Extremely bad gas mileage maybe only 3 to the gallon

Check for fuel leaks.

A tune-up isn't a bad idea. Use a gage and check for proper fuel pressure, 3.1L, key on engine off, 40--47 psi. once you build up pressure, turn off the key, see if fuel pressure holds for a bit? If the pressure drops like a rock, check for a leak. Check fuel pressure regulator, check vacuum line on regulator, if it has gas in it, the regulator is faulty. The regulator is usually located on the fuel rail. Have you checked for fuel line leaks?? Any applicable trouble codes? If your obd1 compliant you should be able to check some codes, yourself, if the cel still works? You can smell of your crankcase oil dipstick, if it smells like gas either you don't change oil and filter very often or you have a fuel leak. Some of the national brand auto stores will loan out a fuel pressure gage, you may have to put up a deposite, but, you get it back when you return the gage.