White smoke from the exhaust
Milky or frothy oil on the dipstick or under the oil cap
Coolant loss with no visible leak
Engine overheating
Bubbles in the radiator or coolant reservoir
Sweet smell from the exhaust
Rough idle or misfiring
Loss of engine power
Coolant in the oil
Oil in the coolant
Persistent check engine light
Compression loss in one or more cylinders
Spark plugs that look steam-cleaned or unusually white
Hard starting or no-start condition
Unexplained engine pressure buildup in the cooling system