Coolant pipe that runs from the front-left of the engine to the heater core inlet, carrying hot coolant into the cabin heater circuit. These metal-and-rubber pipes corrode and weep over time; a leak here shows up as a coolant smell and low coolant level with no visible external puddle since it's tucked against the engine.

Fitment: BMW E39 (525i/528i/530i), E46 (323/325/328/330 Ci/i/xi), E53 X5, E83 X3, Z3 and Z4 - M52/M54 engines.