new thermostat causes oil in water?

1998 Peugot 306 automatic. Car was running cool. Changed thermostat. Car now runs at temp, no overheating. Water was spotless before the thermostat was changed. Now lots of oil in the water. Is this causal or am I just unlucky? Local mechanic advised disabling the oil cooler – thinks it might be cracked inside. Does this make sense?

2 thoughts on “new thermostat causes oil in water?”

  1. It is possible that now the thermostat is working is is allowingt he cooling system to under go the 15 – 17 psi where as when the thermostat was stuck open the system never had that much pressure on it. Now that it has more pressure on it, it may be showing signs that weren’t visable under low pressure circumstances.

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