Troubleshooting a MAP Sensor
If you find that your car is suddenly getting bad gas mileage, or mileage has worsened after a tune-up, then the device responsible could be the MAP sensor. These little devices pick up on the voltage being delivered by the car engine, and then relays this information to the car's computer, which then causes it to decrease or increase the fuel consumption. But if your MAP sensor is feeding the wrong information to your computer, then you will find that combustion is disrupted, ragged or even excessive, and your engine performance is poor. In order to decide if your MAP sensor needs to be replaced, you should try a few troubleshooting steps.
Check the MAP Sensor Pump
Locate the MAP sensor in the car, and look at the accessories surrounding it. You should look at the hose, which the sensor uses to make its calculations. If this is dirty, or there is debris inside, then you need to clean out the hose. You can also remove this vacuum hose and use a manual pump to remove any remaining air in the system in order to check that the sensor is correctly picking up on the flow of air. You should apply a small amount of vacuum, which will produce about 3 to 4 volts. Apply a heavier vacuum, which should produce around 1 volt. Any other readings could mean that the sensor is not working, and you need to replace it with a fresh MAP sensor. Check the wires around the sensor, and make sure that these are all firm, and that there is no corrosion on the sensor itself, or any of the attachments. You should then use a voltmeter, to make sure that the sensor is properly configured.
Checking the Electrical Connection
If you suspect that the electrical connection of your MAP sensor is responsible for the poor communication, then you need to check this out using a voltmeter. Remove the electrical connections from the back of the sensor, and turn your ignition, without starting the engine. Measure the voltage, by touching the loose connections with the meter's lead. If you find that the connection is out of specification, then you need to remove the wiring, and replace it with some fresh pieces.
Using a Tachometer
If your MAP sensor operates using a frequency, rather than a direct current, then you need to use a tachometer to work out the problems. Plug in the sensor's electrical connection, and attach the tachometer to the ground wire. Attach the positive probe of your meter to the signal wire, and check the wires. Your frequency should be only 5 volts of signal. Anything more than this suggests a problem with the sensor.
Check the Computer
You should also check the computer of your MAP sensor. Using a fresh sensor, pump in enough air to give a 3 to 4 reading, and then check the amount of voltage which the computer thinks it is receiving. Discrepancies suggest that the computer connections need to be changed.