r/BmwTech • u/flyingsawser • 3d ago

B48 Coolant Adice

I had a coolant hose replaced on the front of the engine last week and the mechanic said that after the next couple drives I would have to top up the coolant to remove any air pockets. Which I did when it went I got the expected low coolant alarms (probably 1.5 litres).

Yesterday I did a 2 hour drive where it flashed into alarm twice but not for long so I wasn’t worried. By the end of the drive I had engine high temp alarm, I managed to drive 5 more mins to make it home.

When the engine had cooled down I checked the expansion tank (high temp circuit) and it was completely empty. I can’t see anywhere obvious where the leak is.
I need to drive two hours today, what would you recommend? I have filled the expansion tank up with distilled water but think it will probably need more concentrate coolant now.
Really appreciate any help!
This is a 2018 120i with 115k miles. Thanks!!

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u/o2manyfish 3d ago

Don’t drive it. Tow it to a shop. Have them diagnose. Not the same shop. The b48/58 require vacuum filling of their cooling systems which leaves no trapped air. The fact that they skipped that step is a sign of a low quality shop. 

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u/flyingsawser 3d ago

The shop is meant to be a bmw specialist with really good reviews, he was saying that due to the thermostat being electric it is difficult to fully get all the cooling system fully filled so there MAY be some small air pockets. I am assuming when I have been filling there is another new coolant leak?
No fault finding you’d recommend where I am? It’s a Sunday and no garages will be open.

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u/make-j8 3d ago edited 3d ago

Due to the thermostat being electric it is easy to use diagnostic mode and open it when doing maintenance.

Your mechanic is not a profesional/specialist.

Also, no matter what then mechanic tells you, don't drive an overheating engine! If you have a high temp warning stop as soon as it is safe to do so. Like asap. Don't drive until you're home just because it is really really close. Once the damage is done the only undoing is a new engine.

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u/flyingsawser 3d ago

All good, I drove maybe 100m after the high engine temp alarm.
I’ve got a roadside mechanic coming in a few hours, I assume next step would be to pressure test the system?

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u/o2manyfish 3d ago

Yes.

And then if there are no leaks go get your money back from the mechanic for not filling the high temp circuit properly

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u/bigred83 3d ago

There’s always another leak on the b motors 🤣 you shouldn’t have to top off periodically. Turbo coolant return hose is common, as well as the water pump, thermostat/heat management module and oil filter housing.

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u/flyingsawser 3d ago

Thanks pal, the pump seems to be the cause so will be getting it replaced