The MAP sensor has the biggest say in how much fuel goes into the engine. When you disconnect the hose it will see no vacuum which correlates with wide-open-throttle and the need for a lot of fuel. If you're disconnecting the rubber hose, (you didn't say which engine you have), the engine is probably flooding and slowing down. You would see black smoke at the tail pipe. If you unplugged the electrical connector the computer will detect that, set a diagnostic fault code, and turn on the Check Engine light. It will go into backup strategy to calculate the approximate fuel needs but it won't be right.
I prefer looking for vacuum leaks with a squirt bottle filled with water, while the engine is still cold. You'll see it get sucked in and hear the engine slow down when you spray the leak. You can also pinch off rubber hoses. If one affects engine speed, follow it to where it branches off and do that to each one to narrow it down.
Starting fluid takes a while to vaporize so it may not get sucked into a leak. Propane will, and it will cause a further increase in engine speed. Water works well for intake manifold gasket leaks on top. Propane works better for leaking hoses.
You should check for diagnostic fault codes too. If there's one for the automatic idle speed motor there is likely a cut wire going to it or a corroded terminal in the connector. Codes for that circuit usually will not turn on the Check Engine light because the problem won't have an adverse affect on emissions. The additional clue is the motor would not move so the idle speed will always be the same.
Monday, April 29th, 2013 AT 1:22 PM