There is some variation between engines and also between MAP sensors, so the pressures are more realistic when expressed as ranges.

Idle -- 10 to 13 in Hg is typical

WOT -- 27 to 30 in Hg is typical

 

should be pulling 20-22in vacuum at idle and close to zero at wot

 

I've never seen a car that doesn't peg to 0 at WOT? Is this for real? Must be a very restrictive throttle body if so!

 

Why not just measure the voltage? just wondering.

 

That's kind of a simple question, but a useful answer is a little involved.

The PCM (same as ECM) needs to know the difference between the manifold pressure and the local atmospheric pressure in order to calculate the mass air flow rate into the engine. BTW, that difference is usually called the manifold vacuum. MAP sensors are calibrated at sea level where the atmospheric pressure is close to 29.9 in Hg. That means that the map sensor voltage only works to indicate the correct manifold vacuum when the atmospheric pressure is 29.9 in Hg.

Above sea level and/or under some weather conditions, the atmospheric pressure is less that 29.9 and the MAP sensor voltage will drop to indicate that lower pressure. With the engine running, the MAP sensor voltage drops some more, but because the starting voltage is low to begin with, the engine running MAP sensor voltage won't give the correct manifold vacuum.

It's also possible for the atmospheric pressure to be above 29.9 which means that MAP sensor voltage will increase so as to indicate a higher pressure. In this case as before, the engine running MAP voltage will not indicate the correct manifold vacuum.

The PCM takes into account various atmospheric pressure variations by reading the MAP voltage just before the engine starts in order to know the local atmospheric pressure. Then it can do a subtraction and get the correct pressure difference or manifold vacuum.

 

Depends on a lot of factors....elevation, cam, etc.

 

i was recently wondering the same thing, i might just go ahead and replace mine, they are only 100 something at the dealership and 70 and the parts stores. i would imagine they get progressively inaccurate the older they get.

 

Thanks for the info guys. I have check the voltage and it was a little below par. also i was reading the realtime data with my computer and noticed the vacuum pressure at idle was only 10" hg and i assumed it should be much higher. I know it becomes almost zero at WOT.

Just a little correction: standard atmoshperic pressure at mean sea level or MSL is 29.92 in HG. In addition, atmospheric pressure regularly increases and decreases thru 29.92 as a high or low pressure system approaches. good weather = high pressure typically and bad weather = low pressure system.

just the other day the barometric pressure was 30.38 here in RI and the storm before the nice weather preduced low pressure around 29.15. i've seen it as low as 28.10 during a hurricane.

Back the the info request. I was looking for the operating ranges for the map sensor. since i checked the voltage and read the live data to be lower than expected, i assume the MAP sensor is going bad.

Ratmonkey has the right idea. more vacuum is produced at idle, and near zero at WOT.

Thanks guys. I have a new map to install!

 

You said,

vacuum pressure at idle was only 10" hg and i assumed it should be much higher. I know it becomes almost zero at WOT.

The term "vacuum pressure" does not mean anything in this context. Real time data would be the absolute manifold pressure and 10" Hg at idle is about what it should be. That pressure would correspond to a manifold vacuum of 20" Hg which is also about what it should be.

If the manifold vacuum was actually 10" Hg at idle, then you have a much bigger problem than a MAP sensor going bad.

 

You said, The term "vacuum pressure" does not mean anything in this context. Real time data would be the absolute manifold pressure and 10" Hg at idle is about what it should be. That pressure would correspond to a manifold vacuum of 20" Hg which is also about what it should be.

If the manifold vacuum was actually 10" Hg at idle, then you have a much bigger problem than a MAP sensor going bad.

What kind of problem would I be looking for then if that were the case?

when i install the new map sensor today, i'll read the live data to compare.

 

alright, i've installed the new MAP.

Idling in park: 8-10"HG, Idling in gear : 10"HG

reving in park: 6-8"HG, accelerating to WOT: 10"HG up to mid 20's" HG

Why does reving in park produces a lower Maniford Absolute Pressure than idle and conversely why the MAP increases during driving?

 

MAP and engine vacuum are two different things. MAP (manifold absolute pressure) is a reading of engine vacuum compared to atmospheric pressure. A typical vacuum guage is zeroed at atmospheric pressure, so when its reading "0" its actually reading 1 atmosphere or approx 14.6 psi or 29-30 in/Hg. Thats how it can be confusing to relate MAP with Vacuum, and reading MAP values will look inverted compared to a vacuum guage.

Reving the engine in park increases the RPM but not so much on throttle plate or load, so it has the tendancy to increase engine vacuum giving you a lower MAP reading. MAP will increase when driving or under heavy load because engine vacuum decreases and MAP pressure becomes closer to actual atmospheric pressure (typically 29-30in/hg)

 

interesting... thank you

 

I am not understanding your comment. Is this meant to focus on the fact that ratmonkey said "almost" zero instead of zero?