I had the same problem. It was the idle air control valve. It sits on top of the intake just downstream from the throttle. There's a valve rod inside that gets worn over time and starts to stick. If that's what is causing your problem, you need to replace the unit.
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idle is preset at the factory .. but some cars are equipped with an air conditioner adjuster . which will actually speed up the engine when you turn on the AC .. look at it when you start the AC and you will see it is adjustable to speed up the engine some ... thanks Jerry
It's located in the trunk on the drivers side. You should see a black and yellow sticker below a hole indicating that the fuel pump shut off switch is here. Even if your sticker has worn off, if it's a 92 topaz it most likely will be there in that hole. Shine a flashlight in it and you should see a black unit with a red button on top. Push the button down until it clicks in place. If the button is already down, your problem is something else.
1991-92 Mercury Topaz Stereo Wiring
6 1/2" Doors
Left Front (+)
Left Front (-)
Right Front (+)
Right Front (-)
6" x 9" Rear Deck
Left Rear (+)
Left Rear (-)
Right Rear (+)
Right Rear (-)