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I have an overheating issue, it's fine until the oil gets up to temp then the coolant starts to overheat. If I put the AC on full heat then the coolant stays at 90. Oil gets up to around 115 then after 20-40 minutes it comes down to 100 give or take. When oil temp comes down I can turn off AC full heat and it's fine. Doesn't usually go back up.

All fluid levels correct. Serviced and no change.

Anyone had this issue before? Anything to check?
TIA!

Did you check all fluïds?

Oil and coolant are all okay. Serviced it, no change.

bad thermostat?
bad coolant reservoir cap?
bad water pump?