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If it's completely toast your engine won't even run. The MAP sensor measures engine load, and that plus rpm are primary inputs to the ECM to look up spark/fuel maps. If it's sort of working, or if the vac hose is leaking, etc, it might barely run, hunting up and down for idle, stall when you open the throttle.
If you have a hand vacuum pump/bleeder and a multimeter you could test it.
Jeff: It gets darn expensive replacing sensors when you think they "may" be going bad. There is another altenative that is not quite as expensive as just buying a new one and see what happens. Mid America Direct sells sensor testors for about $12 to $15 depending on what sensor you suspect. These along with a good Volt/Ohm meter will "tell the real story". I just bought a set of testers for my '89 for about $85 total. I had a very rough idle and the sensor testors led me to a new IAC motor/valve and my problem was solved!
Regards, Sam