I changed the t-stat in spring, so I don't know if it screwed up or what.

Anyways it was about -35 here, and the truck wouldn't blow heat at all. Vent fully closed, heat on max. It seemed to sort of come and go. But sometimes it was just plain cold. Normally it blows very hot.

Temp is around 175, and didn't drop or anything.

I'm thinking either the t-stat is buggy, or maybe the core needs changing.

 

I put my money on the core but would at least try the HVAC filter. I always try to start cheap and go up.

 

I changed the filter maybe 3-4 months ago. The air flow is there, the heat isn't. Air flow could be better though, damn Volvo vents always come loose.

 

Does the filter also do the inside "recirc" air or just the outside "fresh" air?? If it just does the outside air, how about partially blocking it off with some plastic. Cut down some of that -35 intake. Brrrr....

We are in a nice, balmy 15f and 12" new snow.

 

When you changed the t-stat did you change the little seal it sits in?, if not the coolant is getting by the stat and not letting the engine warm up completely and causing the heat problem.

As far as the core, just blow water and air thru it, if it ain't leaking it ain't bad, check that it is not clogged on the outside with dirt, if it is blow it out with shop air.

Make sure all your ducts are connected, I zip tied mine after 2 kept falling off of the vent.

 

What about the motor that opens the door to allow the hot air into the ductwork?

 

There's no real way to connect the ducts, they're snap fittings and have come completely loose. There is still airflow however, just a lot less heat.

I did change the seal around the t-stat, and like I said, the engine never dropped in temp. It's the same temp.

I won't have the truck for another week or so, it's getting some body work done right now. Thanks for the tips guys.

 

Is there a heater control valve in there, might be not working, shutoff valve for water lines maybe ?

If you have water temp and air flow you need to see if the hot water is getting to the core, might have to go inside the heater box, also make sire the heater box is sealed as to not loose the hot air before it gets to the vents.

 

this is my guess. mine had the same prob, years ago. if water temp is same, it is not thermo's. on mine there was a lever that controlled the heat/door, & it was broke, it was near my heater core, attached to the heater hose. find your heater core & it should be real close to it.