In my experience, most BMW overheating is simply caused by pressure leaks. BMW nearly always uses a relatively high-pressure cooling system. This has several significant advantages: lighter weight, smaller surface area, cheaper replacement costs and higher efficiency.
Since there is no free lunch, these compromises also result in many significant disadvantages: short service lifespan, low tolerance for fatigue, multiple points of failure, poor survivability. BMW’s choice of cheap materials exacerbates the problem: aluminum/plastic radiators with silicone seals, vinyl profile gaskets, square-section sealing rings, and poorly-designed plumbing leads to tremendous problems with their used cars. Mostly all the DOHC cars from 1990-onward suffer from these problems to a greater or lesser degree. It’s a severe design problem that BMW has ignored, shabbily throwing the blame and cost onto owners for decades. They’ve never admitted fault, nor been content to solve these designed-in problems. Despite all that, I don’t mind it. I’ll readily admit that I take advantage of BMW’s massive used car depreciation. If you understand the cause, it’s not a problem. Everyone’s fear of expensive BMW repairs is an essential part of my method to own cheap second-hand luxury cars. That comes with some caveats: I would STRONGLY advise you to avoid the V8s, and stay well ahead of I6 pressure leaks!
It’s a good idea to refurbish the system every 75K miles or so. If this seems ridiculous, just think of the advantages: light weight, cheap replacement costs, high efficiency. There are also some other less savory advantages…the evil reputation that BMW has gained for overheating truly helps to keep values low. Take advantage of this situation; BMWs are very cheap to run if you’re willing to do the work.
So spend the money on a pressure tester. Test and verify before proceeding.
Once you’re sure the system holds pressure, you can proceed. I’ll also let you in on a little secret…Dave’s fool-proof coolant mix! It’s easy. Get yourself a large bucket, one gallon of Zerex G05 European-type anti-freeze/coolant and a bottle of surfactant like RedLine WaterWetter. You’ll also need two gallons of distilled water. Mix them all together in the bucket, adding enough surfactant for three gallons of anti-freeze mix (I use the entire bottle). The end result is a more dilute 33% mixture of anti-freeze that is both cheaper and more effective. Higher concentrations of water actually improve heat transfer, and the lower viscosity makes bleeding a snap. YMMV…I’d recommend a higher-strength mixture if your climate is harsher than Pittsburgh’s relatively mild 40 degrees north latitude.
Another part of Dave’s fool-proof coolant mix is using a bit of Sil-Glyde silicone lubricant (like this NAPA branded version) for all these plastic & vinyl connections. You really don’t need much. It really helps to install connections, spreads hose clamping forces well, and seems to resist aluminum electrolytic corrosion.