Map sensor would be on the firewall, with a vacuum line to the intake manifold connected so it can sense engine vacuum, and thus imput 'load'. Low vacuum == high load, high vacuum == at idle or loafing, thus needing less fuel. A rectangle a little larger than a pack of cigarettes. It varies a Hz signal depending on vacuum.
I would check your fuel pressure regulator as when they go bad, and peak the fuel pressure, you'll get about 1/2 normal mpg. The pressure should rise when the vacuum line is pulled, and fuel should not come out the vacuum port.
The other thing that makes them weaker than normal until you get the rpms up is ignition timing being re - tarded. {some web sites bar the word whole.} Check with the plug removed that timing is at 10BTDC at hot idle.
tom