White exhaust smoke, especially after the engine warms up
Sweet smell from the exhaust or engine bay
Engine overheating repeatedly
Coolant level dropping with no visible external leak
Milky or frothy oil on the dipstick or under the oil cap
Bubbles in the coolant reservoir or radiator
Rough idle, misfires, or poor engine performance
Loss of compression in one or more cylinders
Coolant in the spark plug wells or fouled spark plugs
Engine oil contaminated with coolant
Radiator hoses becoming hard very quickly after startup
Unexplained coolant loss and overheating together
Check engine light with misfire or overheating codes
Exhaust gases detected in the cooling system
Engine cranking but not starting after overheating event