scanning question for 85 owners

Well, intermittent it is.  This afternoon the car started at low idle again, after not doing it for about a week.  I thought it was all cleared up.  I just gave it a little gas and everything was fine, but it's a very odd thing to have happen.  Otherwise, the car is running great.  Just the intermittent starting up at a low idle thing.

Check the P/N switch output to the ECM for intermittent connection. (This is separate set of contacts NOT associated with the starter contacts. Even if it cranks reliably, the ECM contacts may not be 'set'). If the ECM doesn't know the shifter is in Park, it may not properly 'set' the IAC for start-up.

It is an orn/blk wire on pin B10 at Connector 2 of the ECM.  Back-probe this connection with a VOM connected to ground. (A12 blk/wht, C2 is an ECM ground. It's difficult to establish a good ground in a Corvette for test meters).

With key on, engine off, in P or N you should read 0-volt. In any other shifter position you should read nominal 5.0-vot. Wiggle the shifter in Park and watch the meter. If the voltage isn't a constant 0-volt throughut the "wiggle-range", move the shifter to where you get voltage in Park, and try to start it. If it cranks, you have a worn P/N switch assembly that allows the starter to operate, but not send the 0-volt signal to the ECM.

As a further diagnostic, note where this occurs. It will be either at the forward, or aft end of the shifter position. Note which one is "wrong". When starting the car, make sure the shifter is pushed to the "correct" position. If the intermittent low-idle is gone when doing this, you figured it out.