15 inches is a little bit low. Trouble is, a poor idle can cause low vacuum, and a vacuum leak can cause a poor idle So to start with, check all vacuum lines and connectors in the engine compartment carefully. Then disconnect and plug the vacuum line at the rear of the intake manifold, and see if that makes a difference. If it does, you may have a leak in the cabins systems (climate control and door locks).

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe