My buddy has a 06 3500 mega cab, he is losing a overflow tank of coolant about evey three weeks. No sign of it in the oil and no blue smoke out the tail pipe. Anybody have any suggestions and also no external leaks showing.

 

Look at the very back of the motor and see if you see a pinkish brown color. the coolant isn't green. The very back is prone leaking if the truck has a hard life. Also look at the water pump. It is on the left hand side of the motor facing the truck. It has a smooth pully on i that is about middle of the motor from top to bottom. With the truck running and hands clear of the fan look at that pully and see if it is wobbeling. If it is the bearing is bad and it will leak coolent and depending on what stage of failure you may not see the leak. With the truck off feel in behind that pully and see if there an oily sludge. This collant has an oily feel to it. If that is ok it could be a head gasket leaking in the pistons and it will evaporate and you may not see it out the tail pipe if it has the cat and muffler.

 

Probably the small coolant line right above the downpipe that clamp is known to let it leak when temps get cold. I just haven't wanted to fix mine yet. There are alot of articles on it and pics too just do a search.

 

also just behind and under the turbo is a coolant line that is prone to leaking.