Car runs hotter when it's cold?

[SirTanon]

Noticed this today on my drive in. Temps this morning were substantially colder than normal (at least for here in Phoenix), and I haven't driven the car to work since last Friday when it was warmer..

Normally, my drive in to work, the temp gauge (in Torque Pro) is around 199F cruising, and up to around 208F in stop-n-go traffic, and gets down to around 188 or so when I'm coasting downhill or at the offramp. When I get off the offramp and stop at the light at the intersection, the gauge will usually get up to around 206F and gradually drop slowly.

Today, traffic was light, so there was pretty much no stop-n-go, and during my drive, at normal cruising speeds, the temperature seemed to hover between 203 and 205F, and never got down below about 195 while coasting. Sitting at the intersection, it never got higher than 203F.

I'm not really concerned, as such, but it did get me wondering why it would be steadily hotter during cruising and coasting, but cooler at the stop light, when it was colder out.


I know what I THINK it is, but I'd like to hear some other opinions.

 

[ragtoplvr]

In Hot weather the ac compressor has a higher duty cycle, and a fan runs. This fan also draws air thru the radiator. So the fan will often keep the temp below the point where the fan is on to cool the engine.

Rod

 

[SirTanon]

Originally Posted by ragtoplvr
In Hot weather the ac compressor has a higher duty cycle, and a fan runs. This fan also draws air thru the radiator. So the fan will often keep the temp below the point where the fan is on to cool the engine.

Rod


Perhaps, but I haven't run the A/C in this car since.. October? Between today and last week, this has not changed.

 

[dirtymudder]

low radiator fluid. Gets cooler out of the heater when idling but is fine at higher rpm's.

 

[SirTanon]

Originally Posted by dirtymudder
low radiator fluid. Gets cooler out of the heater when idling but is fine at higher rpm's.

Coolant level is fine. No change there. Degas bottle at fill line, and pressurized. Heat is plenty hot coming out at all times.

 

[Kira]

Cold air is denser so more fuel was delivered by your ECM.

 

[dadto2]

Originally Posted by SirTanon
Originally Posted by ragtoplvr
In Hot weather the ac compressor has a higher duty cycle, and a fan runs. This fan also draws air thru the radiator. So the fan will often keep the temp below the point where the fan is on to cool the engine.

Rod


Perhaps, but I haven't run the A/C in this car since.. October? Between today and last week, this has not changed.

If you are running the the vents in defrost mode, the A/C will still cycle to dehumidify the air. Could you have the defrost on?

 

[SirTanon]

Originally Posted by dadto2
Originally Posted by SirTanon
Originally Posted by ragtoplvr
In Hot weather the ac compressor has a higher duty cycle, and a fan runs. This fan also draws air thru the radiator. So the fan will often keep the temp below the point where the fan is on to cool the engine.

Rod


Perhaps, but I haven't run the A/C in this car since.. October? Between today and last week, this has not changed.

If you are running the the vents in defrost mode, the A/C will still cycle to dehumidify the air. Could you have the defrost on?

Nope. Just the floor vents. It's not cold enough here to need windshield defrost.

Originally Posted by Kira
Cold air is denser so more fuel was delivered by your ECM.

This is kind of what I'm thinking.

 

[DGXR]

Originally Posted by SirTanon
Noticed this today on my drive in. Temps this morning were substantially colder than normal (at least for here in Phoenix), and I haven't driven the car to work since last Friday when it was warmer..

Normally, my drive in to work, the temp gauge (in Torque Pro) is around 199F cruising, and up to around 208F in stop-n-go traffic, and gets down to around 188 or so when I'm coasting downhill or at the offramp. When I get off the offramp and stop at the light at the intersection, the gauge will usually get up to around 206F and gradually drop slowly.

Today, traffic was light, so there was pretty much no stop-n-go, and during my drive, at normal cruising speeds, the temperature seemed to hover between 203 and 205F, and never got down below about 195 while coasting. Sitting at the intersection, it never got higher than 203F.

I'm not really concerned, as such, but it did get me wondering why it would be steadily hotter during cruising and coasting, but cooler at the stop light, when it was colder out.

I know what I THINK it is, but I'd like to hear some other opinions.


