Idle Surge / Poor MPG Solutions 93-95 Pickup

[93BetterThanTheRust]

Here's a couple of things to check:

The mount flange of my O2 sensor rusted out, leaving the sensor exposed to outside air. The engine thought it was running too lean and pumped more fuel into the engine causing the MPG to drop from 23ish to 19ish. The idle also started surging because the excess fuel delivery was driving the idle speed above the 12K cut-out limit.

At the same time, the temp was occasionally spiking and causing "oh @&^%!" moments on the highway. I feared a bad head gasket but replaced the thermostat as a first step in the diagnosis. The R22 was built to withstand tough environments with minimal access to maintenance... I was amazed to find a small hole with a "jiggle stick" on the high side of the thermostat that is designed to let bubbles through a closed thermostat. If air builds up under a closed thermostat, the coolant has to boil before it will open... which will result in a blown head gasket.

Once the thermostat was changed, the temp stayed rock solid, the remaining idle surges went away and the idle stays at approx 1K. I think the fluctuation of the engine temp was also messing with the EFI controls.

Be sure to change out the coolant or the air bleeding hole with just get plugged again!