Internal oil/coolant leak. How to determine?

3 replies to this topic
  • umu24013

Posted April 17, 2009 - 03:09 AM


Just rebild the top end of a YZ426.

There are no external collant/oil leaks so far.

How do I know if there is an internal leak (between the cylinder and head / between the cylinder and crankcase)?

As far as I know there may be two scenarios:

Oil in coolant and coolant in oil

How to determine? (without taking apart the engine)

What can happen to the engine if there is an internal leak?

  • grayracer513

Posted April 17, 2009 - 06:16 AM


In the event of cross-contamination, you can see it visually. You will find oil in the top of the radiator, and/or coolant in the oil. In the latter case, the oil may become foamy and "milky", looking somewhat like a rather gross milk shake, and there may be actual coolant at the drain plugs when they are opened.

Coolant in the oil almost totally eliminates the oil's ability to lubricate anything, so you can imagine what happens. It also attacks wet clutch linings.

Coolant can also leak internally from the head gasket or a crack in the head/cylinder and go straight to the combustion chamber without entering the oil supply.

It is normal for the coolant level to drop to about the top of the radiator core due to expansion/contraction.

What makes you believe you have a leak?

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  • umu24013

Posted April 17, 2009 - 07:18 AM


Thank you very much for the complete info on this.

What makes you believe you have a leak?

I've installed a used head gasket :p The base (cylinder) gasket is brand new.

  • grayracer513

Posted April 17, 2009 - 07:52 AM


I've installed a used head gasket

That was not wise.

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