Lately I've been wondering if maybe the thermostat on my truck is going bad. As an example this morning it was 2� and the truck never got above 180� on a 45mile run to work. That is with the radiator blanket on it. On a 45� day it runs about 208� without the blanket.
The truck is an '03 Chevy Duramax.

What do you think? Is the thermostat operating properly?

That would be the first thing I'd look at.

Go buy a new one and switch it out, cheap and easy to see if that's what's wrong.....don't forget the new gasket....

I just watched a youtube video of changing the t-stat. Supposed to be 2 of them in there and there's a bunch of stuff in the way that needs to be moved to get at the thing. Nothing's ever easy anymore....

Edit to add: With the truck being 12 years old and these being the original parts, it makes sense for them to be worn out by now.

I'd check the coolant level....

Coolant is topped off. Drained and refilled when the new block heater was installed this fall.

Ooops,...overlooked diesel....

I know nothing about diesels,...and watching the video makes it even worse...

At least they're in the same area under 1 housing..

Comments from the youtube say there is an easier way, but just looking under the hood of my truck makes it look just as bad as the video. I guess Jay down at Able Auto will be getting a call. Too cold for me to do it right now.

the thermostats in the newr trucks are not like the older ones. they have a by-pass. the one in my 2004 colorado works one day and not the next. it will cause the check engine light to come on if it doesn't work for a while. my truck runs alittle cooler when its not working and it never gets hot. heater puts out more heat then i need even when its not working. i've been running it for over a year and i see no different in the way it runs.

When the temps are more moderate, I don't have a problem with the way this is working. However, when it's this cold I think it could run a bit warmer and put more heat in the truck.

and some of them have electronic thermostats, too...

I know nothing about Duramax diesels but ...

I was having a problem with my gas 98 Chevy taking longer than normal to warm up and it turned out to be a faulty radiator cap. It was only holding 13 psi instead of the required 16 psi. I would never have believed that a slight difference in pressure would make a difference but it did. Pressure and heat go hand in hand.

My first thought was a bad thermostat so I put in a new one but no change. Then I swapped out the radiator cap and that cured the slow warm-up problem.

I have changed out more than a few thermostats. Went to Toyota trucks about 20 years ago, problem solved.

Went to Rock Auto to check prices. The AC Delco thermostat is cheap as heck, but what struck me was that they are set for 180� and 185�. Soooooo, sounds like mine is operating properly.

I don't know anything about the Duramax but I replaced the thermostat on my 2005 Dodge diesel about a year ago when I replaced the water pump. Ever since then the temperature fluctuates. It usually ran a steady 200 degrees once warmed up no matter what the outside temperature was but now it sometimes fluctuates between 200 and 210 whether I'm driving in town or on the highway. Maybe a Dodge guy has an idea of what is causing this. I guess it could be an air bubble but the holding tank is always full and always has been.

Factory temp gauges have a reputation for being inaccurate. I dunno but the normal operating temp shouldn't be drastically affected by outside temps that aren't extreme and 2 degrees isn't extreme IMHO and especially since have you have a radiator blanket.

Like I said though, I know nothing about Duramax diesels so I can't say based on experience if it is operating normally.

If you want a more knowledgeable opinion post a question at ...

http://www.fullsizechevy.com/forum/forum.php

fan clutch can be staying engaged a bit and the t-stat staying open too much...something is amiss

If you have access to an I.R. thermometer, you can check that no warm coolant is getting to the radiator until the engine reaches it's proper temp. Easy to check, anyway.

The cooling system will keep it cool with outside temps over 100 degrees

At 2*, the ambient temp is enough to keep things cool with airflow alone

If the thermostat DOES open, the extremely cold liquid from the radiator will just make it close again almost instantly

Yes they have 2 thermostats. Mine wouldn't warm up properly after 290,000 miles. Replaced both and I have heat. Mine is a 02 GMC Duramax.

Exactly, I wouldn't mess with it.

Just to play scientist for a minute...water can't get any hotter than boiling. At normal pressure, it boils at 212 and that's as hot as it can get until it turns to steam. As the pressure rises, the boiling point goes up.
So, at 13lb, you have a certain boiling point. At 16lb, the boiling point is higher.

Once water turns to steam, the pressure will skyrocket and can become very dangerous. The purpose of a thermostat is to keep the temp below the boiling point and one of the purposes of the radiator cap is a fail safe to release steam before it blows up something if the thermostat fails.