Idle for the 2.5 124's is managed by an idle computer, either EDS or EIS. The computer has a lot of inputs...air mass sensor, rack position sensor, crank position sensor, reference resistor, temperature sensor, barometric sensor, etc. It's power is supplied by the OVP relay. It controls a solenoid on the back of the IP that adjusts rack position to maintain stable idle speed. There are no adjustments: it either works or it doesn't, and it rarely goes bad. The only problem I've ever had with this system is a bad OVP. I think the diagnostic for '91 was still by blink codes, which can be read with an LED. Take a look at the FSM, and try to read the codes.

As for mechanical reasons for a bad idle, there are a few, but mostly seen on high mileage engines: damaged valve seats, sunk lifters, stretched chain, low compression. Less catastrophic reasons are air leaks in the fuel line (I once had this when a hard line began to rust out) and leaky delivery valves. Eyeball the entire length of the fuel system, from tank to injectors and see to any obvious leaks.