[GrabEmByTheBimmer]

Took my 2015 335i (n55/f30) to an Indy BMW shop to get battery replaced. Tech pulls me back and shows me my coolant tank under the hood is steaming (he removed cap). Said it was boiling when I first pulled in. I later confirmed, after some light driving, that yes when I open the cap, it starts to boil violently.

Tech told me it was probably mostly water, someone (bmw dealership is only other tech whose touched this) must have been topping it off w water. He suggested a coolant flush and refill.

Is this a big deal that needs to be addressed asap? I’ve probably been driving it like this for 1.5 years. I’ve never gotten a low coolant or high engine temp warning - so is this nbd or is this a pretty urgent problem?

[Charles B]

Yes you need to address this ASAP. The whole process is not a big deal and it's not a ton of money. I would have the indy shop do it if you're not comfortable doing it yourself.

I wouldn't drive it until this is taken care of.

[Billfitz]

Originally Posted by GrabEmByTheBimmer

I later confirmed, after some light driving, that yes when I open the cap, it starts to boil violently.

Never open the cap when it's hot. The reason it appeared to boil is that it's under pressure, which raises the boiling point. When you took the cap off you removed the pressure. A mechanic should know better. I wouldn't be surprised to find out he's a shyster who pulled the cap on purpose to make it look like something was wrong to drum up some more business. Google how to open the hidden menu so you can see the coolant temperature. It should run between 195 and 220 degrees.

[GrabEmByTheBimmer]

Originally Posted by Billfitz

Never open the cap when it's hot. The reason it appeared to boil is that it's under pressure, which raises the boiling point. When you took the cap off you removed the pressure. A mechanic should know better. I wouldn't be surprised to find out he's a shyster who pulled the cap on purpose to make it look like something was wrong to drum up some more business. Google how to open the hidden menu so you can see the coolant temperature. It should run between 195 and 220 degrees.

Thx. Is the boiling when hot and cap is removed a symptom of there being too much water? Or would a normal, healthy mix of coolant and distilled water (50/50?) also boil? Aka could the whole thing he said be bs, especially since I’ve never got an engine temp warning?

[Billfitz]

If there's too much coolant the car will purge the excess by itself. Water boils at 212 degrees when there's no pressure, but under the usual 15 PSI pressure in a radiator it goes up to 257. If it's a 50/50 mix it's 268. That's why it went nuts when the cap was pulled. The lack of an engine temp warning is also suspect. Open the hidden menu, watch the coolant temperature just in case.

[GrabEmByTheBimmer]

Originally Posted by Billfitz

If there's too much coolant the car will purge the excess by itself. Water boils at 212 degrees when there's no pressure, but under the usual 15 PSI pressure in a radiator it goes up to 257. If it's a 50/50 mix it's 268. That's why it went nuts when the cap was pulled. The lack of an engine temp warning is also suspect. Open the hidden menu, watch the coolant temperature just in case.

Ive got a P3 guage and also enabled the secret menu. With normal driving, on and off highway, pretty quickly got up to 220 F degrees. It hovered there, but after more time I saw it creep up to 225.

I’m guessing this is not ideal?

Now I’m wondering if a top up of coolant is all that’s needed, or if I really do need a flush as the shop suggested.

[Eschmacher]

Originally Posted by GrabEmByTheBimmer

Ive got a P3 guage and also enabled the secret menu. With normal driving, on and off highway, pretty quickly got up to 220 F degrees. It hovered there, but after more time I saw it creep up to 225.

I’m guessing this is not ideal?

Now I’m wondering if a top up of coolant is all that’s needed, or if I really do need a flush as the shop suggested.

No, that's exactly what's expected. 220-230F.

[Billfitz]

BMWs intentionally run hot, especially in highway driving, for maximum efficiency.

[F32Fleet]

Originally Posted by GrabEmByTheBimmer

Took my 2015 335i (n55/f30) to an Indy BMW shop to get battery replaced. Tech pulls me back and shows me my coolant tank under the hood is steaming (he removed cap). Said it was boiling when I first pulled in. I later confirmed, after some light driving, that yes when I open the cap, it starts to boil violently.

Tech told me it was probably mostly water, someone (bmw dealership is only other tech whose touched this) must have been topping it off w water. He suggested a coolant flush and refill.

Is this a big deal that needs to be addressed asap? I’ve probably been driving it like this for 1.5 years. I’ve never gotten a low coolant or high engine temp warning - so is this nbd or is this a pretty urgent problem?

Just buy some test strips to test the coolant.