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My Mechanical Fuel Pump Saga

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ArlingtonRV

Well Known Member
I had been suspecting my engine driven fuel pump of being on its last legs, mostly because it had been installed for 12 years and 1,500 hrs. I asked a couple of questions here in the past weeks and got some help.

The full story is linked below if anyone wants the whole thing. It is a little long, but should cure that insomnia you've been suffering from.

http://rushesroost.com/blog/

The big thing that I wanted to bring to everyone's attention was what I found when I got the old pump out. The bolts that hold the bottom section were loose. I don't just mean you could put a wrench on them and turn them a bit, I mean they were backed out so you could see daylight under the wave washer. It took several turns to get the bolts just snug.

IMG_1095-XL.jpg


Just for reference, this is what the new one looks like.

IMG_1094-L.jpg


The side in the picture is the worst, but they were all loose and they all took several turns to tighten up.

I just finished my first condition inspection on this airplane in March and didn't notice it. I had never actually put a wrench on these before, I just looked them over visually, but I am adding a step to my condition inspection checklist to put a wrench on these in the future.
 
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I'm pretty sure the washers are supposed to have that gap. Every mechanical pump I've seen has had the gap. BWTHDIK
 
Mine came that way too. That ring is a wave spring to maintain preload as the rubber gaskets/diaphragms shrink with age (or swell from the use of 'alternative' fuels, in which case the bolts might break if tightened all the way).
 
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The high pressure pump, for FI engines, has a wave washer on the bottom of the pump and the low pressure one for a carb'd engine has no washer. So the wave washer gaps are normal. That being said the bolts on the wave washer pump should not be loose. In all my years working on aircraft engines I never saw the bolts on that style pump loosen up. So, I think they were tight but you felt they were loose because of the wave washer. I would imagine it would take a lot of excessive torque to get the wave washer flattened out like that.....
https://groups.yahoo.com/neo/groups/lycoming/files for pictures of each style pump.
Good Luck,
Mahlon
 
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I saw ArlingtonRV's old fuel pump, and indeed the bolts on the bottom of the pump were loose. Just compare the picture of the old pump and the brand new pump just out of the box in Post 1 and it's obvious.
 
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The high pressure pump, for FI engines, has a wave washer on the bottom of the pump and the low pressure one for a carb'd engine has no washer. So the wave washer gaps are normal. That being said the bolts on the wave washer pump should not be loose. In all my years working on aircraft engines I never saw the bolts on that style pump loosen up. So, I think they were tight but you felt they were loose because of the wave washer. I would imagine it would take a lot of excessive torque to get the wave washer flattened out like that.....
https://groups.yahoo.com/neo/groups/lycoming/files for pictures of each style pump.
Good Luck,
Mahlon

Mahlon (engine guru) has probably seen more pumps than everyone on this forum put together, I'm pretty sure we can trust what he says!
 
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