Bad intake gaskets won't necessarily lead to low vacuum readings or lean mixture codes. Motronic sees low vacuum due to leak, closes down throttle to compensate, vacuum goes up, motronic is happy- Engine idles rough because incoming air is not metered properly, and MAF meter has little sense of what is happening. O2 sensor sees lean mixture due to excess air, Motronic lengthens injector pulse to compensate for 'leanness'. More rough running, but some cylinders are actually too rich. Driveability problems can be elusive and extremely difficult to diagnose on engines with good EMS's, due to their very ability to adapt to a wide range of conditions. A propane tank with a hose attached works very well for finding vacuum leaks on such vehicles.
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