I have a 1971 boat with a Volvo Penta AQ170 inline 6. The engine runs well, but it overheats rather quickly. After about 5 minutes of running in the driveway, the gauge was bordering on the red zone, and it didnt feel like the thermostat had opened yet. Last time I had it in the water it did the same thing, but fluctuated with my speed, if i remember right, i was able to keep it in the green at about half throttle. <br /><br />I have gone through and confirmed the cooling system is wide open, no restrictions, and the water pump impeller is in very good condition. I have not checked the internal water pump, but I dont see how that would cause such a quick overheat. I also have not qualified the gauge itself, because I dont have a backup. I did remove the thermostat and boiled it, confirmed it does indeed open when the book says it should, and closes when it should too. After running it today, I pulled the thermostat housing and the t-stat was sitting there wide open. I am going to run it w/o the thermostat just to make sure. <br /><br />When I pulled the t-stat housing off this last time, I noticed a sheen on the water that spilled out of the engine. I colleted some in a jar and it did have a slight sheen on it, but no oil actually separated out of the water. There was also some sediment in the water that swirled around when I shook it, but then settled to the bottom. <br /><br />Could this be a head gasket? How can I confirm that in an open cooling system? Any other thoughts? Thanks!
