First the RPM's on the highway are due to gearing not tune-up. Second if you have the vacuum gauge in the red then you have a serious ass vacuum leak somewhere. Third check the rubber boot between the Air Flow Meter and the throttle body, the AFM is that Carb looking flap thing. You are most likely getting that back fire due to no air going in and lots of fuel. Well it has to blow somewhere. The only othe thing it could possible be is the Cat is Clogged or Pre-Cats. Drop the exhaust at the joing between the manifolds and the down pipes. If the exhaust has an issue this will let you know as it will be disconnected. You do not need to drop it but instead loosen it enough to leak like crazy.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....