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PCA Member since 1988
Join Date: Feb 2018
Location: SW Washington State
Posts: 4,627
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I'm not a Motronic guru, but...
Stumbles are usually caused by a lean condition. If you put in a wideband sensor, that's almost certainly what you will find. The question is why? You may have significant vacuum/intake leaks, so make sure your hoses and connections are all good. The usual way to test for intake air leaks is by spraying starting fluid, or CARB cleaner, or propane around the intake manifold and hoses; or a smoke tester. Your car has power brakes, which use vacuum from the intake for power. check those hoses and connections.
However, the oxygen sensor should be operating in closed loop under the conditions you describe, so it should adjust the mixture if it's too lean, unless: (1) the leak is big enough that it can't correct for the additional air, or (2) the O2 sensor is faulty.
After verifying that you don't have vacuum leaks, then think about replacing the O2 sensor. They do wear out. How old/how many miles on it? You could have "pretty fantastic numbers" on the smog check becuase it is running lean due to a faulty O2 sensor.
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1973.5 911T with RoW 1980 SC CIS stroked to 3.2, 10:1 Mahle Sport p/c's, TBC exhaust ports, M1 cams, SSI's. RSR bushings & adj spring plates, Koni Sports, 21/26mm T-bars, stock swaybars, 16x7 Fuchs w Michelin Pilot Sport A/S 3+, 205/55-16 at all 4 corners.
Cars are for driving. If you want art, get something you can hang on the wall!
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