2018 BMW 340i — surface rust underneath a winter daily — trying to narrow this down

Log the conditions before you spend. Two 2018 BMW 340is can feel identical and have different causes, so measure before you copy someone else's repair. If the cheap test changes it, stop and reassess. On “surface rust underneath a winter daily —,” that was the fork that mattered for me. This is from a 2018 BMW 340i in Austin with 146,500 miles. That's the whole play. This is the filter I use when the thread starts repeating itself. Not driving it hard until I know more.

Start with the test that can prove you wrong. If a shop can't duplicate it on the 2018 BMW 340i, bring a video plus freeze-frame data, not just a story. If the cheap test changes it, stop and reassess. Happy to compare notes if your setup is different. Next check is the cheap one I can do in a driveway.

I'd want a measurement, not a hunch. Two 2018 BMW 340is can feel identical and have different causes, so measure before you copy someone else's repair. Don't erase codes until someone who can help has seen them. That's the whole play. I'd sleep better after that step, even if it finds nothing. I'm in no rush to be a hero on it.

This is usually cheaper to test than to guess. Write temp, speed, and in-gear vs coast when the 2018 BMW 340i does it — then decide on parts. If the cheap test changes it, stop and reassess. Do that first. Doing the boring step first on purpose.

If it were in my driveway, capture voltage while the 2018 BMW 340i is actually failing — after it restarts you've lost the clue. If the cheap test changes it, stop and reassess. This is from a 2018 BMW 340i in Austin with 146,500 miles. That's the sequence I'd use. Trying not to diagnose from the couch.