Okay I know my old exhaust manifold is leaking, and is at it's worst when cold so I'm wondering if a leaking manifold could fool the O2 sensor and cause the Cater stepper motor to go too rich, until the engine gets a little warm (at which point the manifold has expanded from the heat and sealed things up a little better) and then leans out.
Would an exhaust leak before the O2 sensor cause rich or lean? In other words would the sensor detect MORE O2 (lean) causing the computer to go too rich?
I'm about to get out and check the stepper metering pins to make sure I've actually got a rich condition (pins all the way forward) and watch it to see if it's backing down as the engine warms up.