Re: 2010 mariner premier question
You can check a MAP sensor by applying vacuum to the port, along with power. The sensor outputs a frequency related to the manifold absolute pressure, hence the name...
If your engine is idling, it will have higher manifold vacuum. There's little load on the engine. When you accelerate, the manifold vacuum drops, because the throttle plate opens and allows air to flow. The MAP senses this, and signals the computer how much load, in general, the engine is experiencing. So, the computer, based on that, flows more fuel than it would otherwise.
Altitude also affects the MAP, so if you were crossing over Loveland Pass, the MAP would sense the lower absolute pressure and adjust the fuel accordingly.
On older systems, the MAP would mess up the idle severely if it were out to lunch, as an indicator of things they can do. In your case, seems as if it is telling the computer your engine is working, so give it more fuel.