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Changing the Radiator Outlet Coolant Temperature Sensor

6K views 61 replies 8 participants last post by  Sapote 
#1 ·
Hi guys I have got myself a new temp sensor the one that goes on the lower radiator hose. I see it's only a 2 minute job, just wanted to know on a cold engine someone said I have to drain all the coolant out first is this correct? Was kind of hoping I could just pop the old one out and replace it. Any help is appreciated thanks alot.
 
#3 ·
It's easy:

Open up the radiator (expansion tank) cap half way. Squeeze the upper hose with all you've got. At the same time tighten the cap. Now your cooling system is under slight vacuum.
Have everything at the READY.

Pop the sensor out and have the new one slightly lubed. Make sure the old O Ring comes out, have a pick ready if not. Pop the new sensor in and make sure it clicks. Give it a tug to be sure it's ALL the way in. Reconnect the electrical connector.
Open up the cap and top off.

Loss should be minimal.
 
#7 ·
Sorry to be a pain my car is an automatic and has been trying to overheat in heavy traffic soon as I'm moving again it's fine I've got no coolant leaks etc. My electric fan isn't coming on and I did have a sensor fault code so I've just replaced it. My air con isn't gassed at the minute it's empty, does my air con need to be filled for it to activate the electric fan or should the fan come on regardless once I press the AC button.
 
#16 ·
I did just change the temp sensor and left it idling for half an hour and it stayed perfectly at 12 o'clock. And the mechanical fan seems to spin fast, I do get the odd squeak sounds like a belt but normally when it's wet for some reason. I've had a brand new water pump done and belts were all changed around 6k miles ago.
 
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#18 ·
I have drove my car for 3 hours this morning normal and traffic driving no overheating and gauge perfect at 12 o'clock. My A/C is empty but when I press the AC button the electric fan now spins for around 30 seconds then goes off, before I changed the temp switch it didn't run at all even with the AC on. My question is the reason it's turning back off with the AC on because it's not cold enough hence not needing to stay on? I have my m.o.t on Friday so I am going to ask the garage to re gas my ac and see if it stays on.
 
#21 ·
Let's look at how many have issues with properly bleeding the cooling system, quite a few....
Now if the above is done properly and quickly, no cooling system bleeding becomes necessary.

However if everyone performed a system bleeding, I'd bet that 50% of the folks would create an issue that did NOT previously exist. This is an unwanted outcome.
 
#22 ·
Yeah I get that it took me no longer than 20 seconds to pop it out and back in. I'd rather do it this way if I ever had to do it again even of for some reason it had to be bled after as bleeding it only takes 5 minutes, rather than draining the whole system. I inspected my old temp sensor and the wires wasn't connected must of been knocked at some point, new sensor has solved my problem.
 
#25 ·
likely issues:

aux fan is not working
mechanical fan clutch has failed - do the newspaper test

we recently had a thermostat fail with symptoms like the ones you describe

fan issues are more common.
 
#32 ·
"cap was opened again, hose returned to former round shape pleasing to our eyes What had filled in the extra volume in the system when the upper hose returned to its round shape as soon as the temp sensor was removed -- ether, coolant or air? both, thru the open ET, should be minimal. "

No, as soon as the lower hose sensor was removed, the upper hose will return to its former glory round shape in less than a second, unless people be able to swap out the sensors in less than a second.
 
#54 ·
Let's assume we have a sealed cooling system here. Let's also assume the upper radiator hose is slightly collapsed indicated a slight vacuum in the system. When you remove the temperature sensor in the lower radiator hose, effectively we have 14.7 psi of ambient air pressure on that small opening. In a physics text book, this would say the air pressure would push the coolant at the small opening back into the cooling system as the system has a slight vacuum. But, don't forget the coolant in the cooling system wants to flow out due to gravity as the opening is practically at the lowest point. Also keep in mind that the opening is small, the volume of coolant is large and there are mechanical blockage of the backwards coolant flow at the thermostat. In the few seconds that it would take to actually replace the sensor, the amount of air that would be introduced into the system is practically zero.

As a physics homework, anyone is welcome to calculate the static equilibrium here with coolant wanting to flow out the opening due to gravity, the slight vacuum in the cooling system, the ambient air pressure pushing the coolant back into the system, the mechanical blockage of the thermostat, and all the other minute leakages in the entire cooling system that have not been talked about.

In re-reading this entire thread, at no time did the OP describe the upper hose being slightly collapsed and then relaxed later. This conversation went down the path of theoretical scenario, which does not help the OP. If it makes anyone feel any better, you are right that if the upper hose was slightly collapse and then relaxed later, something had to have occupied the "space". It would most likely be additional air that was introduced into the system. But in this thread, that did not happen to the OP.
 
#55 · (Edited)
In re-reading this entire thread, at no time did the OP describe the upper hose being slightly collapsed and then relaxed later.
Open up the radiator (expansion tank) cap half way. Squeeze the upper hose with all you've got. At the same time tighten the cap.
OP: - Great. Your a star mate. Didn't fancy draining the whole cooling system. Thanks alot really appreciate it 👍
If OP followed Mr.M suggestion then the hose was not just slightly collapsed but it was squeezed the last drop of ...out .So the vacuum is much stronger than the slightly collapse hose from the cold temp in the morning.

The air probably not traveling through the lower hose with a closed Tstat, but it moves through the radiator and up to the upper hose.
 
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