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  #1  
Old 12-23-2021, 02:37 PM
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Cooling system problem?

2001 M54, 203k miles, head gasket job done at 186k miles, much of cooling system is already pretty new.

Recent sudden major problem(s):
- car otherwise runs fine
- coolant tank cap venting coolant after getting warmed up. This happened also when I had the HG problem a few years ago, fixed. Have done "block test" for combustion gases, which is negative, but I'm cautiously considering that inconclusive due to other issues that may mean the test is not valid.

In starting to diagnose, I took off the covers and air intake stuff for a better view. Did the block test, scratching my head so far, waited overnight for a better look, to re-do, think some more, and here is the real head scratcher I'm looking for help with:

Engine cold, everything off
Coolant tank cap (Rein, I think, 2 bar, a few years old) is off
Expansion tank (Rein, I think, barely a year and 6k miles old, fully intact, not leaking)
Upper Radiator hose (genuine BMW, 17k miles old) appears slightly collapsed as if it is under vacuum [spoiler alert - it still looks that way even after being removed, so whatever shape change happened got baked into it]
Lower Radiator hose (genuine BMW, 45k miles old) appears fine, not collapsed
So I'm figuring there is no pressure anywhere in the system due to the cap being off, puzzled by seeing and feeling the upper hose as if it is under vacuum, so I started to loosen the bleed screw [which is the highest point in the system] on the upper radiator hose, expecting to release the vacuum so air rushes in and the hose returns to its circular cross-section. But instead, the coolant is under pressure, forcefully coming out of the bleed screw hole, so I stopped removing it, wondering WTH is going on.

I have the 3.0i with automatic transmission, so here is the coolant flow diagram.

Name:  m54cooling.jpg
Views: 349
Size:  82.2 KB

At this point, I'm just trying to understand how pressure can exist overnight, even with the expansion tank cap removed. Could a problem with the ATF thermostat at the bottom of the tank cause this condition?

I later took off a bunch of stuff, including the expansion tank, and the ATF thermostat is broken. So I'm definitely planning to replace that, but am hoping there are not more issues. Coolant thermostat (Genuine BMW, 45k miles old) and water pump (Saleri, 17k miles old) both removed and looking fine.

Separately from this (I think), I had got a Foxwell DME code indicating an off-spec cooling thermostat heater. I will replace that as well while I'm at it.

Also, the only "event" that could have triggered a change was that prior to this, my daughter drove the car home from SoCal, about 400+ miles, maybe faster than she should have. A good solid drive like that may have raised coolant and ATF temps higher than normal. But no overheating ever occurred. My daughter is very careful to notice that, and a few years ago I reprogrammed the coolant temp gauge to be mostly linear.
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Last edited by oldskewel; 12-23-2021 at 02:48 PM.
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  #2  
Old 12-23-2021, 04:04 PM
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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.
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Last edited by 80stech; 12-23-2021 at 04:18 PM. Reason: add
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  #3  
Old 12-23-2021, 04:24 PM
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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.
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Old 12-24-2021, 07:38 AM
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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
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Old 12-24-2021, 11:20 AM
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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/
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  #6  
Old 12-24-2021, 02:40 PM
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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.

Quote:
Originally Posted by 80stech View Post
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.

Quote:
Originally Posted by nick325xit 5spd View Post
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.

Quote:
Originally Posted by guntherrex View Post
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 -

Quote:
Originally Posted by X5chemist View Post
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.
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Old 12-24-2021, 03:27 PM
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Are you sure the aux coolant pump wasn't running when you opened the bleed screw?
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Old 12-24-2021, 04:02 PM
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Sorry to hear...Any signs of coolant in any of the cylinders ?

stumble on cold start-up
puff of smoke on cold start up

If either, I suggest you swab each cylinder one morning after a full-temp run the day before.

This happened to me after doing a head gasket on my E53...5k miles and a year later, I found coolant in a cylinder, leading to head gasket #2..
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Old 12-24-2021, 05:52 PM
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I think more likely just head pressure vs. air pressure. I'm not aware of any check valve that could actually cause pressure.
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Old 12-24-2021, 07:57 PM
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Quote:
Originally Posted by 80stech View Post
Are you sure the aux coolant pump wasn't running when you opened the bleed screw?
I'm pretty sure it was not, but you know, that might be more likely than anything else at this point, which is very helpful.

Interesting to see the mentions about HG repairs succeeding but not for long, which might be my situation. In doing my repair, I TimeSerted all the block threads, but it sure seemed that Aluminum cut like butter - seemed very soft and weak, even though it held the torque when installing the head.

I know the ATF thermostat at the bottom of the expansion tank came apart in pieces, and that is a definite problem, getting replaced along with the coolant thermostat (which I don't think is an actual problem, other than the DME code).

When the new parts come in, I plan to test everything again, really hoping to not have to do another HG. If so, and it was a TimeSert or two that failed, maybe I'll need to BigSert them. Not looking forward to any of this.
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