White smoke from the exhaust
Coolant loss with no visible leak
Engine overheating
Milky oil on the dipstick or under the oil cap
Bubbles in the radiator or coolant reservoir
Rough idle or misfiring
Loss of engine power
Sweet smell from the exhaust
Spark plugs that look steam-cleaned or fouled
Coolant in the oil or oil in the coolant
Hard starting or no-start condition
Compression test showing low compression in one or more cylinders
Leak-down test showing air escaping into the cooling system
Persistent coolant contamination after refilling
Engine running poorly after overheating event