Fan blowing cold air on heater mode

[Pawan77]

Guys,

My car is blowing cold air from the dash vents on the heater mode, also the fan speed sounds abnormal, it is not that high when I put the fan speed control on high. Any ideas on what could go wrong? I changed a Final stage resistor 3 years back. Is the same part broke again?

Any diagnosis to figure out which component it is?

Thanks in advance

[danewilson77]

I do not think FSR....unless fan is not working, or is changing speeds erratically...

[nathancarter]

Cold air when the heater is on means one of four things:

- Stuck-open thermostat (very common)
- Low coolant/ air pockets in heater core (very common, but should work themselves out with some high-revs driving)
- Stuck-closed heater valve (moderately common)
- Stuck baffles in the vent ductwork (not common at all)


To diagnose the thermostat, drive around town until the engine is fully warmed up and the temp needle is pointing straight up. Then get on the highway and drive at a moderate, consistent speed and low RPMs - e.g. 60MPH in 5th gear. If the needle moves back to the left (colder), then your thermostat is stuck open. You may also get code P0128.

[Pawan77]

Cold air when the heater is on means one of four things:

- Stuck-open thermostat (very common)
- Low coolant/ air pockets in heater core (very common, but should work themselves out with some high-revs driving)
- Stuck-closed heater valve (moderately common)
- Stuck baffles in the vent ductwork (not common at all)


To diagnose the thermostat, drive around town until the engine is fully warmed up and the temp needle is pointing straight up. Then get on the highway and drive at a moderate, consistent speed and low RPMs - e.g. 60MPH in 5th gear. If the needle moves back to the left (colder), then your thermostat is stuck open. You may also get code P0128.

Nathan, Thanks for the response. Stuck open thermostat should drive the temp needle to red zone right? anyways I will do the diagnosis you mentioned above. Thanks again


Also thanks Dane.

[telijah]

No, Nathan stated the opposite. If the t-stat is wide open, then the needle will go to the left when cruising, into the colder region (blue). The t-stat opens wider when the engine temp rises, keeping the temp balanced. If it is wide open, it is constantly letting the maximum amount of cooled coolant as possible.

Also, it was not mentioned, and I hate to ask dumb questions, but check the fresh air wheel in between your two center dash vents. Make sure you have it turned to the red dots.

AND, in regards to fan speed... when using heat, if the zone setting is on Auto (not direct to face, or direct to feet etc), you generally will not feel the air coming out faster through the dash vents. Try manually selecting the face zone on your control unit and then set the fan speed.

[Critter7r]

Set the thumb-wheel between the center vents to the red dots (to cure the cold face vents) and replace your microfilter (to cure the low air flow).

[Pawan77]

No, Nathan stated the opposite. If the t-stat is wide open, then the needle will go to the left when cruising, into the colder region (blue). The t-stat opens wider when the engine temp rises, keeping the temp balanced. If it is wide open, it is constantly letting the maximum amount of cooled coolant as possible.

Also, it was not mentioned, and I hate to ask dumb questions, but check the fresh air wheel in between your two center dash vents. Make sure you have it turned to the red dots.

AND, in regards to fan speed... when using heat, if the zone setting is on Auto (not direct to face, or direct to feet etc), you generally will not feel the air coming out faster through the dash vents. Try manually selecting the face zone on your control unit and then set the fan speed.

I ran the diagnosis and the temp gauge needle is rock solid in the center. I dont think the t-stat is the culprit.

I gave a couple hard revs nut still cold air is blowing so donno if air pockets are the reason. DO I NEED TO HOLD DOWN THE GAS PEDAL FOR A WHILE IN THE RED ZONE, I PRESSED IT BRIEFLY INTO THE RED ZONE for this diagnosis. Please suggest?

Any codes that it will throw off for the heater valve?

Also the center wheel between the vents is set to the 3 red dots and all the fan settings are manual.

Anyway to diagnose the heater valve?

Thank you once again.

[nathancarter]

The thought processes were this:

The heater draws warmth from the hot coolant. There's a heater core, which is essentially a little radiator, inside the dashboard, in the AC/heater ductwork. The heater core is full of coolant: when the heater is on, a valve opens allowing coolant to circulate through the heater core; when the heater is off and the valve is closed, the coolant isn't actively flowing through the heater core (but it's still full).

Whenever the car is running, coolant is flowing through the engine block, driven by the water pump. Coolant ONLY flows through the radiator when the thermostat is open; otherwise is just cycles around in the block and doesn't flow into the radiator. The thermostat opens automatically when the engine reaches the top end of its operating temp, but can also be opened electronically by the engine management computer, under certain conditions. Coolant ONLY flows through the heater core when you turn on the heater, as noted above.

