The idle speed control shouldn't effect mileage, it can cause the wrong idle speed or an unstable idle though. Do you see any black or dark smoke from the exhaust? Any loss of power or is it just the poor mileage? I would probably do a leak down test on the injectors to see if one or more is sticking or leaking and also check that the fuel pressure regulator on the fuel rail isn't leaking into the vacuum lines. To do that simply pull the vacuum hose on the regulator off while the engine is running, if you see or smell gas in the hose or dripping out of the regulator, it is faulty. That would let the car pull fuel through the intake that wouldn't be monitored.
Also being a 93 I would check the fuel lines under the car for any leaks or issues.
This is a video on one way to test the injectors for leakage:
https://www.youtube.com/watch?v=K7STocr9FBI
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Monday, March 2nd, 2020 AT 10:38 AM