Low MAP sensor voltage at idle
 

Trying to nail down why my engine is running a bit rough. I'm 99% sure my spark plugs are old and fouled from trying to get by with a bad IAC for a few days. I have some new ones, but want to make sure everything is good before throwing in the new ones.

TPS should range from above .2V while closed and less than 4.8V WOT. Mine ranged from .9V to 3.9V. Much further from the min and max listed, but I assume it is fine.

MAP sensor voltages were:
4.65V with engine off (supposed to be between 4V to 5V).
1.75V at cold idle
1.21V after idling for several minutes.

FSM says voltage should be 1.5V to 2.1V with a hot idle. Mine is at least 20% too low.

I figure it should be replaced, but thought some outside input would be good. This is a '96 4.0.

You're overthinking it, change the plugs, wires, cap, and rotor before you start throwing sensors at it.

Map sensors are ratiometric. That means the sensor output voltages at any given state are a percentage of the sensor supply voltage, relative to the sensor's power supply Return wire. If you know that the supply voltage at the sensor is in-fact at 5 volts then it's okay to report the values you have measured (relative to the sensor Rtn line). The FSM is assuming perfect supply voltage and the real world doesn't work that way so, if your supply voltage at the sensor is truly 4.65V then you should expect to measure 1.395V to 2.1V with a hot idle (I extracted 30%-42% from the "perfect world" numbers and applied them to your measured value - may need correction), relative to the sensor return line at/near the connector (not the block).