98 Sirion 1000cc EJ-DE engine problem

Hi everybody

New to the forum. Has anybody had a problem with the MAP sensor from the top of the throttle body of a 98 Sirion? Engine was starting to suffer from an inconsistent erratic idle going sometimes a week or more without this symptom occurring. I've eliminated it (I think) back to the MAP sensor, I've done a vacuum check using a multimeter and a hand pumped vacuum pump connected to the MAP which is showing approx 5.04V which is correct. When you introduce the vacuum in 5lbs psi measures up to a max of 20lbs psi the voltage should drop approx 1V with every 5lbs psi which indicates the sensor is ok. My sensor is showing no voltage drop when vacuum is introduced. Does this mean it is faulty?
(I've no wiring diagram but am taking the middle terminal in the MAP sensor as the signal wire back to the ECU which I'm taking my voltage reading from)
Hope this makes sense :S