We recently acquired a 190E that had some similar problems.

It runs fine till we come into traffic and then the rpm flirts with 5 before dying. Sometimes it acts fine.

On advice, the idle sensor was cleaned (some call it the idle control valve), but to no avail. The engine also had a re-tune.

Eventually, the problem was expertly zeroed in on what was called a decelarating sensor. It is a small plastic (sensor) component located directly behind the vdo (instrument cluster). Detach the small piece of plastic from the vdo and put in a new one (quite cheap) and the problem never re-occured.

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... Kerry

126 tailed by a 203, 129 leading the pack.