I have a 91 tpi w/ a 305. There is a sensor on the bottom of the back of the plenum and I don't know what it is. I tried tracking it down in the service manual but didn't have any luck. The car doesn't idle until it's warm so I figured while I have the plenum and runners off I would replace the sensor that you can't get to while it's off. Any ideas? my guess was an air charge sensor because it is connected to the distributor.