[Gmecm] MAP and Vac

[Andrew Gibson]

Here's the scoop. Manifold vacuum is the best indicator of engine load. Which is why it is used to control fuel and spark. (I'll explain the vacuum/map difference in a moment.) The reason the vacuum changes when the fan comes on is beacuse the fan uses electricity from the alternator which is driven by the engine. Fan comes on, engine load goes up, vacuum goes down.
   
  Here's where the map/vacuum gets interesting. When you turn the key on the computer takes a Barometric pressure reading for alititude spark/fuel adjustment. It is taken from the map sensor. When the car is started the manifold vacuum adjusts the reading at the map sensor according to how much vacuum is present. Your MAP and Vacuum values in the datastream will always add up to the BARO reading. For instance say you have a BARO of 25 inHg, Vacuum of 17 inHg, then your MAP will be 8 inHg. The MAP reading is opposing the Vacuum because it is Manifold Absolute Presssure - The difference between barometric pressure and Vacuum. It is used because of the changes that occur in pressure due to altitude. A vacuum only measurement is not enough for accurate fuel and spark control at varying altitudes.
   
  The reasoning is this: At higher altitudes less air is available. The engine is also less able to pull a high vacuum. With no altitude compensation the ecm will see a lower vacuum, think there is a higher load and more air available, and enrich the fuel mixture. When in reality there is less air and so what the ecm precieves as normal is creating a double power enrichment without needing one. Lower altitudes will create a lean mix and even advance the timing for a precieved cruise condition. Less fuel, advanced timing = engine meltdown. So as you can see all three readings are very important to proper running, fuel economy, power and engine longtevity. Enjoy!

 			
---------------------------------
See the all-new, redesigned Yahoo.com.  Check it out.