Engine Cooling problem
2002 Nissan Pathfinder 6 cyl Four Wheel Drive Automatic 106K miles
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Working on an overheating issue. When the engine is brought up to operating temperature, the heat blows cool unless you race the engine. It will then blow hot, but after about two minutes, it's blowing cool again. All the while the engine is getting hotter and hotter, to the point it will overheat if you allow it to. I already bled the cooling system, however never really got a solid stream of coolant out of the bleeder tube until it was at the overheating point. Replaced the lower T-Stat and no difference. My question is this: Will the upper T-Stat (water control valve) cause the vehicle to overheat if it is bad? Or is it strictly to control the water to the heater core? I'm not about to pull the intake and waste my time if totally unnecessary. Also, the fan clutch, radiator cap, radiator, and water pump are fine. Thanks in advance for any help you can offer and sorry for writing a book on this one.

To me sounds as its low on coolant
or u have an air trapped inside the engine I suggest follow coolant system bleeding steps below and also check the radiator cap


COOLING SYSTEM BLEEDING

WARNING: To avoid being scalded, never change engine coolant when engine is hot. Wrap a thick cloth around cap and carefully remove. First, turn a quarter turn to release built-up pressure, then turn cap all the way. Be careful not to allow engine coolant to contact drive belts.
1.Turn ignition switch to ON position. Set temperature control to maximum heat position. Turn ignition off after air mix door moves to HOT position.

2.Open radiator drain plug at bottom of radiator, and remove radiator cap. Remove reservoir tank and drain coolant. Clean reservoir tank. Remove cylinder block drain plugs and air relief plugs. See Fig. 1 and Fig. 2 . Check drained coolant for contaminants such as rust, corrosion or discoloration. If contaminated, flush engine cooling system.

3.Install reservoir tank, radiator drain plug and cylinder block drain plugs. Tighten fasteners to specification. Fill radiator slowly to allow air to escape. Install each air relief plug when coolant overflows. Tighten fasteners to specification. Fill coolant reservoir to MAX mark.

4.Start and warm engine to normal operating temperature without radiator cap installed. Run engine at 3000 RPM for 10 seconds and return to idle speed. Repeat 2-3 times. Turn engine off.

5.Check coolant level. Refill coolant reservoir to MAX mark, and repeat step 4 until coolant level no longer drops. Warm engine to normal operating temperature. Move heater control between COOL and HOT positions. Increase engine speed to 3000 RPM. Listen for coolant flow at heater valve. If coolant flow is heard, bleed air from cooling system by repeating step 4 until coolant level no longer drops. Check air relief and drain plugs for leaks.

Shouldn't have air in it as it was doing this before the T-Stat was changed. Coolant level is fine. Radiator cap replaced, no change. Still have the question out there of the water control valve. Will it cause the engine to overheat (restrict water flow through the cooling system) if it is bad, or is it more like a bypass in the system to accept water for the heater core??

Its just a gate or a door to allow coolant to the heater core
will not make engine overheat
at this point then if you are sure that you have bleed the system using the correct procedure I would suggest having a headgasket test and water pump
good luck