Do you know for sure the venting is OK? I've been studying plumbing more and more and it can be tricky. I wish I'd apprenticed in it years ago. Good plumbers can easily make 6 figures plus. And the good ones are worth it.

Have you taken it off the floor yet? Several years ago I was doing a lot of work on a house and one of the toilets was prone to issues. I pulled it off and there was a weird place at the bottom exit flange where 3 or 4 Q tips had found a place to get stuck. Blocked about a third of the opening.

Another time I was maintenance man at a health spa, similar issues. I pulled it off and a piece of dental floss had gotten caught on a small jagged bit of porcelain at the exit point, something that should have been smoothed over at the factory. Even so, what an odd coincidence it caught and hung on, and it then caught a condom and it had toilet paper all over it. Weird. I filed that porcelain down Imo tell you what.

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1986 300SDL, 362K
1984 300D, 138K