Climate control not blowing Hot Air anymore

I noticed a few days ago the my E39 520 did not blow hot air from the climate control vents when I set the tempature to 26Degrees and turned the the vent control to hot.
This is a new occurrence so I know something's wrong, but don't know what. I can see the engine temperature gauge right in the middle position which is optimum, and the temp guage works as expected, from start up to a driving, but the vent only gives warm air at most if I set it to hot.
What I did notice though was that I don't hear that little change of flow that you hear when you move the vent control from blue to Red. Anyone who owns an E39 will know what I mean. When you rotate the vent know to the red from blue, you get that gap in which the air flow drops then comes back, like something just redirected the flow. I don't get that anymore. I know cos, even when I turned off my AC I didn't get that weird smell that comes for a few seconds. so something is up. I've used the fault code reader and it came back with no fault codes. Anyone know what it could be cos at the moment it seems ok, but I like to heed the warning signs from the car which isa 98 E39 520i 110Kmiles.
Any help will be appreciated. Thanks
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I'd be looking at either a clogged heater core or a worn impeller on the waterpump. The impeller would appear as temperature rise in standing traffic, and since you do not report this, I suspect blockage of the heater core.

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I've heard alot about faulty waterpump impellers and It might be the waterpump if not for the fact that my temp gauge seems to be working as expected, it stays low and goes up slowly to the mid position where it stays once the car is warmed up, maybe I should idle the car after its been driven to see if there is any weird behaviour from the temp guage.
As for a clogged heater core, do you know where the heater core is and if its something that can be fixed with a little DIY. Or is it an expensive stealer job?
Also I read somewhere about a passenger compartment thermostat not the engine thermostat, and I'm confused as I thought the thermostat was only in the engine. If that was stuck open would that cause the air not to warm as my understanding is that a stuck thermostat results in the car itself not warming up so the temp gauge stays low which is not what I'm seeing.
Thanks
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Passenger compartment temp is maintained by blending cold air from the outside with warm air that has passed by the heater core. How much air is blended is determined by a thermostat. Kind of like the thermostat in your house. You turn the heater on (in your house) and the heater blows how until the thermostat is happy, at which time the heater turns off until the thermostat decides it is cold again. In your car, the action is a bit diffferent, blending baffles open to a specific location to allow the proper blending of air to achieve the desired result. The air that enters the cabin is a constant, unlike the air entering your house due to the activity of the furnace. The car's heat source does not turn on and off, as happens in your house so the thermostat (cabin) changes the baffle position to alter the amount of cold air that is blended with hot air to give you warmth at a desired level. To complicate matters a bit, (my car, and I assume yours too) there is a temp control for both left and right sides of the car. By your description, I assume you are complaining that both the driver side and passenger side are cold. This points me toward the heater core itself, or its associated plumbing, as being clogged. On my car, I can set my side to 70 and my wife's side to 80. As a general rule, the entire car gets hot eventually, but her side gets more hot sooner. This would happen if there was a thermostat, or more than one, in the cabin that operated baffles.
If the thermostat governing your comfort stops working, the result could be improper mixing of the air sources, and you get a cold cabin as a result.
The life support system basically lives behind the center of the dashboard, straddling the transmission hump. I've only seen one, and it was very large. It comes out as an assembly, then the component parts are removed as needed. I don't know how much of it can be serviced with complete removal of the system.
I once had a car ('65 Ford Mustang) that had rusty water in it. The heater blew cold even though the fan whirled around furiously trying to do its job.
I got tired of being cold, and connected my garden hose to the outflow hose -- the hose the sends water back to the engine after it had gone through the heater -- and lots of rust and gunk came out. I connected to the outflow hose because I didn't want to push any blockage further into the core, making the blockage even worse. Today, I would be branded a polluter on the scale of the Exxon Valdez for pumping rusty water onto the street, but it cured my problem of poor output from my heater.
My suggestion is to flush your cooling system and see if you can get your heater to come back. My old Ford is completely different than your modern BMW, but I hope my experience helps guide you to the least costly service activity. I like to do the cheap & easy stuff first, and save difficult & costly for later.
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Jeff Strickland wrote:

Actually - not. There are two small electric pumps that circulate the coolant through the TWO heater cores in the heater assembly. If these go bad - either one - the heater will not function correctly. With your scenario it would be rather difficult to keep the heat from rising if you had the air supply on recirculation wouldn't it?

It's a split core - two halves. One for each side. It is NOT a single core device so it's very doubtful if both cores were plugged up (and plugged up with what?)

My suggestion is to get a Bentley manual and stop listening to troubleshooting of questionable value from self-proclaimed experts on the Internet. Be lots cheaper in the long run betcha..
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I agree, get a Bentley manual.
I never claimed to be any sort of expert. I described the way heater systems commonly function, and a way I would expect BMW heaters to work.
Two cores is definitely a possibility, and if true, then I'd have to agree that having both of them clog at the same time is unlikely. A '98 model year car is closing in on 10 years old, the heater could be clogged with rust and gunk from nearly a decade of inattention, how do I know what it is clogged with? Running clear water through it will tell you if the trouble comes from there or not, and it doesn't cost much to do at home.
PS In Recirc Mode, it would not take rocket science to figure out how to bleed in small amounts of outside air to keep the temp from rising out of control. There is no reason why the baffle could not be placed in a manner that would pull 95% of its air from inside the cabin, and 5% from outside. That is how most heaters work that I am familiar with.
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Jeff Strickland wrote:

It isn't how modern BMW heaters work. They shut off ALL outside air since the E39 has a detector for noxious fumes outside the car - and it does the same as the manual recirculation - shuts off all outside air.
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Dunno where the rust would come from given my 528 has an all ally engine. If you've changed the coolant at the correct intervals and not topped up with hard water I don't think the matrix(es) will block.
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wrote:

I believe you are correct, but we don't know what it might have been topped up with.
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