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I have 2005 yamaha four stroke, 200 hp v-6. I ran it problem free most of last year. In september i changed the oil and ran it pretty hard on about 5 outings. On the last of those five trips the motor ran good all day but on the way back after running at full throttle for about 10 minutes I noticed the rpms drop. I backed of went to full and it ran bit and rpms dropped again. This reoccured 2 or3 more times then I backed of the throttle to 5000 rpms and it ran the rest of the way and never even sputtered. When I got home I checked the and found it to be a lite brown color (about the color of coffee with cream in it.) At that point was thinking I had a bad head gasket but I figure I would try try changing the oil again just case I did not tighten the drain plug properly because I had noticing a light slick of oil on the surface ever since Ichanged the oil. So I changed the oil ran it the rest of the season, (at least five more times) with no problems, checked oil before and after everytime and it looked good. I ran the boat today, checked the oil before I left it was good, on the back in running full throttle acted up just like I described above. Got home checked the oil and it was brown again. I did put a new crush washer on both times I changed the oil. Any suggestion. I'm thinking if it had bad head gasket that show up after a full day running, am I wrong any sure to diagnose that without tearing motor apart
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Are you sure it is water and not fuel? Just wondering as I know some of the 4-strokes had problems with "making oil".....if it is fuel in the oil which will discolor it it could be caused by the motor not getting to operating temperature - check thermostat to make sure it is not stuck open or maybe the the engine is over proped so as motor does not rev out and you could have too much fuel in cylinders and this could by pass the rings. -
Yeah pretty certain it is water, took sample to my inspection mechanic he seemed to have no doubt that there was water in the oil. It is milky in consistancy. I will check on the thermostats for shore though, the problem did arise after running it in cold weather. I don't believe that is over proped, the motor is on a 22ft walk around and it turns 6200 rpms at full throttle and dosent lug getting there. Thank u for replyComment
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