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Hello Joza:
Bad idle on deceleration-- does it idle badly when you decelerate without braking too?
The first thing that comes to mind is the adjustment of the throttle position switch. If you let off the gas, the switch should change position, signalling the ECU to control the idle via the IAC. A dirty throttle body will impede the movement of the butterfly, resulting in intermittent or no switching by the throttle position switch. The switch might simply need to be adjusted.
If the problem persists with deceleration without braking, I'd look for a vacuum leak at the brake booster.
These things can be difficult to track down-- my '92 945T has intermittent poor idling on deceleration that I haven't been able to solve completely, having swapped IAC valves, replaced turbo intercooler hoses, cleaned the throttle body and replaced its gasket and made multiple attempts to adjust the throttle position switch. I have also replaced the brake booster check valve and its seal, since mine seems worse when braking and decelerating.
Measure the output of your 02 sensor before replacing it (although with high fuel costs, replacing a "lazy" sensor will make financial sense). Check the motor mounts and transmission mount carefully before replacing them. Motor and transmission mounts are generally best replaced together, otherwise differential movement can cause strain on the new parts.
Good luck!
--
Herb Goltz, Aurora, Ontario, Canada '92 245 w/130K mi, '92 945T w/200K mi
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