It's an '88. You don't need a dealer. I am actually surprised you even have a functional dealer left.

Under the center console should be the ECM. With it are two single wires. Plug them in, turn the key to on and count the flashes of the Check Engine light. It will start with 12 (flash-pause-flash-flash) and then spit out any codes in the same fashion and then will end with 12 again. If all is good, then all you will get is 12.

I agree with BigSweed above, it sounds vacuum like. Either you have leak(s) on the many vacuum lines, or the vacuum valve on top of the thermostat housing is not working properly.

Is it overheating? Is the coolant full? Does the coolant look normal? These engines will bump up the idle if they get hot, but they can also do it if there are bubbles in the cooling system from either a leak, low coolant or a head gasket issue.

Good luck,
Michael