Blown Coolant Reservoir
 

Ever since I've had my 545i, I've had to top off coolant. Not sure where it goes. System has been pressure tested numerous times. Anyway, recently, I've been getting warnings for engine overheated and to stop the vehicle. Even when it is raining outside and there's no way.

Yesterday, on my way home from work, I hear a pop. Steam starts flooding out of the hood and through the air vents. The coolant reservoir blew a hole the size of a baseball. I talked to the guy who towed it and he said it's probably the thermostat. What are ya'll's thoughts on this? I ordered the parts to replace the reservoir and thermostat. Is there anything else I need to look for?

Could be Blown head gasket pressurizing the coolant system.

And... What does rain have to do with preventing the engine from over heating?

Could be the tank was old and needed replaced.

Does your cooling fan work?

But on the topic of a blown head gasket overpressurizing the cooling system: the rad cap should keep it from grenading, no?

NoQuarter is right about the tank being old. Our E60s use a pressurized cooling system. The coolant reservoir, being made out of biodegradable plastic, will eventually breakdown just like all the natural rubber used on the car. When the reservoir becomes this weak, you can just guess what the pressurization will do to it. The thin coolant line that runs along the top of the radiator is another weak link.

Upon routine engine bay inspection on my 550i, I noticed dried light blue coolant on the side of my coolant reservoir. When I took a closer look, the reservoir formed a tiny crack right along a bulge on the side, and coolant was slowly leaking out. I promptly replaced the tank.

I replaced the tank and coolant sensor in it on Saturday. I ordered a new thermostat and hopefully it will be here today. I don't think I have a blown head gasket. I'm not leaking any oil. I have noticed, however, that I had been losing coolant and I have no idea where. When I'd had it in for work, it had been pressure tested multiple times and they said it was fine. Do you think that replacing the reservoir and thermostat could resolve the issue?

I had also tried replacing the reservoir cap and that did nothing. My radiator fan will run now and it gets loud. There's nothing wrong with it but it will run when I would think it shouldn't but I imagine that's because I'm low on coolant.

I replaced the reservoir and thermostat. I am still getting overheating messages. The code I was getting was P0128. I really don't think it is overheating but I can't be totally sure. Any ideas on what else I could check?
2005 545i E60
N62 engine
automatic transmission
176,000 miles.
https://cimg7.ibsrv.net/gimg/5series...e112516939.jpg

Have you checked your temp sensor. You can be loosing coolant and or the sensor is faulty. It sits on the lower radiator hose.

Popped the hood and sure enough, with the AC running, the cooling fan is not. It tries to but nothing. So there's that. Great

Ordered me a Carly. Should be here on Thursday. In the meantime, here's a shot from the mechanic. It took 20 minutes to let all the steam out so it didn't explode when we pulled the cap. She was angry for sure.

https://cimg2.ibsrv.net/gimg/5series...581221ccce.jpg