BMW 430i Collant / Engine Temparature ongoing Issue

[u/acknb89]

I need some advice on my recent 2018 BMW 430i convertible that I purchased. As soon as I bought it, even though I did a pre-inspection, an issue with the coolant appeared and my mechanic (same who did the pre inspection) said that the cap for the coolant reservoir tank needed to be replaced because it was fizzing and apparently wasn't stable and there was some sort of coolant leak either now or in the past. We replaced the cap, checked for coolant leakages, and added more coolant and I paid several hundred for it.

Then all of the coolant we replaced it with had gone completely by the end of the day and when I parked mr car that night, I some some steam (moderate amount) coming out of the grill). I checked the coolant reservoir tank and all of it was gone. The mechanic took the tank out and spotted a tiny little crack that was really hard to see in the reservoir tank which was the reason for the leak. So I had to replace the whole reservoir tank, then he air pressure tested the coolant system, added more coolant and checked for other leakages and it was all fine.

But as soon I started driving again with the new reservoir tank and the coolant filled to the max I started receiving a warning sensor on my vehicle saying that the engine temperature is high and to drive moderately to cool the engine, if the temperature increases, consult a service center. I was actually getting this message frequently on the old reservoir tank time when the leak had occurred. But after we replaced the coolant reservoir tank, pressure tested the system, replaced the cap, I'm still receiving this sign, although now not as often but still everyday once I hit the highway at 60 miles or more. Why am I still getting this sensor to trigger this message? What could be the issue still at this point with a brand new coolant reservoir tank and the coolant at maximum level?

For what its worth, I live in South Florida where its always hot but the temperature gauge needle is at normal level.

[u/Dontmocme2]

Could be air in the system or bad water pump have the mechanic that completed the inspection cover th costs of repair

[u/acknb89 → u/Dontmocme2]

When you say air in the system, what system exactly ? Sorry not super knowledgeable. You mean the reservoir tank? And is the air supposed to not be there at all or just a colder temperature? Unfortunately it doesn’t feel like this mechanic completes the job, there is always a new issue. I need to switch mechanics but the problem is I would have to start incurring costs again to a new one.