MAP (Manifold Absolute Pressure):

Here is where you can do something about the pressure that forces the air/fuel mixture into your engine. The air cleaner is exposed to atmospheric pressure (BAR). As the air passes through the filter, it loses some pressure. It loses more pressure in the ductwork that feeds it into the engine, the MAF sensor, the throttle body, the intake runners and the restriction of the open valve. Each of these items reduces the air pressure slightly. And gives you a pressure LOWER than atmospheric pressure in your intake manifold. This pressure is called manifold absolute pressure, to avoid using the negative numbers required to measure vacuum.

When a stock LT1 is idling, the "MAP" reading should be about one-third of the "BAR" reading. Check the "BAR", and remember it - it won't change for a while, unless you're in the middle of a hurricane… Then display "MAP". If the BAR = 30.0, MAP should be at 10.0, or below. On my relatively stock LT1, with a BAR = 30, I would see MAP readings at idle of 9.3 to 9.5. The readings would fluctuate slightly with the idle. Maybe as low as 8.9, and maybe as high as 9.8.

If you have a cam that is more aggressive than the stocker, you will see a higher MAP reading at idle. With a fairly radical cam, you might see reading of 18 or so.

The difference between MAP and BAR is your manifold vacuum. If BAR = 30.0 and MAP = 9.5, then your vacuum is 30.0 - 9.5 = 20.5 inches of Hg. If you connect a vacuum gauge to the correct port on the manifold, this is what it should read.

When you start driving, the MAP will fluctuate with the throttle position and the engine load. Close the throttle completely at 2,000 RPM, and the MAP will drop well below 9.0. Floor it at low RPM, and the MAP should shoot up to a number equal to, or very close to "BAR". The output of the MAP sensor is very important, because it tells the PCM where to look in the fuel and ignition timing tables. High MAP = high engine load. Low MAP = low engine load.

Conversion Table - MAP pressure units:

IAC (Idle Air Control):

The Idle Air Control is a little stepper motor that moves a pintle in and out of a hole in the throttle body, and allows air to bypass the throttle blades, and enter the intake manifold. This is required to allow the car to idle when the throttle blades are completely closed (as they should be). The PCM plays with the IAC motor to keep the idle at spec, 800rpm for M6's and 650rpm for A4's. The ScanMaster reads the "position" of the valve motor, which can range from "1" to "255". At idle, you should see roughly "20" to "40" (this is fomr my M6, idling at 800rpm). And, it should be steady. Any lower and you probably have a vacuum leak, or the throttle return stop screw on the throttle body is preventing the blades from fully closing. If you see numbers higher than this at idle, it is possible you have a dirty IAC motor pintle which is not moving correctly, or plugged air passages leading to the IAC motor. If you have installed a throttle boy airfoil, it is possible it is blocking air flow through the opening that supplies air to the IAC system.

When you start to drive, the IAC value will start to rise. This is done in order to keep enough air flowing into the manifold that if you suddenly let off on the throttle, and let the blades fully closed, there will be enough air to keep the motor from stalling, and provides air to minimize pollutants. The counts I have seen under cruising and WOT conditions are in the range of 60 - 100.

The IAC motor is located at the base of the throttle body, on the passenger side. The PCM stores the "learned" idle position, so that it can return to this value quickly, in order to control idle. This stored value can be "lost" if the power to the PCM is lost, and idle may hunt a bit on startup until the value is relearned.