Car running COOLER in warmer weather!?

[wewantutopia]

All winter my temp gauge was running @ ~3/4. Now for the past few days in Chicago it has been 70-80*F and my gauge hasn't gone above 1/2 (the entire time of several hour trips). Is this normal? Has anyone else experienced this and if so why? I would think that it would be the exact opposite.

[Fisher]

I have an 84 Jetta TD and it runs cooler in 90 to 100 degree weather than it does in moderate PNW weather.

I'm not sure why, I think it must have something to do with the Thermostat being open more or something, I also think the Radiator cools better in hot and low humidity weather.

[surfcam]

You might have a sticky thermostat. My thermostat will go to just over half, even on cold days. On warm days it will go to just over half and then return to just over a quarter. It will remain in this position for the remainder of the trip unless I do some big hills. When its real cold out sometimes it can't quite make it passed half way so it sometimes runs hotter for the whole trip. I actually like this since it will warm the car up faster went its cool out. The down side is you have to keep an eye on it because maybe some day it will stick permanently.

[tylernt]

I've noticed the same thing. Was running two marks past the fat mark in the winter, now I'm running less than 1. Thermostat has less than 4,000mi on it, though, so it shouldn't be sticking so soon. Odd.

[VWCaddy]

Could also be heater usage. Heater core stips some heat out of the coolant and perhaps fools the t-stat into "thinking" the coolant is cooler than it really is. The send sender is at the head outlet so would be hotter reading than the coolant returning from the heater.

[tylernt]

Could also be heater usage. Heater core stips some heat out of the coolant and perhaps fools the t-stat into "thinking" the coolant is cooler than it really is. The send sender is at the head outlet so would be hotter reading than the coolant returning from the heater.

Say, that's a good hypothesis. However, my heater cable is broken so even in warm weather, I have hot coolant flow though the side outlet (I just roll my windows down if I get hot ). So I don't think the gauge sender is contributing to this phenomenon. I am using my HVAC blower less though, so the thermostat may be getting hotter coolant flowing across it via the heater core return line.

[Lee]

I've always thought that the reason they run cooler in warmer weather is because in the winter when cold air is hitting the thermostat housing it causes the thermostat to open slightly more which causes more coolant flow through radiator which causes a cooler temp at the sender. Just my idea.

[tylernt]

Yesterday I happened to notice my radiator fan coming on at a cooler temperature too. It's just too coincidental that both the thermostat and the rad fan would both start working at one mark lower than they used to. That makes me think that my engine is the same temp it's aways been but my voltage regulator is causing my gauge to read "off". I can take a reading with my multimeter now, but I'll have to wait for next winter before I can confirm any differences.

[Lee]

Just realized I explained my idea wrong. In winter the colder air hitting the thermostat housing causes the thermostat to close more which causes less flow through radiator which causes higher temp at sender.

[Quantum-man]

In a Quantum if you allow heat into the car; then the coolant temperature rises
It doesn't really its just that coolant flow to the heater is switched off with cold air flow only... and the temp sensor is in the flowline

[JRM]

mine does the same thing, even after I picked up a new stat from jack, it seems as warm weather holds it open- my temp runs at mid way mark in the winter, with ultra hot heat... in the summer it barly breaks off the cold side unless im in traffic

[bignivanut]

I know about my dad's 90 Jetta it had AC but compressor was removed, we didn't change any wiring just remove the compressor. It runs cool if in defrost position in winter time but in heat it runs normal. The electric fan runs in all positions except for floor heat. Any other VW I've had ran warmer in warm weather except when idling around town in the summer when the fan cycles regularly, and it pulls temp way down.