Vacuum issue, maybe? You could start by checking the line from intake manifold to the vacuum modulator. Maybe you have a crack in the boot at the intake manifold.

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1990 190e 2.6 - The only one I can really call "My Car".

1987 190e 2.3 - The donor car's up and running, only mods are Euros and a Sony headunit. My Dad's runabout now.

1990 300e 2.6 - The parents' ride.