Rex wrote:
This confuses me ... I thought the by-product of s stuck (wide) open T-Stat was a motor that was too cool. The T-Stat closes to keep the water in the motor as the motor rises to operating temp. Once "at temp", it opens so the heat can transfer from the engine to the coolant as the coolant passes through engine, into the rad to exchange the heat to the passing air then back into the engine.
Sounds logical right...nope
The VQ uses a reverse flow system with a water control valve to purge hot fluid from the block.
If the engine in a QX4 overheats it's time to pull the top end of the motor off and replace the coolant temperature sensor and water valve. For the OP the damage to your engine is most likely worse and I would not be surprised if you have blown the head gasket, which is the leading cause of a boilover in these cars. You have had recurring issues pertaining to the valve train and intake timing control which is a strong indicator to me that your engine might be sludged and running warm already. Since it is now overheating after some abuse I am thinking you might be in over your head. This is time to stop guessing and get it looked at, hell taking it to a Nissan dealer would be better then blindly guessing. I would start by replacing anything with a rubber gasket in the cooling system and a complete 50/50 coolant replacement using a premium ethylene glycol coolant. Once the fluid is overheated once it is best to remove it and everything that may have been damaged. To do this correctly you will need to remove the intake plenum, upper and lower as well as the fuel injectors and lower plenum to access the water valve. The valve is located at the bottom of the V and is the most likely cause of your problem. Beware though that you will need to ensure that ALL the air is removed or it will overheat once <75% load is applied (acceleration in drive).
While you have the plenum out I would strong recommend a thorough cleaning of the intake tracts. This makes a huge difference in the operating characteristics of the QX4 as they tend to get dirty after the first hundred thousand miles.
Not trying to rant I had the same thing at work recently and several techs had already replaced everything there was to replace with the exception of the two things no one wanted to touch. The water valve is often overlooked because not many people know it is there.