First of all, install a new catalytic converter. You will never get it to run right because the downstream oxygen sensor will be detecting no change in exhaust gas from entering to exiting the converter.
Once that is done, start by looking for a misfire. When a cylinder misfires, it sends the unburned fuel and oxygen down the exhaust. You can smell the unburned fuel at the tail pipe, but the oxygen sensors don't detect unburned fuel. They detect unburned oxygen, and since there's a bunch of it, the Engine Computer thinks the engine is running lean so it adds more fuel to all of the cylinders on that bank.
If the misfire causes the engine speed to be too low, the computer commands the automatic idle speed motor to open the air passage around the throttle blade to let in more air and it commands the injectors to stay open longer during each pulse. The extra air and fuel raises idle speed to the target value. You will hear the hiss of the air when the AIS motor opens.
If you connect a scanner to the truck and view "live data", the AIS steps will be displayed. Normal is somewhere around 32 "steps". For a single cylinder misfire, you can expect to see somewhere around step number 45 to 50 meaning the computer is opening the AIS motor further to let in more air to get idle speed up.
You can also watch the voltage coming from the oxygen sensors. The upstream sensors will switch from rich to lean a couple of times per second. The downstream sensors are expected to switch very slowly, perhaps once or twice per minute, indicating the catalytic converter efficiency is within limits.
Friday, January 21st, 2011 AT 6:18 PM