Cooling system problem?

[oldskewel]

Thanks all for the help so far.

I know something is wrong somewhere. The main thing I'm looking for right now is if anyone has a theory of how significant pressure could be maintained in the radiator hose (behind the bleed screw), overnight while cooling down, with the expansion tank cap removed. Seems that at least one valve must be closed tight. Wondering if it could be the known-broken ATF thermostat, which I will be replacing. If I don't get any ideas, I'll end up replacing that thermostat, also replacing the coolant thermostat (both already ordered), hoping that fixes something and get back to debugging. I do realize I could end up going through all this, perhaps to the point where the block test is believable, and get a positive result.

Originally Posted by 80stech

The diagram doesn't show it clearly but I think the expansion tank is sectioned off so that there is always a bit of backpressure but that should only amount to the head pressure from gravity on the height of the sectioned off portion which should not be that much. I'm guessing the hose is throwing you off because the vacuum deformity is baked in like you say.

I'd bet if you let the engine cool with the expansion cap on you'll be back to having vacuum at the bleed screw.

The blowing coolant out of the cap is more likely to be a head gasket/crack issue since you have already done a head gasket. It sucks that the block test doesn't seem to be as good an indicator as it used to be back in the day.

Yes, I'm now discounting the deformed hose as a clue.

About the head pressure, I really think that would be near zero since the bleed screw is at the top (4* sloping driveway helps even more). BTW, heater set on full hot for all this). So I think there must have been some pressure in there originally, and somehow it was retained.

I do fear you may be right about another head issue. When I fixed it back in 2018, it was actually just a head warp, due to probable overheating, perhaps largely pre-existing from the PO. But I really want to rule everything else out before tearing into things again.

Originally Posted by nick325xit 5spd

Before you start taking the engine apart, I'd try putting a vacuum on the cooling system and filling it that way. (See Air-Lift: https://www.amazon.com/UView-550000-.../dp/B0002SRH5G )

Some cars are just really hard to bleed out and the Air-Lift has solved intractable coolant issues for me in the past.

Good tip - that is cool tech there. I might want to get one some day. But I've never had a problem bleeding this or other cars, and I'm pretty certain I've got at least one other problem to solve prior to worrying about bleeding air.

Originally Posted by guntherrex

did you check coolant temperatures? if your thermostat isn't working properly that could be a cause too

edit: do not trust the gauge in the dash because it's not actually showing a temperature

Coolant temps are good, make sense. A few years ago I reprogrammed the dash gauge so it does actually read proper temps.

https://xoutpost.com/1163393-post54.html
I moved the breakpoints
from
15 - 50 - 75 - 115 - 120 - 125 -
to
15 - 60 - 94 - 96 - 110 - 115 -

Originally Posted by X5chemist

BWM used a 2.0 bar coolant cap for years. All newer models use a 1.5 bar cap. I'm replacing mine to a 1.5 bar as soon as they are available again. Or buy a kit that as one in it. https://germanautosolutions.com/stor...r-coolant-cap/

I'm pretty sure that unless other changes to the thermostat, etc. are made, reducing the setpoint of the pressure release valve will just make the cap vent coolant earlier. So it will perhaps prevent a plastic explosion (yes, helpful ), but if there is some problem like I have that is creating high pressure, it will just vent even earlier than it already is.