You could have a vacuum hose that has become disconnected.

You could have vacuum hose(s) that are cracked.

You could have a bad seal at the intake manifold gasket. I had an idle that went up and down, and my leak was at the intake manifold seal.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".