Can somebody please verify the correct voltage output of the MAP sensor with the key on and engine off? (the voltage on the center wire)

I have a 94Z and noticed it shows 30 kpa on my scantool with the Key on, engine off. Its also putting 4.8 volts out on that center output wire. (also has 4.8 or so in from the pcm on the left wire).

The car will not start, has wet gas fowled plugs and I'm suspicious this is the problem...

Thanks

 

Are you sure that is not barometric pressure you are reading? That would be more like it if it reads 30. That also would correspond to the ~4.8v reading you are getting. Both would be normal.

 

probably your right... Always get the two confused.

Why would it be at 30 before starting? I only hit 30 when at WOT, and then mostly 27-29... That would add way too much fuel for startup, correct?

Thanks
Bob

 

It reads the current barometric pressure when you first turn the key on. That is then the baseline it uses for further calculations. When you start the engine, the vacuum produced inside is then compared to that reading.

When you are at WOT, there is very little vacuum in the manifold and you are much closer to atmospheric pressure.
Fred has some explanation here.

 

Straight from my Scan Tool report

Key ON engine Off

with my old MAP sensor 4.91 volts
Barometric 4.91 volts

With my new MAP sensor 4.82 volts
Barometric 4.82 volts

both scannings done at the same altitude.




30Hg you mean? It should be at 30Hg. The MAP sensor reads the barometric pressure inside your intake manifold. with the engine off there is no vacum and the outside air pressure (BAR) will force air pass the throttle body and will balance your inside intake manifold pressure with the outside air pressure.


Negative my little grasshopper.. MAP sensor reading is not the only one that dictates how much GAS is pumped into the cylinder.
The PCM needs to know How much air is entering the engine first. remember the fuel is calculated by a mass of air and a mass of fuel... the so called air to fuel ratio.

MAP sensor readings indicate the amount of load your engine maybe experiencing.

Check this webpage out. This will give you a better idea of the role of the MAP sensor in an engine:

http://www3.autozone.com/servlet/UiB...3d8004f7cf.jsp


I hope this info helps


Marvin

 

I'm used the turbo buicks... We dont have MAPs that play a role in the tuning (we use Load, LV8)

Thanks guys!

Bob