When my car had an intake leak, the computer seemed to over compensate and it ran rich, a little black smoke on hard acceleration, black spark plugs and a black 02 sensor just after several hundred miles.
An hole in the exhaust system before the 02 sensor that allow fresh air into the exhaust system causes it to run lean. Counterintuitive for sure.

When I did my fault tracing, bought a small package of vacuum stem caps, the box contained various sizes, so I just capped all of the stubs that were on the manifold to see if that helped on the idle speed. Then I went to the propane gas around the intake, that did not show it either, not sure why but, in frustration just decided to do the intake gasket. My problem was the bolt holes were stripped so I had to helicoil all the bolt holes, problem solved.


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DanR