Scenario: Factory new IO550 on my Legacy.  440 Hours.  I'm second owner.  Back from FAA maintenance shop condition inspection recently.   

1) I always noticed a puddle of fuel after boost pump priming so never thought much of it.   Took my cowls off to change oil myself and inside of cowling was blue.  The factory fuel pump was leaking at a cost of about 2 gph based on my calculations.

2) After repairs, I tested the pump but fuel poured out of the drain lines connecting the cylinders to the belly check valve.  Further inspection revealed they were 8 years old, had cracked from the heat from nearby exhaust and literally were dumping fuel within a few inches of the exhaust pipe--unseen with the cowl on.  Brittle as toffee near the exhaust.

3) When I took my interior out of the plane for inspection, I decided to reveal the Andair valves for suspect fuel smell and all 4 banjo fittings were loose and not lockwired and blue stained.  They had worked loose over the years I guess.  They are now torqued and lockwired and no more stains to date. I previously would smell fuel in the cabin on final approach and now that is gone.

My comment to owners is to verify the puddle of fuel next time you have the cowls off.   TCM has a number of pumps that leak like mine and you will never know unless you look specifically for a problem.  Those hoses draining the cylinders should have been replaced much earlier and routed further away from the exhaust.  The Andair banjo fittings should never have been left without lockwire as Andair specifies.

I wanted a separate set of eyes on this condition inspection but the shop missed all of the above issues.

As for air in the system from leaking fittings: the TCM swirl chamber is designed to remove most or all air and return that to the tank in operation.   A small leak of air should not cause the io/tsio engine to falter as the design margins for fuel delivery are easily exceeded.   Personally, I would look at the maintenance basics for our fuel systems and check the integrity of the existing systems spending time searching for a design problem.

Paul

Legacy

Calgary