No coolant flow through radiator?

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Vehicle is 1996 Maxima, 3.0L V6. Basically did a cooling system refresh-- new coolant, thermostat, several hoses, coolant temp sensor, etc.

Pressure tested the system after reassembly, holds pressure fine, no leaks. Filled up with coolant, and while on a steep incline started up the engine and proceeded to bleed the cooling system by letting it warm up with the radiator cap off. Let it run till it got to operating temp and cooling fans came on (approx 205*), heater is blowing hot air. Let it cool down a bit, reinstalled radiator cap and went for a short test drive, temps climbed up past 200F and I stopped the engine before it got to 210F. Upper cooling hose is hot, lower cooling hose is cool, as is the bottom half of the radiator. I see no movement of the coolant through the radiator fill when the engine running at temp.

I'm a bit puzzled, in the past I've had no issues with bleeding air after servicing the cooling system. I suppose it's not outside the realm of possibility to have gotten a bad thermostat, but I'd like to eliminate all the other possible causes as the thermostat is a bit of a chore to change out.

All the other engines I remember working on have had the thermostat before the inlet side of the radiator (installed at the upper radiator hose). This one has the thermostat after the radiator outlet. Could the cool water on that side be preventing the thermostat from opening? Or does it effectively work the same way no matter which side the thermostat is on? Curious if you folks have any ideas or suggestions / things to check.
 
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Thermostat in backwards? The sensor part of the thermostat (the copper bulb) needs to face the engine. This is true whether the thermostat is in the upper or lower hose.
 

92saturnsl2

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Thermostat in backwards? The sensor part of the thermostat (the copper bulb) needs to face the engine. This is true whether the thermostat is in the upper or lower hose.
Thermostat can't be installed backwards. It's part of an assembly that provides the connection for the cooling hose, thus can only be installed one way (see picture).
tstat.jpg
 
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Air bubble behind the 'stat. Are you suspicious that it didn't refill with as much coolant as you drained?

Another tip, point it uphill at thermostat temp and rev it to the moon/ floor it in 1st gear. That might shake the bubble loose, letting water get at the pellet, opening the thermostat, and finally burping that bubble.
 

92saturnsl2

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I went and bought another thermostat just to have on hand and pulled the old one. I put both in a pot of water with a thermometer, and heated the water to boiling. Both thermostats opened around the same time-- began to open at around 180F, fully open by 195F. Pretty much behaving exactly as they're supposed to.

Reassembled car, repeat the bleeding process. Got some air bubbles for a few minutes as temp moved through 180F. Afterwards, same behavior as before. Before, I had shut the engine down at 210F but this time I let it continue to run. Temp doesn't go any higher than that. It will peak at around 205-210F and stays there no matter how long I idle, or even a long drive. Not sure why the running temp is so high, but I'm beginning to think it might be the coolant temp sensor giving off too high a reading-- it's also causing the cooling fans to stay on all the time, since they're triggered at about 204F.

I'm pretty sure the thermostat has to be opening because there's no way I could idle or drive that long without the engine seriously overheating if the t-stat wasn't opening. Strangely, after a 15 minute drive, I was able to remove the radiator cap afterwards and there was little or no pressure in it. I pressure tested the cooling system after reinstalling the thermostat and there's no leaks. Should I be observing significant pressure in the cooling system after warm-up / driving? For grins I put a thermometer down into the coolant fill port after driving it awhile, and temp was 190F.

Suppose trying a new coolant temp sensor is the next course of action?
 

92saturnsl2

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It is a bolt usually on the Dr side. Takes a 10 mm socket and has a sealing washer under it. Should have a sticker next to it that says bleeder or something similar.
I still don't see any evidence of a coolant bleeder. Is it on the intake itself? There's no coolant running through the intake on this engine, so a bolt on the intake wouldn't have anything to do with the cooling system. Perhaps you're thinking of the VG engine on older Maximas?
 
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The T-stat must be working if you have heat. If it's overheating your either low on coolant or have air in the system. Check the level again
Nope. A stuck-shut thermostat will give plenty of heat but the car will overheat since it has no access to the radiator. There are two cooling loops, one for each side of the 'stat, and the heater core gets the bypass loop from the closed thermostat.

OP should drill a tiny hole (1/16") in the top of the thermostat flange. This will give another avenue for air bubbles to escape through, and potentially allow better coolant flow behind the thermostat pebble to better signal it with temperature.
 
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Hot top hose and cool bottom hose points to a radiator issue. Thermostat is opening and sending water to the radiator, but it's the radiator won't allow flow. This happened to a friend's El Dorado years ago. I just don't understand how you're not overheating, as that Northstar got hot and started limping. After 2 thermostat changes, he took it somewhere and they did a flow test on the radiator. If memory serves, their diagnosis was a "collapsed tank". Try unhooking both hoses and run your garden hose in the top of the radiator to see what happens.
 
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