BMW Z3 Overheating: Bypass Valve or Something Worse?

Hey there! I have some experience working on cars, and this reminds me of an issue I had with my 2003 BMW Z4. Initially, I also thought it was the bypass valve when I had similar symptoms. Turned out, in my case, the overheating and heating issues were actually due to a faulty thermostat that was incorrectly installed during a previous repair. The workshop fixed it without charging me since it was their mistake from an earlier job. They replaced the thermostat and properly bled the cooling system. Before jumping to conclusions though, could you share: Have you had any recent cooling system work done? Are you losing any coolant? What's the exact error code you're getting? This info would help narrow down if your issue is similar to what I experienced with my Z4.

Thanks for sharing your experience! I've had my Z3 since 66007 KM, and the last service was pretty recent. You know what's interesting, I did have some cooling system work done about 2 months ago. They replaced a few hoses, but the issue started showing up just last week. Haven't noticed any obvious coolant loss, but I'll definitely double-check. The error code's P0128, coolant thermostat related from what I've read online. Given what you mentioned about your Z4's thermostat issues, I'm thinking maybe I should get that checked first. Did your car also make any unusual noises before the overheating started? Mine's been a bit grumbly lately.

Hey again! Yeah, that P0128 code sounds very familiar, exactly what my Z4 was throwing before I got it fixed. When you mentioned the grumbly noise, it's like déjà vu because my car did the same thing! It was this weird sort of gurgling sound, especially when accelerating. Since you've had recent cooling system work done, I'd put money on it being similar to what happened with my Z4. The tricky thing with these BMWs is that the bleeding procedure for the cooling system has to be done just right. When my thermostat was incorrectly installed, air pockets got trapped in the system, which caused all sorts of chaos, overheating, no heat in the cabin, the whole nine yards. You know what? Given that your symptoms started after the hose replacement, I'd take it back to whoever did the work. They might have either not bled the system properly or, like in my case, messed up something with the thermostat during the repair. These symptoms are textbook signs of air in the system or a thermostat that's not doing its job. My repair bill ended up being covered under warranty since it was their mistake, so definitely worth checking if you're in a similar situation. Let me know what you find out, really curious if it's the same issue as my Z4!

Thanks so much for the detailed explanation! You've actually made me feel a lot better about this whole situation. The gurgling sound you mentioned is exactly what I'm hearing, can't believe I didn't mention that earlier! After reading your experience, I just called the shop that did the hose replacement, and they're willing to take a look at it tomorrow. You're probably right about the bleeding procedure, I did some research after reading your comments, and these BMWs are apparently pretty particular about how the cooling system needs to be bled. The fact that all these symptoms started after the hose replacement can't be a coincidence. I'll definitely push them to check the thermostat installation and proper bleeding of the system first, rather than letting them jump straight to replacing the bypass valve like I was initially thinking. Really appreciate you taking the time to share your experience, saved me from possibly spending money on the wrong fix! Will update you on what they find, fingers crossed it's just an improper bleed job like your Z4. Thanks again for the help!