The International Cessna^® 170 Association

Richard, at the risk of someone misunderstanding my question: Have you used autogas recently? Autogas these days may contain up to 10% Ethanol, and it will attract moisture like a sponge.
I once stored Ethanol in a 5 gallon metal gas can with a gasketed, tightly capped lid. Two months later (approx) I tried to mow the lawn with it, and my mower ran about 5 minutes and quit. I couldn't get it re-started and troubleshot the danged thing for about an hour before I realized the tank was 1/3 full of water. (When I tipped the metal can upside down to fuel the mower, the accumulated water ran to the bottom and was first-out into my mower.)
I also once flew with a friend in his 182 and lost an engine on takeoff out of Dallas Love field at night. Thankfully we landed on the remaining runway. Lots and lots of water in the tanks. (A&P autogas user.)
Not for me.

George-
Good idea. That would be a plausible cause if I had been using car gas, but no I have not. 15D has not had any auto gas in it since probably 1978 when I got out of college and got a job that paid enough to afford av gas. It has only been fueled with 80/87 and 100LL since.

I did use car gas before there was any STC to do it legally. I hope the statute of limitations has run out by now. Anyway in about 1974 Richard Collins of Flying Magazine fame personally told me he would officially deny it but he had run it in his TriPacer for years with no problems. That was good enough for me. Of course that was when regular car gas had lead and no alcohol.

I think it was probably condensation that accumulated during the 6 month overhaul period after the tanks were reinstalled but left empty while sitting outside. I hope so anyway. The tank drains and gascolator were dry on the first pre-flight after overhaul (sitting outside below freezing and probably frozen). Pre-heating the engine overnight with an electric heater did not thaw the water in the tanks. On the second pre-flight after being in a heated hangar overnight there was a full quick drain tube of water. The third pre-flight had about 1/4 quick drain tube. Thereafter it has been about 1/2"-3/4" in the quick drain tube. I'm thinking it is taking some time to get sloshed off the tank walls, drain to the bottom, and make its way to the gascolator. The engine is running smooth as silk so I don't think it has gone that far.

I especially would not use car gas on a fresh overhaul before the valves and seats had a chance to get coated good with lead. The engine overhauler made a point to show me some valve seats he removed from an engine that was fed car gas from day one. After about 200 hours the exhaust valves had receded into the seats about 0.050".

Thanks for the comment

Am fairly certain that all of the auto gas STC's state not to use auto gas during the first hours of a new, remanufactured, etc. engine. The lead is very needed during those first hours.