Bleeding issue where hot air only coming while raising RPM

[cinnamonDillon]

Recently replaced my thermostat in my 1997 M3 (US spec) and everything has gone smoothly, no leaks from housing, car holding temperature fine, etc. Now the issue is when I bleed the coolant, the air from the vents is not hot unless I press the throttle and raise it to about 1500RPM. I also notice the coolant level in the expansion tank starts to drop a little. Then when I take my foot off the throttle the air eventually goes back to not hot and the coolant level starts to creep back up to where it was before. What could be causing this? I raise the front end of the car when bleeding the coolant, have bleeder screw and fill cap off, have HVAC sent to 90F full blower speed. Is this a broken heater control valve or something else?

I will mention that I've tried bleeding the car a few times now. It drives fine and holds temperature. But every time I turn the car off I get a coolant level low warning on the OBC and when I check the coolant level it has dropped to almost the bottom of the expansion tank. Then when I turn the car off it rises back to around the KALT/COLD line. Not sure if this is part of the same issue or not. Any insight or advice is much appreciated Thanks in advance!

[82eye]

a) air lock in the heater
b) coolant leak / bad heater valve / bad heater core.

did you lift the front of the car when bleeding?

just chased the exact same issue down on the e30. persistent cooling trouble with poor heat after a system refresh. replacing the valve / o rings / heater core now. the o-rings had turned to plastic and let go. the others may or may not have been leaking. doing it all.

it began very minor after the cooling system refresh and progressed to the point i had a lake develop in the drivers footwell.

[R Shaffner]

Try having someone rev the engine a little while you bleed it (heater on full hot). Replace the bleed screw when just coolant comes out. (If that's different from what you've tried.)

[Eric93se]

This is almost certainly a bad reservoir tank. Look inside, there should be a plastic tube running from the very top to the bottom, when old they shatter. Only buy top quality, OEM bmw, behr, mahle, beck arnley.

[cinnamonDillon]

@82eye

a) how would I clear the air lock in the heater?
b) I was suspecting heater valve but didn't want to blindly throw parts at the car if I can help it. Is there a way to either definitively rule it out or narrow it down to that?

I did lift the front end when bleeding the car. Not an obnoxious amount but a good couple of inches.

The reason I am kind of skeptical of it being a sudden heater core failure is that everything was working completely fine before I went in and replaced the thermostat. And also seeing how much of a job replacing the heater core is makes me want to really make sure whether it really is that or not ��.

Thanks for the input!

- - - Updated - - -

Try having someone rev the engine a little while you bleed it (heater on full hot). Replace the bleed screw when just coolant comes out. (If that's different from what you've tried.)

I have had someone lift the revs a bit and hold it, however when they let off again the coolant comes back up in the tank, which leads me to think that it is just a air pocket getting compressed and then decompressed. And then moving back to the reservoir once the car gets turned off. Not sure if that's what's actually happening but just a loose theory.

[cinnamonDillon]

This is almost certainly a bad reservoir tank. Look inside, there should be a plastic tube running from the very top to the bottom, when old they shatter. Only buy top quality, OEM bmw, behr, mahle, beck arnley.

How exactly would it be a bad coolant reservoir? Do you have a photo you can show of the plastic tube you're referring to? Mine only has maybe 10k or less miles on it. The brand is Rein

[R Shaffner]

The info and pics in this thread will probably help.

https://www.bimmerforums.com/forum/s...ding-Explained

The system is designed to hold the coolant at a higher level in the radiator than in the expansion tank. The pumbing in the tank is designed to help do that.

[82eye]

@82eye

a) how would I clear the air lock in the heater?

lift the front and bleed proper.

b) I was suspecting heater valve but didn't want to blindly throw parts at the car if I can help it. Is there a way to either definitively rule it out or narrow it down to that?

wait til you see an outright leak, soaked carpets, or do a pressure test.

I did lift the front end when bleeding the car. Not an obnoxious amount but a good couple of inches.

that's not enough to get air trapped in the heater core out. the front tires need to be a foot or more off the ground higher than the rear tires when bleeding. you want your expansion tank to be the highest point of the car. bmw's can take forever to properly bleed.

The reason I am kind of skeptical of it being a sudden heater core failure is that everything was working completely fine before I went in and replaced the thermostat. And also seeing how much of a job replacing the heater core is makes me want to really make sure whether it really is that or not ��.

it's quite common on older cars. you replace or do maintenance on one item and the next one in line fails.

I have had someone lift the revs a bit and hold it, however when they let off again the coolant comes back up in the tank, which leads me to think that it is just a air pocket getting compressed and then decompressed. And then moving back to the reservoir once the car gets turned off. Not sure if that's what's actually happening but just a loose theory.

it'll be trapped in the heater core.

[B328iC]

Had the same issue. On the drivers side there is a connector that the heater hoses run through. Find the return line and loosen the clamp and gently slide it like you are going to remove it. DON"T remove it! Doing this will allow coolant to flow removing the air that has been blocking from flowing. Once coolant is starting to leak from the hose tighten back up the clamp and you should be good.

[82eye]

Had the same issue. On the drivers side there is a connector that the heater hoses run through. Find the return line and loosen the clamp and gently slide it like you are going to remove it. DON"T remove it! Doing this will allow coolant to flow removing the air that has been blocking from flowing. Once coolant is starting to leak from the hose tighten back up the clamp and you should be good.

nice fix/hack. i'm gonna keep that one for future use.
also the coolant bleed funnel kits are a great thing for most cars.