Recently I had a Ox sensor issue with my 85 22re. After installing a Thorley header about four or five years ago I had a harder and harder time passing smog tests. The engine was also not running right, and it wasn't pulling the idle vacuum it used to (13 inches, down from 16-17 inches). The Ox sensor had been replaced recently with OEM.

The Ox sensor reads off one tube on the Thorley header, and this is a common design. The exceptions are Downey which reads off two, and TRD which reads all four. After going round and round trying to get the truck to pass I had a sensor bung welded between the header and the cat, and installed a new screw-in heated Ox sensor. While there I also had them install a new cat. HC numbers at idle went from 1800 to 34. The motor runs signficantly better and vacuum is back up to 16 inches. While I'm sure the new cat is helping, I believe relocating the Ox sensor was the ticket to getting the motor to run correctly.

It was actually a mechanic that figured this out. He noticed that the Ox sensor was going cold at idle because only one tube was reading. The other alternative was to install a heated sensor on the header. While my solution involved more work and money I think it was the correct way to go.