BMW 3 Series Head Gasket Failure Symptoms and Diagnosis

Head gasket failure on BMW 3 Series shows as: white steam from exhaust when warm, sweet coolant smell from engine bay, bubbling in coolant reservoir, mayonnaise under oil cap, overheating without visible external leak, or milky oil on dipstick. Combustion gas test in the coolant reservoir confirms the diagnosis definitively. Head gasket replacement costs £700-1,500 on 3 Series.