2001 Pontiac Montana ~150,000 miles, bought used last summer. The engine has never seemed to warm up very much, in warm weather and cold alike. Any ideas why?
Try changing the thermostat first. It’s a $15 part and that’s the part whose function it is to allow your engine to warm up.
If this fails to help, post back. Add specifics like are you referring to the temp gage not coming up or to the heater not putting out warm air…or both. The devil is in the details.
They wanted $300 at the shop to change the thermostat because on that model they’d have to pull a bunch of stuff off to get to it. Checked a few different places and they all said the same thing, cheap part, tons of labor. Anyway, luckily my guy says he can do it himself. I guess we were just looking for more opinions before he dove into it, so I appreciate your reply, it confirms what he thought it might be 
And the major thing we’d noticed was the temp gauge not coming up very much, much lower than any other vehicle we’ve ever driven. Now that it’s gotten cold we’re not getting a lot of warm air either.
I’m sure mountainbike is correct. But be prepared for a repair that should be cheap but isn’t. The thermostat in this van is very hard to get to, and the part will likely be about $15, but the labor estimate will floor you. (The “official” procedure involves actually removing a chunk of the exhaust manifold).
But it would be good to add specifics. Are you going by the temp gauge alone? Does the van produce adequate heat? If it truly is running too cool your check engine light should be on b/c the car’s computer would fail to enter “closed loop” mode.
Make sure that if you have someone look at it they fully check things out before looking at the thermostat. You could have a fan problem or a problem with the temperature sensor. Or something else.
Blah! My perception stands corrected. I’ve never run into a T-stat that hard to get at before.
Just FYI, if you have a friend or something do it, I managed to replace the thermostat in a 2000 (Silhouette, but its the same van) without pulling the chunk of manifold (the major labor cost). It is not easy - there is precious little room to work - but the bottom bolt on the housing should not need to come all the way out as it is notched rather than a closed hole. Get the top one out and the lower one loose and it can be gently jiggled in and out. Of course, that housing might have changed between '00 and '01 but I doubt it.
I was incredibly surprised - in an incredibly unpleasant way. My brother - long a GM fan - got an earful from me about it (not that its his fault). You know me - I decided to blame it on a corporation 
I think we posted at the same time! Answered a couple questions above. I do also notice that when the a/c is on, we get a lot of air blowing out the vent, but when we switch to heat there is a huge decline.
No, the check engine light is not on.
The intake gaskets have been replaced (a common problem with this vehicle) and we’ve been told that it would pretty likely that they would have changed the thermostat while they were at it, but I can’t know that for sure since it’s used.
If the engine light is not on I’d be really cautious about going straight to the thermostat, though the lack of heat is telling. Make sure the coolant temp sensor is checked, the fans are checked, there is no air in the cooling system, etc.
Best way to tell is to have the actual temperature of the coolant checked.
Ok, I’ll let him know. His brother said the same thing, that he thought it could be changed without removing it. I’ll have BF read your instructions and we might pick your brain again if he has questions!
If you don’t mind enlightening a car idiot (me) - if a fan was bad, wouldn’t the temp be too high, not too low? Sorry if that is a dumb question… girl here 
Sorry - yes - if the fan didn’t work at all then the car would get too hot (especially when sitting still). But the fan is supposed to cycle on and off - if it fails to turn off and runs all of the time then it could keep it too cool. I don’t see that as a strong possibility - these are just the kinds of things that should be ruled out before diving into a $300 thermostat.
You can check the fan yourself - next time you take a decent length drive and have a few minutes just leave the engine running and pop the hood. Make sure the A/C is not on and that the switch is not set to defrost (just turn off all the climate controls). Then just hang out and see what the fan does. It should go on sometimes and turn off sometimes. Keep way though! Even if it doesn’t look like its on - you don’t want any part of you near it when it kicks on. That won’t tell you if it is working exactly right, but you would at least know that it turns off.