Where did the coolant go?

[Wagen]

Story: I'd been driving my 328is for about three hours straight through Wyoming to my destination in Jackson until I started climbing the Teton Pass in the middle of a snowstorm at night. I got stuck third in line behind a Camry that was barely crawling up, which killed my momentum and I could feel my winter tires losing traction on the steep grade (or maybe that was my clutch giving out, I'm not sure at this point). The roads were slick with maybe 2-3" of slushy snow and I think I would have been totally fine except the Camry kept slowing down, making me afraid we'd stop and I'd lose all momentum and traction completely. I had the bright idea of upshifting to power through and maybe make a pass until I realized my wheels weren't spinning in the snow, I was only smoking my clutch at ~4-5k rpm... I know, in hindsight that was so so stupid. I really couldn't see or tell what was going on because it was snowing heavily and a truck behind me was riding my ass with his headlights in my mirror. I downshifted but my clutch wasn't having it, so I gave up and went back down the pass to pull off at a safe spot. I let the clutch cool off for about 10 minutes (during which I watched the Camry also making his way back down) and decided to go for it again. I made it over the pass no problem.

Except when I started down the other side I noticed my heater wasn't blowing hot air even though my temp gauge was right in the middle where it always is. Shortly after at a stop light I noticed steam coming out the the front of my hood so I pulled into a gas station where I popped the hood to see my radiator cap boiling over. Temp was still normal, so after letting it cool for a bit I checked the level and ended up dumping in a whole jug of prediluted coolant. One concerning thing was the oil level seemed high, but I couldn't be sure. I let the car run again and thought it was okay, except the heater still wasn't working. I was anxious to get to my hotel so I got back on the road but about 5 minutes later I noticed the temp gauge getting hot so I quickly pulled over, killed the engine and the temp went right back to the middle. In hindsight I should have kept adding coolant and burped the system so maybe that's what happened there? I really hate working on the cooling system of these things.

Anyway, after 8 years with this car I was officially stranded for the first time and had to get the car towed. The next day I did my best to check for leaks and make sure I hadn't blown the head gasket but everything seemed fine. I topped off the coolant and bled it the best I could, made sure the heater was blowing hot again, and took it for some test drives where everything seemed back to normal. In total I lost about a jug and a half of coolant, so what the fuck happened? I'm thinking the smoke that I thought was the clutch was now obviously the coolant boiling out? Wish I could remember for sure, but why would it do that? Could an old reservoir cap fail and lose that much coolant? I really didn't rev or stress the engine that hard, but now I'm worried that the temp gauge reading wasn't accurate if the coolant level was too low and I ran the engine hot without realizing it. I got a negative coolant hydrocarbon test, but that was only at idle, so I'm wondering if i've still got a little crack or gasket failure that sucked the coolant out the exhaust. Would the elevation and 10% grade be a contributing factor? I drove it home to Michigan over the next two days and 24 hours, doing 75-80 much of the way with no issues getting 28mpg like nothing happened. What gives?

[Eric93se]

Do an engine compression test, if that passes get a new cap and reservoir (behr or rein). Consider doing the pump as well. Make sure your fans are cycling properly, consider the M50 80c thermostat (I use it).

Your head is most likely cracked because you were super low on coolant. The temp sensor will only work if coolant is touching the sensor, once the coolant is super low the sensor will only come in contact with super heated steam.

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You can also get a loaner tool to pressure test the system, if no leaks are found then the coolant may have gone down the tail pipe due to a cracked head. After running the engine the following day pull all the spark plugs and use a flashlight or inspection camera for coolant in the cylinder.