How to Save Gas with MAP Sensors

If your car's model is beyond the 1995 mark, chances are that you are using a fuel-injected, computer-controlled car. For the sake of fuel efficiency and maximizing power, a plethora of sensor suites have become embedded in the car for the ECU (Engine Control Unit) to monitor a car's operation.

Back to front, the car has turned from pure heavy metal to a smart device that can regulate itself with little human supervision. You can never repair a car anymore with just a wrench and a screwdriver. Today's mechanics have to deal with circuit boards, chips, sensors and computer programming as well.

One of the critical sensors used to regulate the air-fuel balance or "stochiometric" in the car is the MAP sensor or Manifold Absolute Pressure sensor. The MAP sensor's function is to measure the difference between the air pressure within the engine versus the outside atmospheric pressure. If you know what a barometer is, this is a barometer in a car.

How does air pressure figure in an engine?

This pressure difference, also known as manifold vacuum, is what can be compared to the venturi vacuum when it comes to non-electronic, carbureted vehicles.

Simple fact is this: the more air you can squeeze into an engine, the more power you can generate.

The rate of air flowing into an engine can be determined by three important factors: the rotation rate of the engine, the engine's size and the density of the opening of the intake stream.

Rotation rate is determined by the car's road speed. This only occurs when the clutch is not fully depressed. Depressing the clutch will have the effect of idling the engine, as if it was standing still.

Engine size is really just the displacement of the engine. The density of the intake stream, such as the throttle, controls the rate of air the engine is able to take in, directly affecting air pressure in the engine.

MAP sensors make use of these factors to measure how much fuel is to be sent into the combustion chamber, depending on the difference between inside pressure and outside ambient air pressure.

NOTE: MAP sensors need vacuum to measure accurately. Any leak in the engine can lead to a false signal that the ECU will interpret to send more fuel to the engine.

Outside air pressure is normally about 28 to 31 inches of Mercury (Hg). Inside the engine, air pressure can range between 0 to 22 Hg. On idle or deceleration, vacuum is always high as the throttle is closed, restricting the amount of air drawn into manifold. When the accelerator is depressed, the throttle opens, and air rushes in to fill the vacuum as much as it can.

A typical MAP sensor will house two chambers. One chamber houses a diaphragm with a movable membrane and another chamber that is used as reference to the outside atmospheric pressure. As the engine operates, the MAP sensor measures the movement of the membrane and outputs a corresponding voltage to the ECU. The ECU then translate this voltage to use to determine the fuel mix and timing of the spark plugs for meeting its programmed stochiometric.

How do I save gas then with the MAP sensor?

Run diagnostics. Regularly run checks with any trusted mechanic in your neighborhood. An ounce of prevention is better than a pound of cure. Before anything happens, jump the gun and get into the habit of regular checkups. You'll find out that the car will last longer for it.

Install monitors. Every modern car has an ODB II (On Board Diagnostic) interface mandated by US law. You can take advantage of this law by installing a monitoring system to see what the ECU is doing. With this you can spot a problem even faster than trying to figure out what happened. There are several 3rd party after-market products that can meet your needs such as ScanGauge II.

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