So I recently replaced a leaking heater core. It seemed to work fine for about a week but then I noticed that after a cold start, the heater would work fine for about 10 minutes. Then it would all of a sudden blow cold for a few minutes and then come back. Now it goes cold and doesn't seem to want to blow hot air again. My logicon seems to be working fine so I was thinking maybe the thermostat is the issue? Also, when I was coming in to work this morning I saw a white cloud behind me for about 5 seconds and then it stopped. I've heard talk about a heater control valve but can't seem to find any information about it when I search. Any advice on what I should be troubleshooting? I'm really stumped on this one...

 

A bad thermostat would show up on the temp gauge while the heater was acting up.
You'll never know though if you only have the stock S5 idiot light temp gauge because it's too numb to show any quick transients in the system.
If it ain't the thermostat, most likely you have a pernicious air pocket trapped in the new heater core.
Thermostats are cheap and easily replaced, so I'd toss a new one in, rebleed the system and see what happens.

 

My car is a S4. I have noticed the water temp comes up to about half sometimes and it used to never get above 1/4 or so. It stops at about half though so I didn't think too much about it. What's a good way to bleed the system? I keep letting air out of the top of the radiator by screwing out that plastic bleed screw. Should I do this while it is running and warm until coolant comes out or something? I always just bleed it after shutting it off when it's warm.

 

Would the heater core really go cold that quickly if the thermostat wasn't working properly?

 

If you have air or low coolant,then the heater will not produce heat properly.

My question is why you are you always bleeding the system?

New rad cap,new thermo,good and adequate amount of coolant and good hoses with tight clamps.basic list.

 

I keep bleeding it because I just added a bunch of air to the system by replacing the heater core. I guess I'm confused because the heater starts to work great for a few minutes. Blows really hot. Then all of a sudden it goes cold. That's what has me confused the most.