Questions and Answers

Pontiac Gram Am DTC P0108 Q. I have a 1996 Pontiac Gram Am SE. It has a 2.4 liter 4 cylinder engine. and 102,000 miles on it. The check engine light has come on. I took it to Autozone and they diagnosed the problem code for me. I am still confused. The symptom is rough idle and some smoke out the exhaust. I have install new plugs and wires. Changed the oil and flush the radiator.

Here is the code they gave me: P0108 - "The PCM has determined that the manifold absolute pressure sensor or the barometric pressure sensor input is higher than expected for the current engine operating conditions" Any help you could give would be appreciated.

Sincerely,
Don
Marion, AR

A. It could just be a bad signal from the MAP sensor or a poor connection at the sensor. Or a bad MAP sensor itself. Maybe the vacuum line is off or broken.

Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which result from engine load and speed changes, and converts this measurement into a voltage output.

A closed throttle on when the engine is decelerating would produce a relatively low MAP output, while a wide open throttle would produce a high output. Manifold absolute pressure is the opposite of what you would measure on a vacuum gage. When manifold pressure is high, vacuum is low. The MAP sensor is also used to measure barometric pressure under certain conditions, which allows the PCM to automatically adjust for different altitudes.

The PCM sends a 5 volt reference signal to the MAP sensor. As the manifold pressure changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage, the PCM can determine the manifold pressure. A higher pressure, low vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low voltage) requires less fuel.

A high or low voltage fault in the MAP sensor circuit should set a DTC P0107 or DTC P0108. If the MAP sensor sends an intermittent zero voltage signal to the PCM, a DTC P0106 will set.