Low coolant warning

[mrgraybeard]

Well, some of the instruments are going to pot as my 540i approaches 300K miles. Now I'm getting an intermittent "low coolant" warning even though the reservoir is full and the sensor works as it should. I'm thinking it's probably a cracked solder joint in the check control module. I might try reflowing some of the connections in the module if I get annoyed enough. Has anybody tried that?

[shogun]

maybe resoldering the CCM helps, here are many pages of instructions with pics, German language, but the pics help

http://www.e34-welt.de/tips_tricks/Reparaturanleitung%20CCM%20e34%20e32.pdf

[R Shaffner]

There might be another reason for that kind of warning, one that almost killed my M62TU.

This past summer I kept getting intermittent low coolant warnings. I'd check the coolant level when the engine was cool again, and the level was fine. (I'm happy to let it go a little below full anyway, but I made sure the red indicator was floating and just a little low each time.) That went on for a week or so of in-town driving. I finally dismissed the warnings, thinking they were from a bad sensor or loose connector/wiring.

Then I took it on a road trip. It did fine on the highway for several hours, but when I got off the highway in stop-and -go traffic, I got a warning and saw the needle heading to the red area. I pulled over immediately, stopped the engine, and opened the hood. When I did I saw steam shooting out of the bleed screw (not the expansion tank cap). Luckily the overheat episode was benign. After letting it cool and getting more coolant in it, the engine has been fine.

The next day, I refilled and bled the system, and I let it get fully warm and the cooling system pressurized in summer heat. Then I noticed that I had a slow leak out of the bleed screw.

Here's what I think happened:
Over the previous couple of weeks, some coolant kept leaking out of the bleed screw a little at a time. When the coolant got low enough, it would give me a low coolant warning. While the engine was stopped and cooling down overnight, some air would enter the system through the poor bleed screw fitting. When that happened air moved into the top of the radiator, which let coolant partially refill the expansion tank.

(One can see the same effect by opening the bleed screw on a properly filled and bled system. When air enters the top of the radiator the coolant level in the expansion tank will rise. If one then puts the bleed screw back in and drives the car, it will self-bleed enough to push air out of the top of the radiator and lower the coolant level back to the previous level.)

In summary, I was slowly losing coolant while driving, and air was also making it back into the top of the radiator when the car cooled overnight. Eventually, I ended up with a system that looked OK, but was under-filled with a lot of air in the radiator (like someone had filled the tank and not bled the radiator). When I was on the highway, I think high rpm, high coolant flow, and high airflow kept my engine cool enough. But as soon as I got in hot stop-and-go traffic, all that changed and the engine started to overheat. Later I replaced the expansion tank and all was well. No more low coolant warnings. No leaking out the bleed screw.

So...your low coolant warnings might be a sign that you actually have low coolant, and it's being disguised by air that is making it into the radiator.

One way to check: When the engine is cold, do the test I described above. Undo the cap to the expansion tank and check the level. Then loosen the bleed screw and see if the coolant level in the tank changes. If it does, then you might not have the problem I had, and it might be the sensor or wiring. But it the level does not rise (or does not rise much, then you already have air in the radiator, and you have a problem like I had.

[mrgraybeard]

Thanks Erich! I'll look it over.

[mrgraybeard]

Thanks for the input. The M60 in the E34 doesn't have a bleed screw. It does sometimes harbor air bubbles in the cooling system, but a drive at highway speed has always worked them out quickly in my experience.

[R Shaffner]

Wow. I'm amazed by that. Have to wonder why BMW didn't stick with that approach. (Along with the timing chain sprocket in the middle of the V.) Sorry for the long digression.

[shogun]

M70 in the E32 also does not have a bleed screw. First production year had it and then they changed it. Self bleeding. Instead of the bleed screw they made a connector and attached a hose # 7 which goes to the rad top left side and thru a y-piece back to the expansion tank.
older version: 11531710495 vent screw #11 up to to Sep '89 http://bmwfans.info/parts-catalog/E32/Europe/750i-M70/L-A/jul1987/browse/engine/waterpump_thermostat/#1710495_11