Can anyone tell me what the small grey plastic fitting is that sits right behind the oil filler neck on a Olds 400 motor.
It looks like it should have two 1/4" hoses attached to it.

 

That's a thermal vacuum switch and there should be THREE hoses attached to it. The ports should be labeled "C", "M", and "D". The D port goes to the distributor vacuum advance, C goes to the ported vacuum port on the carb, and M goes to a straight manifold vacuum port. Your car uses ported vacuum to operate the vacuum advance. That means that there is no vacuum and thus no advance at idle and small throttle openings. This can cause the car to overheat in stop-and-go traffic, so this TVS senses coolant temp and if the coolant gets too hot, it switches the distributor from ported to straight manifold vacuum. This increases advance at idle and cools the motor.

Of course, you can ask yourself it that is good, why not simply bypass the TVS and run straight manifold vacuum all the time...

 

"Of course, you can ask yourself if that is good, why not simply bypass the TVS and run straight manifold vacuum all the time..."

Highly recommended for any performance "tune" or as I like to say super tuning." Basic Super Tuning = dialing in all 3 timing events and the fuel curve.

Yank that TVS and install pipe plug in the hole.