White exhaust smoke that continues after the engine warms up
Coolant loss with no visible external leak
Engine overheating repeatedly
Milky or frothy oil on the dipstick or oil cap
Bubbling in the radiator or coolant reservoir
Sweet smell from the exhaust or engine bay
Rough idle, misfiring, or hard starting
Poor engine performance or loss of power
Coolant in the oil or oil in the coolant
Persistent low coolant level after refilling
Pressure building in the cooling system soon after startup
Compression test showing low compression in one or more cylinders
Leak-down test showing air escaping into the cooling system
Spark plugs that look steam-cleaned or unusually white
Check engine light with misfire or overheating codes
Visible external coolant leaks ruled out but symptoms remain