coolant leaking into oil without bike being started

:p:cry: My nephew just got a 98 YZ400f we knew there was water leaking into oil so we replaced all the internals on the water pump. He finished putting bike together and had another issue and when he pulled the clutch case back off he found fresh antifreeze in oil. My question is what the hell could let coolant leak into motor without even being started?:lol: :lol:

In the first place, coolant cannot get from the water pump to the oil supply. There are two separate seals for coolant and oil, and a gap between the two that vents to the outside. Coolant leaking at the impeller shaft goes straight out.

There are 4 possibilities for a source of your leak:

  • The O-ring at the coolant passageway from the crankcase cover to the crankcase.
  • The O-ring at the coolant passageway from the crankcase to the cylinder.
  • The head gasket.
  • A crack in either the head or the cylinder.

In the first place, coolant cannot get from the water pump to the oil supply. There are two separate seals for coolant and oil, and a gap between the two that vents to the outside. Coolant leaking at the impeller shaft goes straight out.

There are 4 possibilities for a source of your leak:

  • The O-ring at the coolant passageway from the crankcase cover to the crankcase.
  • The O-ring at the coolant passageway from the crankcase to the cylinder.
  • The head gasket.
  • A crack in either the head or the cylinder.

We are going to replace o-ring at coolant pass from cover to crankcase. Also if there where a crack would it leak without pressure?

Possibly.

Could the cylinder base gasket also be a potential problem?

Gray just a question. If the bearing was bad for the water pump could that cause water to leak into the oil? I'm not questioning your experience just wondering before start working on the same problem.

I don't think that a bad water pump bearing would leak water into your oil. Here's a pic of a water pump and the seals associated with it showing how it goes together.

The water pump shaft goes through the bearing (#8) then through the oil seal (#7). Then there is about 1/2" of air space then the water seal (#6). The gap between your oil and water seal is there to keep your oil and water separated. If one of the seals goes bad and starts leaking it will drain out of the drain hole in the bottom of your case and onto the ground. If both seals go bad you will have oil and water on the ground. The oil and water won't mix and contaminate each other through the water pump.

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