yet another m52 coolant bleeding problem

[diesekte]

I searched and read a ton of posts, but nothing exactly like my situation, so here goes.
My temp gauge used to always be all the way in the cold/left/blue area. The one time I did a 45min highway drive, it finally popped into the middle of the gauge after about 30 minutes.
I recently sent my whole cluster off to be worked on and had my SI board replaced, and all gauges were working when they left the shop. I just swapped my dash and when the cluster was out made sure the grounding nut was clean and tightened down.

I flushed the cooling system and refilled through the expansion tank. Drove it a few miles and the gauge would kind of jump but always stayed left/blue/cold.
I tried to bleed the system via the jacking up the front, leaving the cap off, massing hoses, bringing RPM up to 2k, etc. I tried this on two different occasions and only got a small bit of air the second time when RPM was at 2k (only bringing it up for about 5 seconds at a time, about 15 seconds apart, for 3-4x).
After I got some air out the gauge moved about 1/4th of the way.
The coolant in the expansion tank is warm/hot (I can touch it and its hot but doesn't burn me).
The top radiator hose is hot and I can feel it getting pressurized if I leave the cap on.
The radiator is hot.
The thermostat housing is hot.
The bottom radiator hose is cool and I can feel some air in it, also it doesn't seem to get pressurized. I keep massing it and the air doesn't ever seem to decrease. Its stuck right around the bend right before the hose connects into the radiator.
The block it hot but I haven't run it long enough to burn me if I quickly/lightly touch it.
The air from the heater is hot.
If I open the bleed screw at the bottom of the radiator, no air comes out, but the coolant that does is cool.

Do I need to just keep trying to bleed the system over and over? I can feel air in the bottom hose. Am I just not letting the engine get hot enough to open the thermostat? Is my thermostat busted?
I don't have a bleed screw in my thermostat housing.

[NickL]

Your problem is the car isn't warming up? If there was air in the system the car would overheat. Sounds like your thermostat is stuck open.

[jink0619]

What coolant are u using? If not BMW coolant then theres your problem.

[diesekte]

The reservoir, top hose, radiator, thermostat housing, air from heater, and block are all hot.
Bottom radiator hose and coolant from radiator drain plug are cool. Bottom radiator hose has air in it that I can't get out no matter what I try. Its right at the "bend" right before the hose goes into the radiator. I can massage it up to the thermostat housing but it never gets any smaller.

I am using BMW coolant.

[DesertBMW]

Cylinder head probably developed a crack and combustion is leaking into cooling system, just a little to create some air bubbles in the system. This is common on these engines.

[justin_l_]

What coolant are u using? If not BMW coolant then theres your problem.

My gage must be wrong on all my builds cause the temp was always fine...

[diesekte]

Cylinder head probably developed a crack and combustion is leaking into cooling system, just a little to create some air bubbles in the system. This is common on these engines.

Unfortunately I didn't feel the bottom radiator hose before I bled the cooling system to see if it ever got pressurized or if there was any air in it before. However at this point my coolant and oil both look like they should. If there was a crack, even a thin one, that had been there for a while, shouldn't I expect to see some mixing of the fluids?

If the thermostat is stuck open or closed, should the "take the top hose off and fill it until overflow" thing work or no? Can someone explain to me how doing that doesn't introduce more air into the system? If an air pocket is trapped somewhere and is significant enough to block coolant from flowing via the water pump, it seems like a not very tall column of fluid in the disconnected hose wouldn't have enough force to push the bubble anywhere.

I never really let the car idle all that long (maybe 10 mins tops) both times I've tried since I get nervous without having the gauge working. Is it possible I just haven't let it heat up enough?

It seems most people with a similar problem don't have heat from the vents, but my heater works just fine.

[NickL]

So your gauge doesn't work? A clear indicator of the system not being bled correctly and having air in it is you wouldn't get heat from the vents.

When I bleed I always backfill the system through the upper hose. Then massage with the cap off until operating temp and it's always fine.

[diesekte]

Thats what I dont get. I can feel air in the bottom hose, but I get heat from the vents just fine. The guage should be working. I sent my whole cluster to Bavarian restoration and saw pics of it all working properly...I just haven't gotten much movement out of it. If there's any temp reading problem I'd be more likely to think it was something besides the guage, but the cars wiring seems pretty solid everywhere else and the temp sensors dont tend to fail often if I remember correctly

[AndrewBird]

If the bottom hose is cold, the thermostat is either not opening or the radiator is plugged.

[diesekte]

I think I wasn't letting it get hot enough. I let it idle for about 20 minutes with cap off and did house massage and revving to 2k about 15 times. The bottom hose is now a bit firmer and is heating up. I tapped the cluster a few times and the temp gauge is working and seems accurate. It was just taking a while to heat up to proper temp at idle I guess

[e30austin]

Always wanna fill your cooling system through the upper radiator hose. Makes things a lot easier.