Heater isn't very hot

[BSOD2600]

This morning it was 20F when I started the car up. First try, it died. Second, I gave it a little gas and it fired right up. I let it idle for ~5 min in the driveway with the fan on 4 and in the 'heat' position. Noticed that after all that time, it didn't make a dent in the frost on the windshield. Anyways, drove ~2 miles to school and the entire way, it never really blew warm heat at all. By the time I got to school, the engine temp gauge showed regular operating temps, yet the heat really wasn't that warm. I tried pressing different settings on the HVAC, and it did change to the selected setting; just no change in the heat. Tried sliding the temp up and down; no change in the heat.

I replaced the thermostat last month and pretty sure the coolant system has been burped (damn that was a PAIN!), since I don't overheat anymore.

Any ideas on why the lack of heat?

[Brat4by4]

The adjuster cable is not adjusting the flaps that uncover the heater core inside the heater box. The cable goes down the right side under the glove box IIRC. Get your car warmed up again and then sneak your head under there and look for a cable going into a plastic slider piece. Move that piece manually to both extremes and see if that changes the heat coming out of the vents. (And make sure the cable is inserted...)

[Legacy777]

When you replaced the thermostat, did you replace it with a subaru thermostat?

Have you replaced the coolant temp sensor recently?

The "heat" setting really isn't a good setting for defrosting the windsheild, use the "defrost" or "def/heat" setting.

Also check the cable like brat4by4 mentioned.

[soloracer]

The cable that goes to the mentioned flap can be easily found just underneath the glovebox. On the very left hand side of the glove box door just an few inches towards the front of the car is where tha cable attaches, if you move the control slider back and forth you should easily see and feel the cable moving. the end loops over the peg on the cam and the cable clips into a plastic clip. With the car warmed up you can also try manually moving the cam back and forth to see of you get anything. Good luck.

[BSOD2600]

Yes, I replaced the thermostat with an OEM subaru one. I hooked the ECU up to a computer and the temps showed 193-203F after driving around town and then idling in my driveway. Thats the right range, right?

I'll check out that cable Sunday when I get home. Suppose I should take off the glove box when I do that too 'eh. I'm fairly sure the cable is at least partially working though because if I moved the temp to the cold setting, it would get cold for sure.

[Legacy777]

Yeah that's about the correct temp range. The ECU kicks the fans on around 200F

[Brat4by4]

Hmm, possible a plugged heater core or the flaps are not opening all the way. You definitely want to check that cable. I don't think its absolutely necessary to take off the glove box, but it wouldn't hurt.

[BSOD2600]

Alright, I checked the temp cable and it properly moves all those vent flaps under the dash. Was 20F this morning and the car fired right up (yay). Used defrost this time and it actually got the window partially clear this time (yay). By the time I got to school, there was some heat but I wouldn't call it hot. Maybe because it's only ~2 miles, even though the engine temp says its fully warmed up, it's not?

What would be the other signs of a plugged heater core? Trickling / water sounds coming from under the dash, per chance?

[Legacy777]

Have you replaced the coolant temp sensor recently?

Have you flushed the coolant recently (I would assume so if you did the t-stat)?

[Brat4by4]

Try driving your car longer than 4 minutes. Let it idle for a while after reaching full temp with the heat on full blast. Go inside, wait 5+ minutes or so and then go in your car and see if it feels warm.

My Legacy puts out the least heat of any of my cars. It takes a while to get the car warm, but after it gets there it stays nice and toasty. My other Subarus throw out much hotter air in a shorter period of time.

[Legacy777]

That's weird because my legacy, even when it was N/A put out very decent heat in a short amount of time, and once it got going, it would cook you to death if you left it on full blast.

[Brat4by4]

My car and Snowboarded's are funny with the heat. You would think it would be phenomenal with the turbo adding all the heat to the system.

It takes a while to get going, but once the car is warm you have to keep it on halfway heat so as not to roast.

[BSOD2600]

Have you replaced the coolant temp sensor recently?

Nope. But since the ECU is reading around the correct temp at least when the engine is warmed up, I'd think it's functioning properly.

Have you flushed the coolant recently (I would assume so if you did the t-stat)?

Yea, I did that. Also bleed the system as much as I could. Every once in a while, I'll take the cap off the coolant tank when the engine is cold to verify there are no new air pockets -- never have been any more.

[Legacy777]

Have you replaced the coolant temp sensor recently?

Nope. But since the ECU is reading around the correct temp at least when the engine is warmed up, I'd think it's functioning properly.

Not necessarily, the response of the sensor may be slow which may impact how the car runs or warms up. The coolant temp sensors are pretty difficult to test. Even with one that tests out ok, it can be bad. Ask anyone who has had starting issues, replaced the sensor and had the car run fine.

it's a $20 item, and IMO worth replacing if you are having any cooling system abnormalities.

[R-Inge]

Sounds like the cooling system still has some air in it, specifically the heater core. Crack your radiator cap when it is *cold* and check the level. Add as needed, also bring the level up slightly in your reservoir. Then run it up to normal temps and hold it there for a few, and shut it off and let it cool overnight. Theoretically as the system cools it will pull coolant out of the reservoir into the system to displace any air in there. May take a couple of shots. Either that or get a Blue Point universal coolant refill tool and use that to refill the system.

[BSOD2600]

I opened up the radiator cap (remember turbo so its a small reservoir on top) with the car cold. After hearing a small sucking sound (pressure difference, I'd guess) I saw the coolant level rise up a little bit. The overflow tank is about half full too. In the past when I did have a lot of air in the system, when I would pull off the radiator cap, the level would drop down a lot -- now its just the reverse.

Guess I'll go run the car for a little bit with the radiator cap off and see how much coolant I get all over the place...

edit:
Alright, started it up cold and let it run for a while. Coolant in the rad cap was always right at the top. Didn't really see any bubbles coming out. I did notice though when I'd rev the engine, the coolant level in top radiator would drop. Thats normal?

[Brat4by4]

Yep.

My car is the same way. I can't check the fluid without it spilling everywhere. And once I filled it up by revving the engine and topping it off before capping it.