I have a 89 Sidekick with 1.6L manual... Have only been getting around 15-17 mpg. When ever an electrical load is turned on the idle air control valve will chatter loudly. The check engine light was on constant and would not flash a code, after having the ECM "repaired" it was the same. Traced printing on the instrument cluster and removed a screw that was grounding out the light. Will now just flash the 12 code saying nothing is wrong. Anyone have a thought on why the valve chatters and why such poor gas mileage??? Thanks

The chatter is normal, I would start with the basics, air filter spark plugs
and wires, then check the cat for plugging and the O2 sensor, I bet the O2
is not working and causing poor fuel economy.

What size tires do you have?

Wild

Tire size is 205/75R 15. When I say chatter, the valve cycles about 5 times per second. The O2 sensor has been replaced fairly recently but I can check it for proper function.

OK stock size tires, so it's not a tire issue, yes the chatter at 5 cycles per
second is normal. If the O2 has been changed, then it could be defective,
and I have found them at times, or the timing could be off, also you could
have a slipped timing belt, that will also affect the timing and the engine will
run out of valve time which could cause poor economy

Wild