No Heat When Idling; Full Heat When Moving
 

Hi All,

So I've been having some issues with my heat as of late. About a year ago, I noticed that my vehicle really didn't heat up until it was moving. It wasn't really a huge deal for me however, so I just let it slide. Once in a while I would see the dreaded book symbol, but it would go away soon enough and I wasn't too concerned.

Today I got the book symbol again. Just as before, when the vehicle is idling (at startup, or at a red light), the heat goes away and it just blows cool air. However this time, when I started moving again, all it would blow is full hot air. When I tried turning both temp. controls down (all the way), the passenger side would get cool air, but my driver side would still get full heat (really hot).

I was/am able to sort of deal with the problem by just adjusting the fan speed, as now it's getting cold enough that I really only want heat. However the issue is going to have to be addressed at some point.

What do we think I'm looking at here? Any ideas?

Thanks in advance,

John

at the very least , check the coolant level, and bleed the coolant system....if it's low on coolant it wont circulate through the heater core until the engine speeds up. If the coolant level is ok, and the system is bled properly.....the heater core may be partially clogged. If it turns out that you were low on coolant...you'll need to find out where the leak is..

The thermostat is bad, it is stuck partially open and not allowing the coolant to stay inside the engine long enough to get heated by the engine.

I had a similar problem some years ago when, after a journey, whilst the engine was idling queuing for a ferry I noticed the heater was blowing cooler/cold when it was OK during the journey. It turned out to be an air lock in the heater matrix and was obviously low on coolant. After bleeding the cooling system and topping up the problem disappeared. Hence it seems to have been an air lock in the system.

You have no heat because you have a coolant leak causing air to fill the heater core.
First thing to do is fill the coolant, bleed it and then do a coolant pressure test to find and fix your leak.

Typically that results in the opposite, heat when idling, no heat when driving.

This is one of those bypass stats, so it has two valves, one for "main" flow, and another for flow at idle speed to warm the interior. Lots of hoses coming out of it. When water pump speeds up, the pressure is supposed to be enough to open the smaller valve to send heat to you. When main valve opens, it shoves smaller valve closed.

So Spike and Tom can both be right, because it would depend on which section was sticking.

Thoughts to ponder -

if an air pocket, would you hear it moving when speeding up (the gurgle thru the heater core)?

If a sticking stat at idle, would banging on the stat housing free it up?

Seems like you could feel the line going toward heater core (maybe you have hot water and not enough pressure to push it thru a clogged heater core at idle?).

Pix of D2, RR is a similar concept.