Inspect the Cooling System
 

• Start by visually inspecting the entire cooling system for any obvious signs of leaks. Look for puddles of coolant under the car and check around common leak points such as the radiator, hoses, and water pump.
• With the engine off and cool, open the hood and check the coolant level in the expansion tank. Low levels can be a sign of a leak.
• Inspect the radiator cap for wear or damage, as a faulty cap can also cause leaks.

Pressure Test the System
 

• If no obvious leaks are found during the visual inspection, perform a pressure test on the cooling system. Use a radiator pressure tester to pressurize the system to its rated pressure, typically around 15 psi.
• Observe the pressure gauge. If the pressure drops, this indicates a leak. Check all hoses, connections, and components for any signs of leaking coolant.

Repair or Replace Damaged Parts
 

• Once the source of the leak has been identified, proceed to repair or replace the damaged component. Common repairs include:
  • Hoses: Replace any cracked or leaking hoses. Ensure that the new hoses are correctly fitted and clamped securely.
  • Radiator: If the radiator is leaking, it may need to be replaced. Drain the cooling system, remove the old radiator, and install a new one following the vehicle's service manual instructions.
  • Water Pump: A leaking water pump usually requires replacement. Drain the coolant, remove the drive belts and pulley, unbolt the pump, and install a new pump according to the manufacturer's guidelines.
  • Gaskets and Seals: Inspect and replace any leaking gaskets and seals, especially around the thermostat housing and radiator.

Refill and Bleed the Cooling System
 

• After completing the necessary repairs, refill the cooling system with the appropriate type and mixture of coolant. For the 2007 BMW Z4 M Roadster, a mixture of BMW-approved coolant and distilled water is recommended.
• To ensure there are no air pockets in the system, bleed the coolant system properly. Open the bleed screws and run the engine until it reaches operating temperature. Allow the coolant to cycle and check the level in the expansion tank again.

Perform a Final Inspection and Test Drive
 

• Once the cooling system is refilled and bled, perform a final inspection to ensure there are no leaks. Start the engine and monitor the coolant levels and operating temperature.
• Take the car for a test drive, keeping a close eye on the temperature gauge and checking for any signs of leaks after the drive.

By following these steps, you can effectively diagnose and repair cooling system leaks in your 2007 BMW Z4 M Roadster (E85).