Got a car in today for an over heating problem. Found a t fitting was broke by the resivore that was leaking coolant. Fixed it then filled back up with antifreeze and water. Still over heats but only when driving it. Cab sit for hours and be fine but as soon as I drive down my driveway it gets hot again. Has anyone had any issue line this before? I don't usually work on German cars like this so I'm lost on this thing. Any input would be appreciated.

 

There is probably air in the system. Just behind the coolant reservoir, there are a couple of hoses that go to the heater core through the firewall. One the engine side, there are fittings on the hoses that have screw caps on them for bleeding the system. Take them loose continue filling the reservoir with coolant until you see coolant welling up and ready to overflow. Put the caps back on and fire up the car. Let it run for minute or so as not to get hot. Turn it back off and loosen the caps on the bleeders. If coolant starts to run out, quickly tighten it back up. Do this for both.

These cars are complex but really no more than some of the others. Don't let it intimidate you. It also helps to jack the car up on the left side to get the reservoir higher than the engine. It is critical to get all of the air out of the coolant system to prevent it from overheating.