For the last 2 weeks, ive noticed my cars been running HOT in traffic and even on the highway when crusing at above 2500rpm, the needle raises past half. Otherwise it runs just fine and doesnt get hot, only seems to do it on really hot days, with alot of idling I guess.

Well tonight I attempted a few brake stands, being silly, and 2miles down the road where I parked my car, I seen steam coming out of my hood, pop the hood and sure enough the overflow is foaming/full and the bubbles are leaking out of the top of the overflow, fluid had even made it onto my hood/grill/bumper.

I popped the top off of the overflow and it looked like just bubbles, no water/coolant mixture to be seen, and not within a fingers length of the neck.

I could hear a boiling/bubbling coming form under the rad cap. I then checked the temp gauge and it was about 2/3 up the gauge

Now I was going to simply change the rad as I thought it may just be old and not cooling good anymore. But does this overflow incident indicate something else?

Very well may be the thermostat

I was just thinkin the 8 year old rad just might not be able to cool the car anymore

But I dont get why I pushed fluid out today

this is the last thing i would check... my radiator is 25 and still cools my truck way more than it should... i rarely go above 1/4 of the way up the temp gauge with my ceramic coated headers and modified intake

The car never runs "hot" to me, only creeps up past half, on my daily commute, which is only 5 min, its fine. I just felt like idling and talking tonight, and ended up doing a burnout, and she got hot and pushed coolant

Didnt know if that pinpointed a certain problem

No weird sounds or anything.

Id buy a new thermostat tomorrow if they were a hundred freakin bucks

To a dead cold engine, 90degees outside though. I top up the coolant, put cap on, start car, try to remove the cap again, and there is water pressure against the cap when uncapping the rad and coolant would spray all over

True, but the cap is functioning as it's opening. The coolant steaming indicates it's opening under excessive steam pressure.

The issue still occurs at highway speeds. So it's not a fan issue. You're getting plenty of airflow.

I would first drain ALL fluid. Remove the thermostat. Flush the cooling system completely. I personally went through 6 gallons of distilled water flushing the system. Install a new thermostat. Refill system with a 70/30 water/coolant mixture of your choice, I recommend Zerex G05 due to its enhanced lubrication properties.

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The radiator system is under pressure. The reason being the boiling point of water/coolant increases as pressure goes up. Usually the cap is designed to release between 14~16PSI. With plain water this works our to ~250degrees. With coolant it is bumped to ~270 depending on the mixture and type.

Removing the cap while the system is hot is definately not recommended. You WILL be burned! If I ever have to do it I put a thick rag on the cap and spin it.

Modified by PoorManQ45 at 8:52 AM 6/25/2009

this is a key term people. take notes. the retard that owned my s30 before me was using regular tap water. now my freeze plugs are rusted and leaking

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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.

If possible remove that temperature valve! You'll improve reliability. You'll lose a percentage of MPG for the first couple of minutes until the engine gets a chance to fully heat up, but it would prevent that crap.

If I ever had to pull the "top end" That valve would not be reinstalled

Maybe 50+ years of proven reliability wasn't enough...

Seriously though, this design is different just to be different.

The coolant system moves in reverse completely disturbing the flow and allowing fluid to flow not intended by the manufacturer is a risk that is not worth the $50 savings. This is a large restriction that moderates and directs flow once the valve is open and completely restricts irregular flow through the cylinder walls. Why would you want to risk the internal cooling capabilities of an internal combustion engine?