Explain to me how a thermostat works

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Ok, get yer laughs out first. My brother's 2009 Cobalt SS turbo runs around 197 F in normal driving and his cooling fans always seem to be running. He says he's got a 180 F thermostat in it and he swears, sometimes profanity-laced, that the engine should never get hotter than what the thermostat rating is. I on the other hand have tried to tell him repeatedly that the rated temperature is only an indication of when it would be fully open, and any engine will get warmer under a load. Who's right? Me, I hope.

His car has some cooling system or related sensor issues. For example, even in the winter the cooling fans are almost always running which seems to overcool the radiator and the upper hose stays cool as well. With both fans unplugged, the upper hose gets hot as it should and the coolant temps seem to be a bit lower for some reason. Also when the heater in his car is on, his coolant temps seem to be a bit higher than if the heater is off which seems totally backward to me.
 
Yes you are right, if you have a thermostat that opens at 197F it will open at that temperature, and it will get hotter than that but not too much ( under normal circumstances )
 
Which thermostat... there's one for the cooling water and another for the fans. The fan "Temp switch" may be failing. Many are wired "fail safe" where faults keep the fans running, needed or not.
 
Coolant temp shouldn't go above stat temp in theory, but the bottom of the rad is relatively cold and water jackets near the exhaust valves very hot so the location of the sensor plays a part. Airlocks can also confuse the CTS.
Maybe his fanswitch is broken making them run all the time?
Indeed, you would expect lower temps with the heater on - strange that his seems opposite.

I'd be tempted to drain it, new pump & stat, backflush rad and heater core then fill bleed and pressure test.
If the strangeness continues then look at the CTS and fanswitch, and associated earths.
 
Engines can get warmer than the thermostat, and fans may not come on until something higher than the thermostat. The thermostat temperature is the point at which coolant is going through the radiator. Now cooling fans have another function though, they usually come on when the air conditioning compressor is running to help the condenser work effectively. Turning the defroster on usually engages the compressor to dry the air.

My Traverse has a peculiarity in that the compressor seems to come on automatically around 30-40 degrees, even if I'm not requesting the defroster. Perhaps that's why his fans were running in the winter? When the A/C is off, I can see my temperature get up to 210 before the fans come on, and I know the fans have two speeds. There's a temperature where the fans will kick up to high speed and it sounds like there's a Shop Vac under the hood.
 
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His car has a bit over 200000 kilometers on the odo and I assume (although I would hope not) it's still running the original fill of Dexcool. Another issue is that he's got an oversized intercooler mounted in the stock location which I suppose could be blocking a bit of airflow
 
A thermostat opens at the rated temperature. + or - a couple of degrees. It may take another 15 degrees to fully open. If the load on the engine, and the cooling capacity of the system, call for the thermostat to be half open in order to provide adequate cooling, then engine will run at a higher temp than the thermostat is rated for. A 190 stat might be half open at 195 and this is where the engine might normally run. On a hotter day and under more load it may be normal for the engine to run at 200.

If the engine computer suspects there is something wrong with the indicated temperature. It will default to maximum cooling and turn the cooling fans on. This will happen with a thermostat that is stuck open slightly. If the computer does not see the temp rise at a predetermined, after start up, it thinks it cannot trust the temp sensor at will again default to maximum cooling. Fans on.
 
A thermostat won't prevent an engine overheating if there is a problem. Thermostat operation is usually stated at an opening temperature and a full-open temperature. For my 04 Camry V6 it is 182F and 203 F. The filler cap will prevent boil over to about 265 F, but the engine software will probably start some kind of corrective action before boil over.
 
Originally Posted By: Jetronic
Corrective action is limited to turning off AC on most cars....
Turning ON the AC will cause the fans to run continuously Turning OFF the blower control will reduce the heat the AC has to dump into the condenser. Often it helps cooling.
 
Thermostat in a first gen isx(Cummins) cold thermo is closed so coolant stay in the front(close to the fan. At a specific temp it open and coolant get to the oil cooler of the engine to cool the oil. You get a funny idea you remove thermo . Yep coolant in isx is gravity based so coolant can't ever reach oil cooler . My exemple is an extreme exemple of , thermostat is there or engine fail
 
Dismount turbo intercooler and radiator and close olinlet and outlet . Then from the fan zide send pressurise air it's the reverse flow might be junk and flow is empeded. Coolant change is also a fmgood idea. If a truck stop is close get the stuff there.
 
Thermostat temp is what should be the steady state temp on the highway, with plenty of ram air over the radiator. In traffic, it can creep higher to fan temp.

When they test cars for the EPA loop there's a 30 second stop here and there. They like the temp creeping up before the fans come on, as it helps MPG slightly. They test with the HVAC off which rarely happens in the real world; both AC and Defrost use the AC and run the rad fans for purposes of making the condensor more efficient.

Since this is a turbo cobalt your friend might have some half baked tuner ideas like rigging the fan switch, so watch out.

Finally I had a dodge spirit with the gauge temp sensor on the other side of the engine from the thermostat. When it was under heavy load the t-stat opened wider which meant more flow through the rad which meant more cold water coming in the other side, and my gauge would drop. But the average engine temp was probably pretty steady. This might be going on with him, though they've gotten better at putting the gauge sensor by the t-stat, and at using the computer to deaden the gauge reading in the normal range.
 
A wimpy radiator could cause a temperature fluctuation too.

Combining this with AC would only make things worse.
 
With every t-stat I've dealt with, rated temp is the temp where it starts to open. Fully open is around 15* hotter than that. So a 195* t-stat will start to open at 195 and be fully open by 210, meaning that 210 - 215 is the upper end of the normal range we could expect to see.
 
I do know that the thermostat on my Fusion is technically rated at two temperatures - 185* F and 210*F. The 185* number is when it starts to open, and the 210* number is supposed to be when it is FULLY open.

Once I get up to operating temperature, and depending on how I'm driving, the grade of the road, traffic, etc.. my Torque Pro app reports that the coolant temp sensor registers somewhere between 186* and 208.6*F, so it would seem that the thermostat is roughly doing what it's supposed to.

Also, I've noticed that if I've been driving @ 70-75 on the highway, and then hit the gas station and shut the car off, after I get back in and start it up, the sensor reports as high as 230*F, due to heat soak, but that quickly drops back down into the low 200's as the system does what it's supposed to.
 
On some cars the thermostat when fully open (not it's rated opening temp) it will also seal off a thermostat bypass when the foot of the t-stat is fully open. If there is a gasket on the bottom of the T-stat this is the case.
 
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