I have a 2006 F90TJR Yamaha 4 stroke that I have been having problems with fuel in the oil. It started 2 years ago when I had the filter between the low pressure fuel pump and VST tank plug up which was causing the motor to cut out until you pulled back on the throttle and then it would run fine again. After a quick investigation when I got home I found that the filter was plugged and the engine oil was full of fuel. I changed the plugged fuel filter and replaced the engine oil and oil filter. It was good for a couple trips and then started having fuel in the oil again. The motor has never cut out on me since but numerous times after checking the oil when I got home it has had fuel in it. Last year I changed the VST needle and low pressure (mechanical) fuel pump, along with the tenth oil change. Ran the boat a few times last fall and didn't have any problems. This spring I changed the oil and took the boat out for a half day of running the next morning the oil was 1/2" over full. I thought maybe I had put too much oil in when changing so I drained some out, this put the level right at the top of the operating range. I took the boat out for a long day the very next weekend and sure enough the next morning when I checked the oil it was 1/2" over the top of the operating range again. Last night I pulled the cover off and was looking things over and checked the oil again, it is now 1-1/2" over full and smelling of fuel, the boat has not been started and it is higher on the stick than it was last weekend. Anybody out there ever come across something like this? I am at a loss of what could put that much fuel in the oil. How does one check the injectors for leaking into the cylinders? The one thing that I notice is after the boat sits or is trailered for a while the first start of the day has a large bluish cloud of smoke and sometimes has to be re-started other than that the boat runs great and starts great the rest of the day. This motor has a jet on it and runs in a shallow river, so props and lots of idling are out of the question, it is always running full throttle when moving. Any help would be greatly appreciated. Thanks.
Might do a leak down test to make sure cylinders are sealing properly and test thermostats to make sure they are working properly. sticking rings or cold running motor would cause this.
Have you owned and ran this motor since new and did it have any problems before?
What RPMs does this set up run at WOT?
I will do a leak down, anything that I need to unplug before I do that? I have checked thermostat operation and visually inspected as well, sorry failed to mention that. I purchased the boat when it was 10 months old and it had 10 hours on it. I never had a problem until fall of 2010. Not sure what RPM's the motor runs at as it does not have a tachometer. G3 boats are owned by Yamaha and there is no way to change the pitch of the impeller and there is at least a dozen other boats exact make and model as mine (Boat and Motor) in this neck of the woods that run the same waterways and they do not have this problem, so I think we can rule out the RPM issue. Any other feedback would be greatly appreicated. I will post leak down test results. In the meantime I still have a gut feeling on a injector filling a cylinder with fuel after the motor is shut off, hence the big puff of smoke on the first start of the day and the issue of the oil level rising from taking out of the water to the next day at home on the trailer.
haven'y been running motor with diluted oil, just did a quick start to pressure the rail and see if it would add fuel when it sat overnight.
As for the stat operation I removed it and inspected it, looked like new. Reinstalled and started motor, hose from stat ran warm water within a few minutes of the engine running. Do I need to do something else to check the stat?
As far as a leak down test I was going to unplug the coil packs and install a compression gauge on each cylinder and crank it over. Is that suffiecent?
When you reply to this post and let me know that the leak down procedure I have mentioned is ok I will do a compression test. Thanks.
What you have stated is a compression test not a leakdown.
Leakdown will tell you more about a motor and what the problem is if it has a problem and does not pass the leak down %, like rings, intake valve, or exhaust valve leaking pressure by.
It uses compressed air at a regulated pressure and does not require spinning motor
Stats should be tested for opening and closing at spec temps and opening all the way, in pot of water while heating and measuring temp
WE HAVE A WINNER! Called a friend of mine who had a leak down set, we did a leak down on each cylinder with all cylinders passing (holding at set) Did a compression test 3 cylinders at 175 1 cylinder at 170. Pulled the stat and it had a stick stuck in it keeping it open. During highwater there is tons of debris in the river. Removed the stick and put it in a pan on the stove with a digital thermometer, opened at 142 degrees Feirenheit. I think we have solved the issue. Thanks for your help.