What Are the Symptoms of a Bad Map Sensor?

by Tom Pace
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The Manifold Absolute Pressure, or MAP, sensor is a common component on modern vehicles. The purpose of the MAP sensor is to measure the pressure within the intake manifold relative to the ambient barometric pressure. This information is sent to the engine computer, which adjusts other variables such as the rate of fuel injection to maintain optimal performance. While a vehicle will usually continue to run with a faulty MAP sensor, engine performance may suffer significantly and result in increased emissions.

Check Engine Light

The most obvious confirmation of a faulty MAP sensor is that the engine computer will send out a diagnostic code causing the "Check Engine" light to come on. Most cars made since the 1990s can be be connected to a diagnostic tool, commonly called a "code reader," that will display these codes and interpret the underlying causes. Some cars can display these codes without a code reader by following certain steps, but this will only give a numeric code that must be looked up.

However, sometimes an engine code will incorrectly indicate a faulty MAP sensor. This is commonly caused by a leak in a vacuum hose or port attached to the intake manifold. Prior to replacing a MAP sensor, inspect the vacuum lines and repair them as necessary. This alone may solve the MAP sensor problems.

Poor Engine Performance

If a MAP sensor is not sending the correct signals to the engine computer, an imbalance of pressure may result in rough engine performance. This will be most noticeable during acceleration or slowing down, as these are the times when there is the greatest difference between atmospheric pressure and the pressure within the manifold. A rough idle is a common manifestation of faulty MAP readings.

Increased Emissions

Many states require that vehicles must pass emissions tests prior to their registration being renewed. If the vehicle was found to release an excess of emissions and other causes have been ruled out, the cause may be a faulty MAP sensor.

Engine Surging

A faulty MAP sensor may result in poor synchronization between the air intake and the release of gasoline from the fuel injectors. Delayed acceleration followed by a sudden surge is a common sign of this type of problem.

Fouled Spark Plugs

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If the engine does not properly regulate the flow of air, the engine will regularly run "lean" or "rich." This refers to the engine performance in relation to whether the fuel is burning with too much or too little oxygen. While an engine running "lean" will experience depleted horsepower, an engine running "rich" will be obvious from an inspection of the spark plugs. The spark plugs will become fouled, meaning that they are coated with residue resulting from inefficient combustion.

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