Steaming exhaust

Q I have a 1986 Mazda RX-7. The engine starts and runs very well until it becomes warm. After around 10 minutes, when the thermostat opens, it starts boiling – exhaust gas mixes into the cooling water and makes a cloud of steam.

There is no oil in the cooling water and no water in the oil. I wonder if I should use the product K-seal to repair the fault? Odd Knudsen

A Because of the design of the rotary engine in your Mazda RX-7, the heat transfer dealt with by the cooling system is far higher than a conventional piston-driven engine.

You mention that it only stards boiling when the thermostat opens, so I would first ensure that it is exhaust gas in the cooling system and not simply overheating coolant due to a faulty thermostat or water pump. The thermostat in your RX-7 is a doubleacting unit and should block off the bypass as it opens to the cooling system. The O-rings do fail on this engine and normally give additional symptoms, such as difficult starting, white smoke from the exhaust and loss of coolant. If you have none of the other symptoms, I would recommend testing the coolant header tank to see if carbon monoxide gases are present, confirming the failure. If this is not the case, you’ll need to check for other causes of the overheating.

Having confirmed that an engine O-ring has failed, the only usual recourse is a complete engine rebuild, but I would certainly try K-seal, which may effect a satisfactory repair.