There's just one on my '87 with the same engine and trans. Right there where the Y pipe comes together.
Bad mileage? Two main things on my car were stretched timing chain and a bad thermostat.
Your temperature guage should run in the middle, with a little bit of oscillation back and forth - which is the action of the t-stat opening and closing.
To check for a stretched timing chain, get underneath the car if necessary, and grab the crank pulley. See if you can rock it back and forth, without any resistance from the valve train. I had about 5 degrees of slop. New chain was TIGHT.
What happens here, is that the cam timing events get retarded in relation to the piston's position. This will also show up as retarded ignition timing, since the distributor is run off the cam, and you're reading the advance at the crank.
Wouldn't hurt to have the injectors professionally cleaned. Shops use a special solvent for getting all the crud out of the injectors - a bottle of something in the gas tank won't do the same job!
Gas is reformulated in the winter months, too. Expect less MPG during cold weather.