BMW X5 Overheating: Bypass Valve vs Heating Issues

Hey there! I have some experience working on BMWs. I actually had a similar issue with my 2003 BMW X3 (same engine family). Like you, I had overheating problems and no heat. Had it checked at my regular workshop, and turns out the thermostat housing had cracked due to a poorly done cooling system repair by the previous owner. Since it was their mistake, they fixed it under warranty at no cost to me. Before I can give you more specific advice, could you share: Are you losing any coolant? Does the temperature gauge behave erratically? Which error codes are you getting specifically? For reference, I also own a BMW 5-series, so I'm quite familiar with these cooling system quirks.

Thanks for sharing your experience! Yeah, I'm seeing coolant loss but can't spot any visible leaks. The temp gauge starts acting up after about 15-20 mins of driving, jumping between normal and hot. Getting P0128 code mainly, which points to the cooling system. Just had my latest service done about 2 weeks ago but they didn't catch this issue. At 153396 KM, I know these things start happening, but still frustrating. I was hoping to avoid a workshop visit, but from what you're saying, it might be worth getting it properly checked out. Did you notice any warning signs before your thermostat housing completely failed?

Hey again! Yeah, I totally get your frustration. When my X3 started acting up, I actually noticed similar symptoms before the thermostat housing completely failed. The intermittent temperature gauge behavior you're describing is exactly what I experienced, it's like déjà vu! Before my housing cracked completely, I ignored some warning signs for about a month (not my smartest move). The main ones were exactly what you're describing, that P0128 code, the temperature fluctuations, and the mysterious coolant loss without visible leaks. The cabin heat issue was actually my last warning before things got really bad. In my case, I initially thought I could fix it myself since I've done some BMW work before. But honestly, with these symptoms and especially that P0128 code, you're probably dealing with more than just a bypass valve. Mine ended up being a cracked housing that was leaking internally, which explains why you can't see the leak. The fact that your recent service didn't catch this is interesting, though these issues can develop pretty quickly. Given your mileage (mine was around 160,000 km when it happened), I'd really recommend getting it checked by a BMW specialist. These cooling system issues can cascade into bigger problems if left unchecked, learned that the hard way with repair costs!

Thanks for that detailed response! You're right, the similarities between our issues are pretty striking. I definitely don't want to make the same mistake of waiting too long and ending up with bigger problems. Think I'll bite the bullet and take it to a specialist this weekend. The last thing I need is to end up stranded somewhere with a completely failed cooling system. Really appreciate you sharing your experience, helped me realize this isn't something I should try to fix myself. I'll probably look for a good BMW specialist rather than going back to my regular service center, since they missed this during the recent checkup. I'll update once I find out what's actually causing the problem. Thanks again for the help!