Cooling System Bleeding Question

[rslifkin]

I replaced my radiator and expansion tank a few weeks ago (01 740i M62TU) and had no issues filling and bleeding the cooling system. It seemed like I got all of the air out, no signs of an issue anywhere, no overheating, etc. This morning I checked the fluids and noticed the coolant level in the expansion tank hasn't changed at all (still a little overfilled, coolant just below the baffle). But the upper radiator hose seemed to be empty when I gave it a squeeze. I listened closely when I started the car and I did hear a tiny bit of air go through the heater core. I've got no signs of a leak anywhere (can't smell or see any coolant leaking).

Car still has good heat and isn't overheating or anything, so I'm not sure what's going on. Haven't gotten any low coolant warnings either. When I squeezed the upper rad hose after driving, there's definitely what seems like a normal amount of pressure in the system and at least some fluid in the hose. The replacement tank, cap and radiator are all Behr parts. Do I just need to re-bleed the system and see if I can get more air out of it or is something probably wrong? I'm almost wondering if it's purging the air into the tank when running (like it should) but managing to suck air back through the vent hose as it cools instead of keeping the upper hose full.

[georgebest]

I replaced my radiator and expansion tank a few weeks ago (01 740i M62TU) and had no issues filling and bleeding the cooling system. It seemed like I got all of the air out, no signs of an issue anywhere, no overheating, etc. This morning I checked the fluids and noticed the coolant level in the expansion tank hasn't changed at all (still a little overfilled, coolant just below the baffle). But the upper radiator hose seemed to be empty when I gave it a squeeze. I listened closely when I started the car and I did hear a tiny bit of air go through the heater core. I've got no signs of a leak anywhere (can't smell or see any coolant leaking).

Car still has good heat and isn't overheating or anything, so I'm not sure what's going on. Haven't gotten any low coolant warnings either. When I squeezed the upper rad hose after driving, there's definitely what seems like a normal amount of pressure in the system and at least some fluid in the hose. The replacement tank, cap and radiator are all Behr parts. Do I just need to re-bleed the system and see if I can get more air out of it or is something probably wrong? I'm almost wondering if it's purging the air into the tank when running (like it should) but managing to suck air back through the vent hose as it cools instead of keeping the upper hose full.

There should be water in that upper hose when you squeeze it. You need to bleed it properly, Timm has the best guide for it:
https://www.meeknet.co.uk/E38/E38_Bl...ing_System.htm

[rslifkin]

There should be water in that upper hose when you squeeze it. You need to bleed it properly, Timm has the best guide for it:
https://www.meeknet.co.uk/E38/E38_Bl...ing_System.htm

That's the guide I followed when I initially bled it. I'll bleed it again and see what happens, I'm guessing I had an air bubble trapped somewhere that burped itself out later but was too big for it to self-bleed the upper hose fully and draw down the fluid level in the expansion tank. Fortunately, the air didn't seem to get stuck anywhere that caused issues, considering the car has had good heat ever since the original bleed and it hasn't had any overheating issues even after 45 minutes of driving.

[red740is]

After years of monkeying around and waiting to bleed air out of the cooling systems of my cars, I finally bought a vacuum fill kit and am sorry I didn't do it sooner. Fast, no air and ready to go with out worry a proper bleeding or overheating somewhere. Well worth it if you will be doing this again a couple times or on other vehicles.

It was ~$60 .. google schwaben coolant vacuum fill tool

[georgebest]

I think most of the people having issues with bleeding have a faulty auxiliary pump. That is necessary for proper bleeding cause otherwise nothing is really moving the water when you are filling it up (before you start the engine).
I have never had an issue and never used any tool, but also have a working pump.

[rslifkin]

I left the hood open when I got home from work today so the car would cool faster. Once it had cooled enough to have no pressure in the system, I re-bled it. I'm not 100% sure my aux pump works, but it sounds like it runs and moves coolant. Coolant level did drop when starting the engine with the cap off, so I re-capped it with it running, cracked the bleeder screw until I had steady coolant and called it good.

There's definitely fluid in the upper rad hose at the moment, so I'll have to keep an eye on it to see if it stays that way or if the upper rad hose starts draining after shutdown like it seemed to be doing before.

[Timm]

I think most of the people having issues with bleeding have a faulty auxiliary pump. That is necessary for proper bleeding cause otherwise nothing is really moving the water when you are filling it up (before you start the engine).
I have never had an issue and never used any tool, but also have a working pump.

I completely agree - the engine is self-bleeding and will bleed without the nose up - but BMW decided to fit a cabin heating system where the heater valves and aux pump are above the water line! Any air in the system ends up slowly emptying the heater matrix - and as a standard E38 runs at 108C there is too much air space to create coolant pressure - and it boils. To compound this, a slightly gunked-up heater matrix or dodgy heater valves makes it really difficult to bleed the air out - which is why my routine says to rev the engine to 3000RPM - 4000RPM as that gets the coolant past the aux pump and through the matrix.

Although I also agree that using a vacuum system helps it really is the heating system that should be sorted first....

[georgebest]

I completely agree - the engine is self-bleeding and will bleed without the nose up - but BMW decided to fit a cabin heating system where the heater valves and aux pump are above the water line! Any air in the system ends up slowly emptying the heater matrix - and as a standard E38 runs at 108C there is too much air space to create coolant pressure - and it boils. To compound this, a slightly gunked-up heater matrix or dodgy heater valves makes it really difficult to bleed the air out - which is why my routine says to rev the engine to 3000RPM - 4000RPM as that gets the coolant past the aux pump and through the matrix.

Although I also agree that using a vacuum system helps it really is the heating system that should be sorted first....

That would explain why I have no such issues, replaced both the pump and the valves, now going over $600 but I've paid "only" around $400 a year ago.

[BillConn]

After years of monkeying around and waiting to bleed air out of the cooling systems of my cars, I finally bought a vacuum fill kit and am sorry I didn't do it sooner. Fast, no air and ready to go with out worry a proper bleeding or overheating somewhere. Well worth it if you will be doing this again a couple times or on other vehicles.

It was ~$60 .. google schwaben coolant vacuum fill tool

+1 on a Coolant Vacuum Fill tool. While not necessary on the E38, it makes the job painless and quick. However I have some newer vehicles (ie. 2011 Range Rover) where it is essentially the only way to get all the air out of the system due to intercoolers, multiple zone climate controls, multiple aux coolant pumps, etc.

[georgebest]

+1 on a Coolant Vacuum Fill tool. While not necessary on the E38, it makes the job painless and quick. However I have some newer vehicles (ie. 2011 Range Rover) where it is essentially the only way to get all the air out of the system due to intercoolers, multiple zone climate controls, multiple aux coolant pumps, etc.

Cheaper than buying a new aux pump & valve combo and you dont really need the aux pump in normal usage. Heater valve is kind of important tho.