BMW M4: S55 coolant expansion tank and oil cooler housing crack

The F82 M4 with the S55 engine uses a plastic coolant expansion tank and a thermoplastic oil cooler housing that are known to crack under thermal cycling, causing coolant loss and potential overheating. The expansion tank plastic becomes brittle with age and heat exposure, developing hairline cracks at the seam and around the cap neck. Separately, the oil cooler thermostatic housing can also crack or develop sealing failures. Both are common on cars over five years old or 80,000 km.

• Coolant warning light or low coolant level with no visible puddle undercar
• Coolant smell from engine bay after a run
• Visible crack or weeping at coolant expansion tank seam or cap area
• Overheating if crack advances and coolant loss is significant

1. 1

   Inspect the coolant expansion tank closely for hairline cracks, particularly at the seam and around the pressure cap neck.

2. 2

   Check the oil cooler housing for seeping coolant.

3. 3

   Inspect all coolant hoses for brittleness or cracking.

4. 4

   Check coolant level both cold and hot.

Parts involved: Coolant expansion tank.

$400 to $1,200 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Has the coolant expansion tank been replaced?”
• “Have you noticed any coolant consumption or unexplained level drops?”
• “Has the car ever overheated or triggered a coolant warning?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.