I have a 1990 toyota 4 runner with a 22RE. It blew the head gasket and i had it replaced and ever since, ive noticed my fuel milage sucks. When it idles, it doesnt idle steady, it jumps between 1-2000 rpm as it sits. I was out 4xing and the exhaust busted off of at the header. Oddly, it produced instant fuel milage when i expected it to get worse.... so when i got home, i checked exhaust and removed the plugged cat and welded the exhaust back together. Nice and quiet, i noticed a small increase in power, but lost 100km per tank of gas! Reset codes after cat was cut out and its not producing a code now but is still idling funny and sucking fuel like crazy. Im guessing o2 sensor is hooped but was wondering if anyone has any ther ideas.

 

If your o2 sensor is working as intended it should be setting a fuel mixture out of bounds rich, meaning the ECU can not.trim the AFR any leaner.

Along with that you've maybe altered the exhaust system enough you don't have proper expected flow. (Part of the exhaust sizing equations account for what is know as back pressure, this is the tuned length of the exhaust causing exhaust pulses to suck the chambers clear. It's way above my pay grade, and its past bed time.)

 

Idle is the easy bit. If it's not a mistimed ignition or other imbalance it's going to be an air flow issue. Air must be properly regulated, this can be a worn throttle body sticking the throttle plate, a general vacuum leak, or.. misbehaving idle valve either due to gunk, failure, improper coolant levels that Lodge air against the sensors (part if the idle valve reliefs on the coolant flow thru the iacv)

 

Have them adjust the idle correctly. Its the bigger flathead screw.
Find the manual except and give it to them, most shops will use the incorrect screw