were is the problem behind the firewall? Well I removed the boost pump and made a connection between the fittings and the engine was running fine again. So the boost pump is faulty and restrictive, blowing into the inlet proves clearly the restriction.

Idle is much better now. I had to enrichen the idle mixture quite a bit and it started to smooth out.
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  #14  
Old 07-22-2016, 04:03 PM
BillL BillL is offline
 
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Quote:
Originally Posted by SuperCubDriver View Post
I connected the aux tank to the engine driven pump inlet and the engine was running on full power with 25 psi and 17 GPH. Then were is the problem behind the firewall? Well I removed the boost pump and made a connection between the fittings and the engine was running fine again. So the boost pump is faulty and restrictive, blowing into the inlet proves clearly the restriction.

Idle is much better now. I had to enrichen the idle mixture quite a bit and it started to smooth out.
Herman, it does not sound like the problem is solved, but the diagnostics have isolated the problem. You might have a suction leak. A pressure test of the section of the fuel system between the firewall and wing root is suggested. If you have access to 30 psi air, then carefully purge that section, then block it at one end. Then pressurize at the other with a gage and use a valve to shut off the air supply. If the pressure drops more than 1 psi in a few hours, then you have a leak. You can find it with a soapy solution that will make bubbles at the leak. A friend and I have each found a leak this way. It still maybe a restriction, but eliminating a suction leak first will allow the more difficult task of quantifying the cause of a pressure drop.
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  #15  
Old 07-23-2016, 12:51 PM
SuperCubDriver SuperCubDriver is offline
 
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Good point with the suction leak. I might check but I really think the boost pump is the problem. Two reasons: I should smell fuel with the boost pump running - ok only if there is a leak downstream of the pump. The other reason should be the final answer: I replaced the boost pump with a piece of hose and the engine is running fine; and blowing into the boost pump inlet shows the flow restrictioin.
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