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Flying my M20K 231/Rocket 305 through eastern Washington this New Years weekend at 14500 MSL in VFR conditions, I felt some light turbulence. After about a minute, it occurred to me that the turbulence was not changing in amplitude like turbulence usually does. I considered whether it could be a small prop imbalance due to icing, but that would be strange since I had been in clear air since takeoff an hour earler. I considered whether it could be a cylinder misfiring, but EGTs were normal on all cylinders, and TIT remained where I had dialed it in. Manifold pressure remained at 30 inches, engine speed was still 2400 RPM, and I wasn't losing airspeed. Then the engine sneezed. That got my attention. I aimed towards the nearest airport, pulled out three inches of manifold pressure, pointed the nose down, and popped the speedbrakes. The engine sneezed one more time on descent, but I stayed focused enough to run all my checklists and grease the landing. Maintenance wasn't available on the weekend, so the aircraft sat on the ramp, shivering ignominiously in sub-20F temps. First thing Monday morning, we brought her into the hangar and immediately pulled a belly pan and opened the gascolator/sump to look at the fuel strainer screen. It was partially coated with what looked like a very thin paste - imagine spreading the thinnest possible layer of white or maybe grey toothpaste over the screen, then wiping it off so you could see through it. Over the next 30 seconds, in the warmish hangar, the paste melted and turned into beads of water. This topic has been covered on the forum in the past (e.g. So whats too cold for the plane and IPA Winter Ops), but I figured a fresh PIREP is always a good PSA. In reading through those topics, I noticed a couple comments that deserve a response. kortopates wrote "I really don't believe there is risk of fuel becoming saturated with water unless there is more moisture than just humid air in the tanks" but then, in a very gentlemanly and helpful way, he retracted his statement and shared Mike Busch's observations: He [MikeB] told me there is a real risk of dissolved (undrainable) water coming out of solution at low temperatures and freezing into ice crystals that can cause engine stoppage. He knows of several emergencies that occurred because of this, although it's definitely not common. Mike says these incidents don't occur in the wintertime as far as he knows. He says the major risk is taking off from a low altitude airport where the temperature and humidity are high and the fuel has become heat-soaked because the aircraft has been sitting in the sun for some hours, then climbing up to the Flight Levels where the OAT is very cold. It takes very hot fuel that can hold a lot more water in a dissolved state (as you pointed out before). The conditions leading to this issue have always been summertime conditions though, not winter. I will simply add that 1) it's winter now, and very very cold for the Pacific Northwest, and 2) I fueled (almost 60 gallons to top off) from an above-ground tank that was at below-freezing temperatures. Thus, I have an existence proof that it can happen in winter with cold fuel. jlunseth wrote of his experience never having this problem: "Fuel gets put on board at ground temps which are warm, and in relatively humid conditions. Then I go to altitude, stay there for 4 hours at -25C, and nothing happens. " Well, this happened to me after only 1 hour at altitude, and yes the OAT at that altitude was -25C (=-13F). I do have many hundreds of trouble-free hours flying this aircraft and another Mooney at temperatures significantly below freezing, though usually never lower than -10C. This happened to be the first time for me in this aircraft flying at -25C. I had done a thorough sump/drain before my flight and there was no evidence of liquid water at all. After observing the ice crystals in the strainer screen, we drained half a gallon from the wings and the sump and there was still no evidence of water. For those who haven't followed the threads, the culprit is water dissolved in the fuel, not free liquid water. 100LL can't dissolve much water, but water is not completely insoluble in fuel, and having tiny amounts of water (on the order of 30 parts per million) dissolved in the fuel is OK so long as it remains dissolved. Sub-freezing temperatures and nucleation sites (perhaps even localized turbulence as fuel flows through a fine mesh) will cause the water to precipitate out as microscopic ice crystals. Once an ice crystal forms, it's going to continue to grow as more fuel flows past it and dissolved water preferentially attaches itself to the crystal structure. These crystals can attach themselves to rough surfaces, get caught in fine mesh screens, or plug tiny injector orifices. For you chemists who are concerned about the proper use of the word "dissolved", I admit I don't know if the water is truly dissolved in the gasoline (= in solution on a molecular scale) or simply finely divided and in suspension (= extremely tiny water droplets that are still orders of magnitude bigger than molecular scale). AC 20-125 discusses "entrained" water and describes it as the latter. I will note that our white bucket test revealed the gasoline in my tanks to be both "clear=clean" and "bright=dry", with no cloudiness that would indicate entrained water. The solutions to the problem, as have been discussed elsewhere on this forum, are: 1) If the fuel is dry, ice can't form. Read AC 20-125 to understand the various ways water can get introduced into fuel. Use fuel from trustworthy tanking/pumping systems that are properly maintained. Make sure your gas cap O-rings are in good shape. Always sump your fuel to check for water. 2) If the fuel has water but remains above freezing as it travels through the aircraft fuel system, then small amounts (30 ppm or less) probably won't have any effect on operation. 3) Adding anhydrous isopropyl alcohol (IPA) to the fuel, in amounts not to exceed Mooney SI M20-64, encourages the water to dissolve into the IPA, which lowers the freezing point of the water, hopefully enough to prevent it from precipitating out as ice. 4) (my recommendation) After taking off from hot and humid conditions, or even cold and dry conditions, when flying for as little as M minutes in temperatures below N degrees C, choose an altitude and a route that give you good descent and landing options in case, like me, you encounter fuel system icing despite all the normal precautions. In my case, M was 60 and N was -25. I am congratulating myself on electing to perform that flight on that day only because it was VMC and, in the event of a problem, I wouldn't have to descend through several thousand feet of icing conditions. I am also considering myself lucky that it didn't happen 30 minutes later over the Cascade mountains.