cooling system, thermostat question

I've got a 1990 GMC Suburban, automatic, 5.7 V8, 150,000 miles

I'm in interior Alaska, and this fall as it got colder I noticed that my heater wasn't heating, but I attributed it to the colder weather and the fact that I only drive this rig 7 miles to work and back, no longer trips.

It was -40 recently last week and one morning I drove to work, 7 miles (with a 5 minute stop at the store where I kept the engine running), and my rig overheated: cooling fluid coming out the overflow container and steam venting off the radiator cap.

A friend gave me advice: start with the easy things first, change the thermostat.

I put in a new thermostat, and voila, problem solved, and I immediately noticed that my heater was working again.

Here's my question. If the thermostat isn't functioning correctly, and is stuck closed, how does the overheated coolant get to the overflow container and out the vent cap?

Here's a link to where I was reading up on cooling systems:
http://auto.howstuffworks.com/cooling-system2.htm

This I think I understand: the coolant in the engine doesn't get routed to the radiator (because the thermostat is stuck closed) and instead gets sent back through the water pump and back through the engine, so it overheats.

2nd question: why would a bad thermostat affect the heater?

3rd question: I would guess that my thermostat has been faulty for some time now because the heater wasn't heating my rig as far back as November. Why didn't my rig overheat before this? Did the cold -40 have anything to do with it overheating? Was it partially working before and then finally went bad?

thanks for input

 

Did the heater work when the vehicle was overheating? Maybe if it was the thermostat could have been stuck open, and while driving a short distance, it(cooling water) might not of had enough time in the engine to get warm enough to feel it through your heater and then when you came to a stop it just kept circulating through your radiator without having enough time to cool off then it started overheating.

 

Your commute is not long enough to allow that 5.7 to reach operating temp, never mind the cold isn't helping
That could hide a problem for a while

The thermostat operates on pressure, not temperature
So it could be stuck shut until over-heat

I suspect yours was stuck open, hence no heat

It will now idle for 15 minutes with no over-heat?

 

I haven't let it sit and idle for 15 minutes, but I idled it for 5-10 minutes with no mishap, and then started to drive it normally again.

According to my engine temperature gauge it does heat up to operating temperature in my commute (about 3 miles at 25 mph and then 4 miles at 55 mph).

Slickshaft, your idea that it was stuck open accounts for no heat in the heater, but not overheating. Maybe it was stuck open for awhile and then somehow stuck shut on the morning it overheated.

Anyway, thanks to both of you (ltrapper and slickshift) for your replies.

 

It could stick either way. Remember that there are two hoses and, if the coolant is not making a complete cycle, the over heated coolant expands and will come out the lower hose. Then into the radiator and out the overflow.

The Alaska interior is beautiful in the summer, but I saw that Fairbanks had -51 last week. Way too cold for me.

 

Thought everyone up there had block heaters! :wmann3: Might be a good idea; would almost certainly be better for the overall life expectancy of your engine if it was at operating temperature longer than the timeframe you describe.

 

Did you have coolant tested. The colder the richer the mix. I live in canada and I change coolant every 2 years and test for -60 If coolant starts to freaze Engine will over heat.