I had never read about a MAS but I was going to look into it. My excellent mechanic buddy in Redwood City suggested a new O2 sensor and I put one in. The other one was only four or five years old but why not - no change. Oh well.

The web article I found about vapor capture said that when that system messed up you can get a lot of check engine soon light action, presumably linked to rich fuel mix.

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