If the thermostat is stuck open, coolant always flows through the radiator, and the engine is cooled off more than needed. If the thermostat is stuck closed, coolant never flows through the radiator, and you overheat (fortunately, this is rare on the E46).

If the heater valve is stuck open, coolant always flows through the heater core, and you always get hot air out the vents (though if you run the A/C it'll cool it off quite considerably). If the heater valve is stuck closed, you'll never get warm air from the vents.


Now, the heater core is always supposed to be completely full of coolant. If you drain the coolant for service, it'll drain out of the heater core - however, when you refill the coolant, you have to make sure the heater valve is OPEN, or the replacement coolant won't completely fill the heater core; the air pockets will just hang out there and cause trouble.

Revving the engine will make the water pump spin faster, and make coolant flow faster through the block (and maybe the radiator, and maybe the heater core). So if you open the heater valve and rev the engine, the fast-flowing coolant will hopefully "burp" the system, forcing the air bubbles around into other parts of the system; when those bubbles eventually get to the expansion tank, they don't leave - though you'll need to top up the expansion tank. You won't have to do it much - or at all, if you haven't had any work on the cooling system recently. Note that if the heater valve is stuck closed, it won't work anyway.


Further diagnosis: If you're cruising at low RPMs, the engine isn't generating much heat. If the thermostat is stuck open, coolant freely flowing through the radiator will discharge heat faster than the engine makes it. If the temp needle is sitting quite solidly at the 12-o'clock position, even when cruising on the highway, then the thermostat is quite likely NOT stuck open; it's cycling closed and open like it's supposed to.

To diagnose the heater valve, the test will be to find the valve on the driver's shock tower in the engine bay, and feel the temperature of the hoses to and from the valve, while the car is warm and running with the heater on. If I recall correctly, there's one hose going from the engine (cylinder head) to the valve, one hose returning from the valve to the engine (heater off), and one hose going from the valve to the firewall and heater core (heater on). There's also a return hose coming from the heater core/firewall into the expansion tank. If the hoses going in and out of the firewall are cold, even when the heater is turned on, then the valve isn't opening like it's supposed to.

[aka fonz]

DO I NEED TO HOLD DOWN THE GAS PEDAL FOR A WHILE IN THE RED ZONE, I PRESSED IT BRIEFLY INTO THE RED ZONE for this diagnosis.

what are you talking about here? I pray to the bimmer Gods above you are not talking about the operation temp. DO NOT EVER GO INTO RED or you will blow headgasket, warp head, scar piston walls, and a gajillion other horrible things.

[nathancarter]

what are you talking about here? I pray to the bimmer Gods above you are not talking about the operation temp. DO NOT EVER GO INTO RED or you will blow headgasket, warp head, scar piston walls, and a gajillion other horrible things.

Nah, I think he just means he revved it up to redline (6000+ RPM) for a sec. Good for burping air out of the heater core, *IF* the heater valve is open, and *IF* the original problem was air in the heater core.

[Pawan77]

what are you talking about here? I pray to the bimmer Gods above you are not talking about the operation temp. DO NOT EVER GO INTO RED or you will blow headgasket, warp head, scar piston walls, and a gajillion other horrible things.

I am talking abt the revs not the temp gauge.

[aka fonz]

I am talking abt the revs not the temp gauge.

lol, okay i was a little worried for a second.

[Pawan77]

OK, the blower started blowing hot air since yesterday. yesterday it blowed hot air but not as it used to be, little warm air but not with speed. Since this morning it is working normal.
Could it be a bad heater valve? or just the air bubbles?
I will diagnose for the heater valve this weekend and will post. Thanks

[Richard Stutsma]

This is an old thread, but a new problem for me. I did not find any posts dealing with my situation. Just bought a '04 325it with 39,000 miles on it- not bad for a 15 year old car- but the vents always blow warm air until I turn on the A/C, then it is moderately cool air. I tried pinching off the top hose on the heater control valve per previous threads, but that did not make any difference. If I move the temp to the hot position, it is still only warm, probably because the heater hose is pinched off. But why is it still blowing warm air- not hot, just warm?? How could that happen if there is no coolant flow into the heater core? I did not pinch it overly tight so as to not crush or tear the rubber hose, but it is 99% closed if not 100%. I hope I do not have to start looking under the dashboard. Any ideas??

[Moogaroon]

Not sure if its relevant or not - we have had our X5 for about a year now. Mostly used for short trips under 20 min, and it makes good enough heat in winter then. But if we take it on long trips, we find it starts blowing cool air. Looking through this thread, I figured it might be bubbles in the heater core (heater valve was replaced since we got the vehicle). But the fan inside the heater control unit may be the source of the problem. If it does not work - on long trips, that unit begins to heat up (in winter), and no longer reads the ambient temperature inside the vehicle well. As a result, we get cool air blowing out of the vents. Still have to confirm this... Have a new unit to swap in.