Got this heating issue when I drive my 5k corolla on a cold winter Hobart night.
When I first drive the corolla, starting with a cold engine, until it warms up 5-10 minutes later, the temperature gauge goes up to 1/3, then the thermostat opens up and lets cold coolant into the engine. The temperature rapidly goes down. Then the engine starts to run rough and I have to keep the revs up so it won't stall at the traffic lights. It last around 20 seconds. I suspect the sudden flow of cold coolant from the radiator warps the head on the block and starts to let coolant to seep through the head gasket. When I check underneath the oil cap after this event, there is a creamy looking substance; likely indicating that coolant/water is entering into the engine oil.
The thermostat and water pump is a couple years old. It only happens in winter time.
Not a major problem, but just curious if anyone has encountered this before, and if its just a matter of planing the head and putting on a fresh head gasket on?
Or if its a K-engine head design characteristic with hot spots? Perhaps the air-bleed valve in the thermostat needs to be opened up more?