Strangest Thermostat problem. Opposite of overheating - Subaru Outback

hal y0 · 11-30-2010, 02:10 AM

I am up and down my local mountain pass 3-4 times a week and every time i have gone down the mountain the thermostat drops less then a quarter. Just like on start up. After i get down the pass it goes straight back to normal.

1. I keep it in third gear mainly down the mountain to keep from gaining lots of speed. RPMs stay 2,500 consist usually.

2. I have about 2,000 miles on an engine rebuild. With new waterpump, thermostat, fuel pump, injectors, timing belt, headgaskets, radiator.. I dont know what it could be.

3. My mechanic told me because of the altitude and the cold air when you drop quickly in elevation it tricks the thermostat?

Red Duc 996 · 11-30-2010, 07:21 AM

Sounds reasonable. The sudden rush of cold air through the radaitor combined with the reduced load on the engine. That will certainly drop the coolant temp in a hurry. Might take a little time for the thermostat to react. Hence the drop in coolant temp.

CNY_Dave · 11-30-2010, 11:50 AM

Could just be that the engine does not make enough heat to heat you (I assume heater is on) and keep the coolant at full temp, even with the thermostat fully closed.

Dave

subiesailor · 11-30-2010, 12:17 PM

you running synthetic oil? That also reduces the heat generating capability. Many diesel cars have a ceramic heater element to heat the cabin on cold days given the small diesel engines have a hard time generating enough heat to heat the cabin.

OldBay · 11-30-2010, 12:30 PM

could be a thermostat that sticks, too.

Dodik · 11-30-2010, 12:33 PM

what temp rating is on your tstat and what is designed temp should it be? if you got a stat that opens even few degrees faster plus cold air rush could explain your symptom, also what is your coolant mix you are not running 100% are you?

JM Anthony · 11-30-2010, 01:27 PM

Quote:

Originally Posted by OldBay

could be a thermostat that sticks, too.

This would be my bet. Used to have that problem with my motorcycle until I replaced the stat.

John

Outback Oz · 11-30-2010, 05:06 PM

I don't think that that is okay at all . the temp gauge should not move Under any conditions . The T stat is not working properly Oz

Timberwolf530 · 11-30-2010, 05:52 PM

Thermostats can stick open (underheating) as well as sticking closed (overheating).

hal y0 · 12-01-2010, 04:49 PM

yes i am running fully synthetic everything in this car. I have a brand new thermostat though. And it works 100% fine. Did the boiling water trick. The car runs amazing as well so its not really worrying me. Do you think maybe i should replace the already new thermostat with another one?

11-30-2010, 02:10 AM	#1 (permalink)
hal y0 Registered User Join Date: Aug 2010 Location: southern california Car: 99 OB Legacy. 2.5L Posts: 52 Feedback Score: 0 reviews	Strangest Thermostat problem. Opposite of overheating I am up and down my local mountain pass 3-4 times a week and every time i have gone down the mountain the thermostat drops less then a quarter. Just like on start up. After i get down the pass it goes straight back to normal. 1. I keep it in third gear mainly down the mountain to keep from gaining lots of speed. RPMs stay 2,500 consist usually. 2. I have about 2,000 miles on an engine rebuild. With new waterpump, thermostat, fuel pump, injectors, timing belt, headgaskets, radiator.. I dont know what it could be. 3. My mechanic told me because of the altitude and the cold air when you drop quickly in elevation it tricks the thermostat?

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11-30-2010, 07:21 AM	#2 (permalink)
Red Duc 996 Registered User Join Date: Oct 2010 Posts: 80 Feedback Score: 0 reviews	Sounds reasonable. The sudden rush of cold air through the radaitor combined with the reduced load on the engine. That will certainly drop the coolant temp in a hurry. Might take a little time for the thermostat to react. Hence the drop in coolant temp.

11-30-2010, 11:50 AM	#3 (permalink)
CNY_Dave Registered User Join Date: Nov 2007 Location: central NY USA Car: 2003 LLBean H6 Outback Posts: 3,356 Feedback Score: 0 reviews	Could just be that the engine does not make enough heat to heat you (I assume heater is on) and keep the coolant at full temp, even with the thermostat fully closed. Dave

11-30-2010, 12:17 PM	#4 (permalink)
subiesailor Registered User Join Date: Jun 2008 Location: San Francisco Posts: 11,448 Feedback Score: 0 reviews	you running synthetic oil? That also reduces the heat generating capability. Many diesel cars have a ceramic heater element to heat the cabin on cold days given the small diesel engines have a hard time generating enough heat to heat the cabin.

11-30-2010, 12:30 PM	#5 (permalink)
OldBay Registered User Join Date: May 2006 Location: Fairfax, VA Car: 2001 Outback 2.5L MT<== Gone. 2001 Audi A6 Avant till I can get back in a decent subie Posts: 153 Feedback Score: 0 reviews	could be a thermostat that sticks, too.

11-30-2010, 12:33 PM	#6 (permalink)
Dodik Registered User Join Date: Apr 2007 Location: Brooklyn NY Car: 02 Outback 3.0 sedan Posts: 451 Feedback Score: 0 reviews	what temp rating is on your tstat and what is designed temp should it be? if you got a stat that opens even few degrees faster plus cold air rush could explain your symptom, also what is your coolant mix you are not running 100% are you?

11-30-2010, 05:06 PM	#8 (permalink)
Outback Oz Registered User Join Date: Jan 2010 Location: Australia Car: 2001 outback 2.5 i Posts: 54 Feedback Score: 0 reviews	I don't think that that is okay at all . the temp gauge should not move Under any conditions . The T stat is not working properly Oz

11-30-2010, 05:52 PM	#9 (permalink)
Timberwolf530 Registered User Join Date: Jun 2010 Location: Indianapolis, IN Car: 2003 Legacy L Special Edition Wagon, AT Posts: 27 Feedback Score: 0 reviews	Thermostats can stick open (underheating) as well as sticking closed (overheating).

12-01-2010, 04:49 PM	#10 (permalink)
hal y0 Registered User Join Date: Aug 2010 Location: southern california Car: 99 OB Legacy. 2.5L Posts: 52 Feedback Score: 0 reviews	yes i am running fully synthetic everything in this car. I have a brand new thermostat though. And it works 100% fine. Did the boiling water trick. The car runs amazing as well so its not really worrying me. Do you think maybe i should replace the already new thermostat with another one?

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