[13000rpm]

What is the biggest reason for the coolant to enter the engine?

I did replace the engine oil with a new one.
Checked the engine oil gauge.
It turns green + brown oil.
The engine oil gauge went up.

I think caused a problem with the water pump
Is there any other reason?

 

[airhead83]

A cracked head or bad head gasket, water pump or leaking 0-ring or gasket, probably a few more I'm forgetting.

 

[Weltap]

I would check your water pump seal first. If everything checks out there I'd look more into a head gasket failure. Pulling the plugs and doing a compression test might narrow it down, too.

 

[GoFaster]

First suspect is the gasket for the right (clutch) engine cover, particularly if that cover has been off recently for whatever reason. Part of that gasket seals the coolant passage from the water pump, and it is a royal pain to get everything lined up during assembly. If it isn't ... it will leak coolant into the oil.

Second suspect is the water pump shaft seal, but it isn't reported to be a common problem area.

Third suspect is blown head gasket, but these engines do not have a reputation for blown head gaskets. If the engine has recently been grossly overheated (e.g. due to coolant leaking out), perhaps this could happen.

There are no other places where coolant passes directly next to engine oil.