Coolant level question

[lbrott]

Yesterday I replaced the hoses and coolant on my 1994 325i and I've been checking the coolant level after each trip to make sure things were well bled.

However, I've noticed that the level in the expansion tank is actually lower when the engine is hot and slowly rises as the engine cools. Is this normal? Seems counter-intuitive to me.

Thanks for enlightening me.

[blkongry]

That would seem pretty counter intuitive to me, too. I mean, even if you did have air left in the system, and that was slowly making its way out when the engine was hot (which could explain why coolant isn't coming into the expansion reservoir when the engine is warm/hot), it would still get drawn back in when the engine cools. Having the reservoir fill as the engine cools seems very strange. Now, when it has completely cooled down, and you open the radiator cap, is your coolant level still completely topped off?

[lbrott]

The car runs perfectly...that is, the coolant temperature needle is rock solid straight up and I don't get the dreaded "check coolant level" error message. However, I'll drive around and when I get home, I'll check the level and see that it's 2 inches below the "cold" line. I'll think "yikes, I need more coolant" but I'll wait to let the engine cool before removing the radiator cap. When things have cooled off, the level will be back up to the "cold" line.

Perhaps my car has always done this and I'm only noticing it now because I changed the fluid. I thought it may have to do with the cooling system being under pressure....

[lbrott]

After a week of no problems, I bled the system again. This seems to have solved my varying coolant level problem. Apparently the extra air in the system would compress under pressure, giving the impression that the coolant level was dropping.

[TexasBMW]

i've noticed that when engine is warm, if you release the cap pressure the coolant level will rise.

The most likely reason is the radiator hoses are swollen by the coolant pressure 10-15 psi. When pressure is released the hoses relax and squeeze the added "swell" volume back into the reservoir. It could be something else, but this seems most likely.

This can fool you into thinking you have a leak.

Regards

[lbrott]

This whole project started because I was getting the dreaded "check coolant level" error message. After reading everything on the forums, I thought I should change the sensor. But on the M50 engine, it's a pain. I decided to remove the hoses to get better access to the sensor, but I was also afraid of damaging the radiator neck. So I cut the hose off and replaced with new hoses.

Before working on the sensor, the hot and cold coolant levels were different. After removing the old sensor, the sensor still looked fine (two years old and not gunked up). After thoroughly bleeding the system, the hot and cold coolant levels are now the same.

However, now I'm questioning whether the sensor was really the problem. I think all I needed to do was change the hoses and completely bleed the system. Because of the swelling of the hoses and the air in the system, the apparent level was dropping and the old coolant level sensor was correctly sending an error.

At least the sensor wasn't that expensive.