Heat Only Works Under Throttle...

Can you help me figure out what is going on?

Recently I noticed that my heat only likes to work when I'm under throttle. Temp gauge shows normal all the time including when I'm warming the car up. When I'm accelerating, the heat works great. Once I coast or stop, the heat slowly goes cold - blower motor continues to work as usual, but the temperature gradually cools off to the point where I have no heat, just cold air.

A couple other things I've noticed:

• My radiator fan has been kicking on after light driving (7 miles) in cool temperatures (20F)
  
• After normal driving (on occasion) I've heard my coolant boiling over into my over flow tank. It has never spilled over that I'm aware of
  
• After I shut my car off, I usually (not always) hear a repeated clicking noise from the middle of the dashboard that eventually goes away (after 1-2 mins). It sounds like the CD player is switching CD's continuously, but I have the feeling this might actually be something within the HVAC system that is trying to open/close, or maybe even something else
  

Background:

• Car ('05 LGT) has 140K, stock minus AP
  
• All maintenance, including coolant and water pump, has been serviced on time
  
• Replaced the radiator within the last 20K miles
  
• Pretty sure the thermostat has not been replaced
  

Can you help me diagnosis this issue? I'd love to stay away from the dealership if I can, but without help, that's exactly where it's going (when they replace my damn airbag that's being recalled).

TIA!

The repeated clicking noise is quite common it seems. Related to the HVAC door that isn't actuating properly (as you suspected): http://legacygt.com/forums/showthread.php/bubbling-noise-after-turn-off-cari-227004.html?t=227004

I'm curious if when you coast/stop and rev the car (throw it into neutral or hold in the clutch, depending on AT/MT) if the heat returns. I'm curious if the problem is RPM related or load related... I'd suspect RPM related. If that is the case, I'd suspect that something is keeping the cooling system from staying correctly pressurized at low/idle RPMs.

Really appreciate the reply, glad to learn more about that damn clicking.

Strangely enough, the heat problem appears to be intermittent as it recently has started working all the time. We're in for some nasty cold in the next few days so I'm sure there will be plenty of time to test. I'll try as you suggested (I have a manual) and will let you know what I find, though if I'm remembering correctly, I had no heat at a stop, rev'd it and the heat came back without moving.

Make sure your coolant is full in the rad.

If its full make sure it has no air in the cooling system.

Worst cause is head gasket is blown.

Low on coolant

Air in the system

Coolant system is plugged (i.e. heater core, radiator)

Water pump is failing

Have you put stop leak or any type of sealer in the coolant system?

I'm curious if the problem is RPM related or load related... I'd suspect RPM related. If that is the case, I'd suspect that something is keeping the cooling system from staying correctly pressurized at low/idle RPMs.

Confirmed, the issue is RPM related (heat blows cool once the RPM drop and rise back up with the RPM).

Also checked the coolant levels. I was able to put maybe 6-8 ounces in? Wasn't thinking this was the issue, but it did have a little bit of air in the radiator.

Wouldn't coolant be mixed in with the oil if the head gasket was toast?

Finally, I have not used stop leak.

Just dropped it off at the dealership for the airbag recall, they said they might be able to clean and lube the HVAC actuator tracks while they are in there! I'll pot an update if they figure out what's going on with this.

Thanks guys.

Just got the car back from the dealer (yes, 20 calendar days later ).

Lucky for me, they agreed to lube the HVAC actuator tracks while they had the dash off for the new air bag. Haven't heard the clicking noise since, but it was intermittent. No charge, so I don't really have any room to complain.

As for the real issue, apparently I must have switched the two coolant reservoir caps (the one on the radiator with the one just above the passenger side valve cover). They bled the coolant, topped everything off and told me to see what happens (but that it could be a head gasket:mad:). Crap. Only good news is that I haven't seen the issues repeat themselves yet, though it's only been 15 miles.

I really hope it was just a low coolant issue, which would make sense to me. I'll keep an eye out for elevated temps, coolant smells, steam from the hood and changes to the coolant overflow tank. If you know of any other signs of head gaskets going out, I would appreciate hearing from you.

Thanks all.