About a year ago several things started with my 1991 Ranger (180,000 miles, and has always run just like it did when I drove it off the lot). Maybe they are related, maybe not. 1) significantly reduced mileage, 2) surging at idle from 1000 rpm up to 1300 rpm and then right back down - very rhythmic, 3) Check Engine light comes on every now and then, but goes off in less than 30 minutes, or if engine is stopped and restarted, 4) way less heat available through the heating system, 5) somewhat reduced power. I have changed the thermostat and replaced the plugs and ignition wires, but nothing has changed in the way the truck runs. Thanks for any ideas!
What code is the check light throwing. The truck is most likely telling you what is wrong with it. My guess is a vac leak or bad/dirty IAC. But it can be a bad injector or 101 other things. The codes are key
The surging idle is very characteristic of a vacuum leak.
How old are your fuel/air filters? O2 sensors?
Next time the check engine light is on use these instructions to find out what your error codes are. (Or find someone with the right kind of scanner).
The odd one out is the lack of heat - but this didn’t start after the thermostat replacement did it? Did you make sure you purged all of the air from the system? Does the truck get all the way up to temperature? What does the temp gauge say? Maybe get your hands on an infrared thermometer & get a temp reading off of the thermostat housing.
Check the EGR valve. Pull the vacuum connection at cold idle. There should be no vacuum applied then. After a cold start up the EGR exhaust supply pipe should not get hot until the truck is driven up to a low cruise speed. It that pipe heats immediately while idling the valve is bypassing.