In my experience, when the ambient air is significantly cooler, this means the radiator sheds more heat in normal driving and the coolant entering the engine is colder. This means the thermostat will hold the coolant inside the engine longer, because it needs more time to reach the specified temperature before the thermostat cycles. Depending on the location of the coolant temperature sensor, the sensor may reach a higher temperature because the thermostat is closed for a longer time, which will indicate a higher coolant temperature on the gauge. Not sure if I am explaining this right, but I see the exact same behavior in my 1995 Corvette. The highway temperatures are consistently 2-3 degrees higher when the outside temperature is below 50F.
Somewhat related is my experience with thermostat settings... my 1992 Thunderbird tended to run hot in summer stop and go traffic, even though everything was in great condition (meaning I maintained everything well). I assumed the thermostat was OE, but when I checked, in fact it was a colder setting 160F. Factory is 180. So counter-intutive I put in a 180 thermostat and the temperature stayed rock solid right in the middle. So when the thermostat stays closed longer, the coolant in the radiator has time to cool down more, so it cools the engine more effectively when the thermostat cycles. Common sense might say that the thermostat staying closed longer means the engine will get hotter, but in the real world this is not what happens in a healthy cooling system.
We might also consider the increased cooing effect caused by colder ambient air passing through the engine bay and across the engine block, and everything else under the hood.
My $0.02. Hope this helps you.

 

[SirTanon]

Originally Posted by DGXR
Originally Posted by SirTanon
Noticed this today on my drive in. Temps this morning were substantially colder than normal (at least for here in Phoenix), and I haven't driven the car to work since last Friday when it was warmer..

Normally, my drive in to work, the temp gauge (in Torque Pro) is around 199F cruising, and up to around 208F in stop-n-go traffic, and gets down to around 188 or so when I'm coasting downhill or at the offramp. When I get off the offramp and stop at the light at the intersection, the gauge will usually get up to around 206F and gradually drop slowly.

Today, traffic was light, so there was pretty much no stop-n-go, and during my drive, at normal cruising speeds, the temperature seemed to hover between 203 and 205F, and never got down below about 195 while coasting. Sitting at the intersection, it never got higher than 203F.

I'm not really concerned, as such, but it did get me wondering why it would be steadily hotter during cruising and coasting, but cooler at the stop light, when it was colder out.

I know what I THINK it is, but I'd like to hear some other opinions.


In my experience, when the ambient air is significantly cooler, this means the radiator sheds more heat in normal driving and the coolant entering the engine is colder. This means the thermostat will hold the coolant inside the engine longer, because it needs more time to reach the specified temperature before the thermostat cycles. Depending on the location of the coolant temperature sensor, the sensor may reach a higher temperature because the thermostat is closed for a longer time, which will indicate a higher coolant temperature on the gauge. Not sure if I am explaining this right, but I see the exact same behavior in my 1995 Corvette. The highway temperatures are consistently 2-3 degrees higher when the outside temperature is below 50F.
Somewhat related is my experience with thermostat settings... my 1992 Thunderbird tended to run hot in summer stop and go traffic, even though everything was in great condition (meaning I maintained everything well). I assumed the thermostat was OE, but when I checked, in fact it was a colder setting 160F. Factory is 180. So counter-intutive I put in a 180 thermostat and the temperature stayed rock solid right in the middle. So when the thermostat stays closed longer, the coolant in the radiator has time to cool down more, so it cools the engine more effectively when the thermostat cycles. Common sense might say that the thermostat staying closed longer means the engine will get hotter, but in the real world this is not what happens in a healthy cooling system.
We might also consider the increased cooing effect caused by colder ambient air passing through the engine bay and across the engine block, and everything else under the hood.
My $0.02. Hope this helps you.


I had not considered this. It definitely makes a lot of sense, especially since the temperature gauge in this engine is a CHT (Cylinder Heat Temp) sensor, mounted at the top center of the engine, and not in/near the thermostat.

 

[PiperOne]

Your thermostat isn't having to open as fully as it does in hotter weather. So instead of your engine getting big gulps of cool coolant flow from the rad when the thermostat opens..it's getting little sips. Your engine is just running at a more even consistent temp....nothing to worry about.

