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Discussion Starter #1
Bottom Radiator hose is cold. Just replaced alternator and power steering pump. Removed upper radiator hose while doing job. After bleeding coolant system I noticed that the bottom radiator hose was cold after running engine. Seems as though thermostat isn’t opening. Tried many times to burp system. Decided to drive and see if it worked itself out. Have driven around 150 miles over the last couple days with no overheating and heater is blowing hot air. Still cold bottom hose. Thought maybe it might be on the verge of overheating but can’t tell due to the stupid dummy temperature gauge (gauge stays in middle). It seems crazy to me that the hose would be cold, but am I over looking this and maybe it is just normal?

Thermostat and water pump was replaced last year while doing timing chain. No I did not use OEM thermostat. Just seems weird that there are no overheating issues.
 

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You could be running hot, but not overheating.In my experience normal coolant operating temperature is around 195 degrees +/- about 10 degrees.You should get one of those OBDII scanners that hooks up to your phone so that it will show you real-time values like coolant temperature. The dashboard needle won't start moving upward until you get to about 230 degrees and it won't redline until over 260 degrees. If you are idling in your driveway at more than 210 degrees, you are running hot and that's not good for your engine.
 

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Just means your radiator works. Water cools from the top down. Cold water out the bottom is because all the heat was removed in the radiator. Thermostat directs water coming out of the engine to either recycle back into the engine to warm up or to the radiator to cool it off. It is not an on/off either, it throttles. If it is warmed up but not hot, you might get 80% of the water recirculated back into the engine and 20% into the radiator. The 20% coming out of hte radiator will be cold, mix that with the 80% recirculated warm water and it is just right to keep the engine happy.
 

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Discussion Starter #5
Thanks for the info. I thought I had
A good beginners level worth of understanding but this has got me
Confused. When the thermostat opens, does the coolant flow out of the engine or in. If it flows out of the engine, then I would think that the bottom hose (going into the thermostat housing) should get hot from the hot
Engine coolant coming out.

I’m going to get a OBD tool to see what the actual running temp is. Also thinking about installing a temp gauge that reads actual temps. What is the proper way to wire a new gauge? Can I pigytail the wires for the new gauge straight off of the wires coming from the temp sensor?
 

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The thermostat acts as a gate to keep the engine warm, but not too warm. when it reaches a certain temp, it opens and the water pump pushes coolant up the upper hose, (cooling) through the radiator, and out the bottom hose, back into the engine.

Probably the easiest test is to let the engine idle for a while until temps hit "operating temperature" basically where the gauge doesn't climb anymore.
At that point, the thermostat will open and your fan clutch should kick on and off to blow air through the rad. It always does at so.e speed but you should hear in come on faster and slower. At that point you should feel the lower hose at least warm, if not hot... but less hot than the upper.
 

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Normal operating temp is 190F so your lower hose should reach that temp.
Wrong.

The UPPER hose will reach that temperature. The radiator cools the water and it comes out colder. The lower hose is where water leaves the radiator and goes into the engine.

If the lower hose is 190° that would mean that no heat has left the radiator. Water in and out being the same temperature. If no heat is removed, and heat is added from the engine running, the temperature will just rise until it boils.
 

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It is going to feel hot to touch regardless of upper or lower. The temp difference isn't enough to notice by hand. One hose will not be cool. If you have one hose that is only warm and one that is definitely hot, then you most likely have a thermostat issue
 

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Discussion Starter #9
Just put a new OEM Nissan thermostat. Same problem. Pulled
Every trick out to try and burp/bleed system. Finally after about an hour of idling and a hour of driving, the bottom radiator hose going to the thermostat was warm. Checked levels and all seemed fine. Took it to work next day (about 20 miles of freeway driving) and bottom hose is cold. No overheating signs at all. Hot air from HVAC. Water pump is definitely flowing. Radiator is definitely not clogged. Doesn’t make sense to me. Gonna keep driving and hope for the best. Maybe I am just overthinking this and the hose is supposed to normally be cold.
 

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Are you up north in a colder environment? After it sits overnight, pop the radiator cap and check the rad level. Also, do you have the cap with the spring on the tank and the plain cap on the radiator?
 

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Discussion Starter #11
The caps are correct. The spring loaded cap is on the expansion tank and the plain cap is on radiator. I do live up north. Outside temps are around between 30 - 50F. I have also always been running the heater. I wonder if the heater fan is cooling the water enough to the point that the thermostat doesn’t need to open. There is a bypass hose coming off of the thermostat housing. Could this may be enough to circulate the water enough to not open the thermostat???
 

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Yes, you are over thinking it. The purpose of the radiator is to make cold water. The water pump pulls the cold water into the engine by the lower hose. The thermostat controls if hot water goes to the radiator or cold water goes back into the engine to be cooled again. If the hot water leaves the engine, clod water has to take it's place. The radiator is very efficient, keeps cool when fully loaded pulling mountain grades in 100°+ heat and will handle V8 swaps just fine as well. In your near freezing environment it just rocks.

if the lower hose has hot water in it, you have major cooling issues.
 
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