Gunk clogging the throttle body blade not letting it close all the way,.. tons of reasons.

If nothing has changed,..

Check the spring on the throttle body to make sure its returning all the way. Manually open and close it to make sure its returning. If still not right from there I would check for vacume leaks.

If you still have many of the original vacume lines,.. you gotta figure a 21 year old car,..those rubber hoses are well past their intended service life. On my girls 87 I replaced almost every single vacume hose on the car after we got it. Most were gummed up so bad that they would collapse on themselves. The ones that werent like a gummy bear were dry rotted.

In most cases the car will run a lot better with new vac lines. 87 cars used air density,.. so most of the EFI parameters are governed by vacume. Check the vac line to the MAP sensor. Its a pain in the ass because it runs from under the upper intake. If it feels squishy and soft,.. replace it.

Hurst