How can fuel get into oil?
The engine was run up to bottom of green arc on oil temp – maybe 10 mins – so the oil drains easily.
Peter wrote:
How can fuel get into oil?
Past the piston rings?
Past the rings is certainly a possibility, but on most aircraft engines it’s not the same (classic) situation as a gravity flow fuel system with a leaky fuel valve, feeding a flooded carb located above the cylinders.
If you primed the cylinders with fuel heavily before starting the engine, some fuel may have drained past the rings and not completely evaporated from the oil in the time available.
Another possibility on engines with a cam driven mechanical fuel pump is an internally leaking fuel pump.
Airborne_Again wrote:
Past the piston rings?
Most probably, and actually reflected in the lead contents in your analysis.
Silvaire wrote:
internally leaking fuel pump
Which should show by fuel out of the vent line on the pump, assuming it ain’t plugged 
Interestingly they say a ground run before sampling can do this, which suggests you always get some, and not getting the oil hot for long enough will fail to evaporate it.
I did once find a defective fuel pump; the lever return spring was broken. It still worked… They are only about $300, and can be changed easily.
But… the pump should obviously not leak at all. I went back 10 years and have not had fuel reported at more than a “trace” until the last few samples.
I think I will change the pump.
This is interesting – from 4 years ago
The pump was actually changed largely because I was getting 1.5% fuel in the oil, on the last oil analysis. Normally it is around 0 to 0.5%.
It is P/N AF15473 or 62B26931 and they give trouble
I will get a Tempest version – AF15473.
Another oil sample:
Another hypothesis is a different priming technique by my son who flies the plane more than I do. He has apparently sometimes not started the engine until some tens of seconds after priming, which might give time for fuel to seep past the pistons.
Everything else looks good. Calcium is a marker in Camguard; I use up 1 bottle of CG for 12 quarts of W80. 9 bottles go into the engine at the service and the extra 3 live in the back; 3 of them get used up over the next ~30hrs and then maybe 2 more go in (over the 50hr interval) without CG added.
The fuel in oil has disappeared so I reckon it was the starting technique or the fuel pump.
I will never know, but it doesn’t matter 
