Coolant level drops but no external leak is visible; possible white residue in exhaust or milky oil.

Safety: Internal leaks can lead to rapid engine failure. Do not ignore unexplained coolant loss.

• Check engine oil: Inspect dipstick and oil filler cap for milky or frothy oil, indicating coolant contamination.
• Inspect exhaust: Persistent white steam and sweet smell from exhaust may indicate coolant entering cylinders.
• Pressure and leak-down tests: Perform cooling system pressure test and cylinder leak-down test to locate the leak path.
• Professional repair: Internal coolant leaks typically require head gasket replacement, head repair, or cooler replacement by a qualified Volvo Penta technician.