Yamaha 4-stroke, fuel in oil

[MrBay]

Hi folks.

Engine is a 1996 50hp fourstroke highthrust yamaha.

Ok heres the story

Bought this engine second hand for a boat i have rebuilt. First time out, there was a problem with the choke in that it was fully on all day (a screw had come out of the linkage) the choke problem was causing starting issues once warm, and cutting out once started etc.

Got home, found there was the missing screw, replaced it, and everything seemed fine. Starting issues gone, cutting out problem gone....

A few days later i decided to check the oil level, as soon as i pulled the dipstick, what i thought to be oil started spewing out of the dipstick hole. It transpired it was actually mainly fuel.

Did a little reading on here (the thread called "making oil" in particular) which suggested that over choking could cause fuel to enter the crankcase, so with that i assumed this was the problem, replaced the oil, ran it in a tank for a good while on tick over, then replaced oil again.

Went out last wednesday to try it, and everything went fine, engine started fine, ran fine etc....

Came in, checked the dipstick, same thing happened, loads of fuel in the oil, spewing out of the dipstick hole........



Someone has suggested that it is likely to be the fuel pump diaphragm leaking fuel into the crankcase. However i have today taken the pump off and the diaphragm looks ok to me....

Is there anything else this could be??? Or, is it simply that it is the diaphragm is knackered, just not to the human eye....

Any ideas much appreciated!!!

Thanks

Paul

 

[MrBay]

Re: Yamaha 4-stroke, fuel in oil

No ideas folks????


UPDATE, just checked Tstat and it has been sticking open, therefore running cool, think i have read somewhere that this could cause "blow-by" where the rings dont seat properly, allowing fuel to pass them and enter the crankcase. Could anyone confirm/deny this???

Cheers

 

[jhebert]

Re: Yamaha 4-stroke, fuel in oil

Based on many reports of problems with the oil lubricating sump being contaminated with gasoline in marine four-cycle outboards, and your narrative of the symptoms, I'd say your Yamaha motor has this problem. If the engine cooling system is not properly controlling the temperature and the engine runs cold all the time, you will likely see more fuel contamination of your lubricating oil.

The engine will generally run better with the cooling system operating properly and maintaining a warmer engine temperature. Repair the cooling system. It should also help reduce the oil contamination problem.

 

[MrBay]

Re: Yamaha 4-stroke, fuel in oil

Right cheers for that...

I have just dropped the oil (fuel) from the sump. I put 2 litres of oil in before my last trip out, there was around 3 litres of fuel/oil in there after approx 1 hours running at 5000-5250 revs ish.....

Does this sound extremely excessive, given the thermostat problem?!? i guess what im asking is, obviously im going to replace the sticking tstat, but could the sticking tstat cause THAT much fuel to enter the crankcase???

What sort of quantities are we talking about in the "making oil" thread??!