My 1991 560SEL idles rough, but smoothes out during acceleration. I replaced rotor, cap, wires and plugs but I still get severe idle. The transmission shifts sporadically. Dealer suspects leaking intake gasket causes both issues - incorrect oxygen mix and vacuum loss to engine and transmission. They want to replace the intake gasket ($1100), then test again. In case I don't feel like doing this expensive trial and error approach, can anyone point me in the right direction? Compression tests well in all cylinders.

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1991 560SEL (254K miles)
1980 Lincoln Mark VI - Givenchy
1979 Lincoln Mark V Collector's Series
1979 Lincoln Town Car Collector's Series
1979 Ford "Effie" Pickup (in gradual transformation to a show/street truck)
1969 Lincoln Mark III (waiting for "Overhaulin" to call)
1989 Lincoln Towncar
2001 Lexus GS300 (spousal unit's daily driver)

You can have an indy shop does a vacuum test to see exactly where the leaks are. They inject smoke in the system and if there are leaks, smoke will come out. The cost to the the test done is about $80.

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1991 420SEL Project Car (Completed and rides beautifully) http://www.peachparts.com/shopforum/tech-help/112321-free-1991-420sel.html
2000 BMW 740IL
2000 Integra Type R - Track Car - SOLD
2005 S2000 - Track Car
06 C230 Sport
89 560SL