 

[eljefino]

+3 on DGXR's answer. The dash gauge on my dodge spirit was like this-- contrary to outside temp. The sensor was in between cyls 2 & 3 while the water outlet and thermostat were beyond cyl 4, water came in under cyl 1.

 

[tony1679]

I could be spewing nonsense here

, but the only thing I can add is maybe it's your coolant mixture playing a role? I know lots of people in Phoenix and South Florida use 100% water. If so, maybe the lower temps combined with the lower boiling point is affecting your reading? I understand whatever your mix is hasn't changed, I'm just suggesting it may play a role. And again, this is assuming you're running 100% water. Someone please correct me if I'm wrong.

 

[SirTanon]

Originally Posted by tony1679
I could be spewing nonsense here

, but the only thing I can add is maybe it's your coolant mixture playing a role? I know lots of people in Phoenix and South Florida use 100% water. If so, maybe the lower temps combined with the lower boiling point is affecting your reading? I understand whatever your mix is hasn't changed, I'm just suggesting it may play a role. And again, this is assuming you're running 100% water. Someone please correct me if I'm wrong.


I suppose some might, but I would only do such a thing in the process of a coolant flush, never for normal operation. I run a standard coolant/water mixture in approximately 50/50 ratio.

 

[zorobabel]

Originally Posted by DGXR

Somewhat related is my experience with thermostat settings... my 1992 Thunderbird tended to run hot in summer stop and go traffic, even though everything was in great condition (meaning I maintained everything well). I assumed the thermostat was OE, but when I checked, in fact it was a colder setting 160F. Factory is 180. So counter-intutive I put in a 180 thermostat and the temperature stayed rock solid right in the middle. So when the thermostat stays closed longer, the coolant in the radiator has time to cool down more, so it cools the engine more effectively when the thermostat cycles. Common sense might say that the thermostat staying closed longer means the engine will get hotter, but in the real world this is not what happens in a healthy cooling system.
We might also consider the increased cooing effect caused by colder ambient air passing through the engine bay and across the engine block, and everything else under the hood.
My $0.02. Hope this helps you.

I'm willing to bet the 160F thermostat was malfunctioning.

 

[The_Eric]

The most likely explanation is that the thermostat is working properly now for some reason.

The remarks about colder ambient temps and colder water entering the engine is considerably less of a thing now that most manufacturers have gone to INLET side thermostats. They now control the inlet temp to the engine to better stabilize coolant temps.

 

[SirTanon]

Originally Posted by The_Eric
The remarks about colder ambient temps and colder water entering the engine is considerably less of a thing now that most manufacturers have gone to INLET side thermostats. They now control the inlet temp to the engine to better stabilize coolant temps.


Thing is, having an inlet-side thermostat would be MORE likely to align with the explanation about the more effective radiator cooling compared to outlet-side.

 

[The_Eric]

It very well could make the rad do a better job or increase it's efficiency, but since the stat sets the minimum temp that is allowed to enter the engine, it shouldn't matter how cold the coolant is. The stat will open only as much as necessary to blend cooled incoming coolant with already warm/hot coolant in the engine.

 

[Surestick]

A hotter thermostat would mean hotter coolant going into the radiator. Heat transfer increases with a greater temperature difference so the radiator should work more efficiently with a hotter thermostat.

The OP changed the thermostat and the overheating problem went away. Bad thermostat, end of story.
If the OP really wants to test this they should buy a new 160 degree thermostat and put that in. Dollars to donuts there's still no overheating problem.

 

[SirTanon]

Originally Posted by Surestick
A hotter thermostat would mean hotter coolant going into the radiator. Heat transfer increases with a greater temperature difference so the radiator should work more efficiently with a hotter thermostat.

The OP changed the thermostat and the overheating problem went away. Bad thermostat, end of story.
If the OP really wants to test this they should buy a new 160 degree thermostat and put that in. Dollars to donuts there's still no overheating problem.


No - The OP did not change the thermostat. That was someone else who talked about having changed their thermostat. The thermostat in the OP's car is the same one that's been in there for a good 90,000 miles or so.

Also, a 160 degree thermostat would be VERY wrong for the OP's car, which requires a 180 degree thermostat (starts to open at 180 and is fully open at 208). If the OP ran a 160 degree thermostat, the computer would start throwing codes about the engine not coming up to temp.