I have noticed when I scan my car that the Baro reading is not correct. It is reading 27.75Hg which seems low to me. My friends LT1 reads 29.75 which would seem more correct at sea level. My MAP voltage reads 4.78 - 4.76 with engine off. We swapped MAP sensors with eachother and this made no change. Car runs fine otherwise. Should i be worried about this? LT fuel trims look good...very close to 128 when driving but can get down to 118 when idling....does this seem ok?

 

I don't really think this is much of an issue. If I read Injuneers page correctly, that voltage equates to 28.8 inches of mercury, pretty close to the standard reading for 1000 foot altitude. Your friends reading of 29.75 In of mercury indicates close to sea level....

What's more important is the difference between the BAR and the MAP readings. the MAP should be about a third of the BAR and will affect the timing and fueling
mapsthe PCM uses. Since your fuel trims seem normal, everything is probably OK. Here's the disclaimer.....I am not the expert that others are. Injuneer knows this like the back of his hand as do several others......seek their council to be absolutely certain.

 

Remember, while the "standard" barometer at sea level is 29.92"Hg, the actual barometric pressure is variable. Pressure will drop, for example whan a storm approaches.

Pull the harness connector off the MAP sensor and verify that the PCM is supplying a full 5.0V reference signal (gray wire to black wire). Check the harness connector... they tend to dry out and crumble, resulting in erratic contacts.

 

Ok, so I checked again this morning....now its reading 27.50...pulled the fuse for the PCM overnight. Reference signal accross the gray and black wire with the map unplugged according to my multimeter is 4.72 volts? However my scan tool says map volts is 4.76???

 

I also noticed the PCM says my batt voltage is 11.9 volts (Car off) but when measured with multimeter across the batt terminals i show 12.5....is this normal?

 

There's always a voltage drop when measuring at a distance from the battery.

I'd recommend cleaning the connections for all the battery and ground on the fender...it can't hurt.

 

It's normal for the PCM to read system voltage lower than the dash gauge or a measurement at the battery terminals.

Why did you pull the fuse for the PCM? There's nothing in volatile memory that will affect the reference voltage or the sensor reading. Your MAP reading is low because your reference voltage it low. Check the reference voltage on another sensor, like the TPS (gray to black wires) or a temp sensor. In all cases, reference voltage has to be measured with the connector unplugged.