Hi, guys! I've had problems with the cooling system of my '13 Cayenne Diesel for some time. First, had to replace some of the plastic pipes, since coolant starting pouring from the car. Then, I still had a leak under the front passenger. Turned out the tank and cap needed replacing. This seemed to fix my problems until I had the chance to drive the car faster for 20 minutes at 90-120 mph. Coolant started coming out of the cap. I tried the test two times - with the new cap I lost some, but not all. I decided to try the old cap, and pretty much my whole tank leaked out (had to pour in 2 liters of 50:50 mix to get the proper level back). I replaced the lost coolant, and drove the car around the city at normal speeds, not exceeding 70 mph - not a single drop from the cap. What could be the problem here? I cannot grasp why it only happens at highway speeds.

 

Guess I baffled you as much as my mechanic

 

Have you checked there's no combustion gases in the coolant? As in, a crack somewhere forcing gases into the cooling system, causing enough pressure to cause the coolant to be forced out of the tank. I've never used one, for neither gas nor diesel, so I can't say which ones work.

That and how's the water pump?