Overheating - bottom hose ice cold, top hose very hot

[mrathell]

I have a 2008 Q7 3.6L.

So this weekend I was dealing with an overheating Q7.


The radiator is less than a year old and this weekend I installed a new water pump, thermostat, coolant tank and coolant tank top to try and resolve an overheating issue that happens every once in a while.


The old water pump had the plastic propeller and one of the fins was missing. When the car is running and getting up to temp the upper radiator hose is hot but the bottom radiator hose is as cold as ice to the touch. The car eventually overheats. I've tried burping the system to see if air is stuck in there somewhere.


Next I reversed flush it with a water hose to try and force out what ever is blocking it. I reversed flushed the coolant system in the radiator and in the engine block with the thermostat removed. The water flowed through both with no issues or appearance of reduced flow.


When I connected the hoses back up minus the thermostat the same issue was present.


I'm thinking the missing fin on the old water pump is logged in one of the passages around where the water pump mounts restricting the flow generated by the water pump. So I decided to take the water pump back off and try to see if I can find any obstructions in that area.


So I took the new water pump off and reversed flushed the entire engine block and the radiator. Everything appears to flow fine from a flush perspective. When I re-installed the new water pump, connected the hoses and didn't install the thermostat the results were the same. The upper hose was very hot and the lower hose was as cold as ice. The pressure built up in both hoses when the car was running but the lower hose and radiator never got hot. I even remover the upper hose while the car was running expecting for water to constantly flow out, but it didn't. The water coming from the engine block never poured out the hose even though there was water in there and the engine was running. Steam escaped but no water flowed out. It eventually overheated.


Any clues?

[mrathell]

So apparently there is an aux electric water pump on these cars as well with the part #3D0965561D.

Does anyone one know where i't located on the car. I couldn't find a fuse for it it the owners manual.