MAP stands for "manifold absolute pressure." The MAP sensor reads the vacuum levels from an inlet in the intake manifold. This vacuum level data then gets picked up by the vehicle's computer, or ECU, which makes fuel adjustments according to load and speed. MAP sensors that have obstructions or leaks can cause a rough idle, stalling and fouled spark plugs from overly rich mixtures. Troubleshooting a MAP sensor requires a few steps and tools that can be accomplished by the do-it-yourself repair person.
Place the vehicle in park or neutral with the emergency brake set. Raise the hood. Remove the air cleaner lid with a socket and wrench, or unclasp the fasteners by hand. Remove the air cleaner element and make sure it looks clean and porous. If in doubt, replace it for the MAP test. Hook up a vacuum gauge to a hose that inlets into the intake manifold. Start the engine and take a reading. Be certain that your vehicle does not have any pre-existing vacuum leaks before you prepare to test it.
Connect a trouble scanner to your vehicle's scanner connector switch, located at the bottom of the dashboard on the driver's side. Start the engine and turn the scanner on. Read any trouble code that appears on the gauge. Write the number down. Refer to your trouble code book. If you see a code number that references a faulty MAP sensor, you have narrowed the problem. Continue with the next procedure.
Locate your MAP sensor by referring to your vehicle repair manual. It should be located on the side or top of the intake manifold. It looks like a small plastic box about the size of a cigarette pack. It will have a vacuum line and small wire connection jack. Use a socket and wrench to loosen and remove the two small mounting bolts on the sensor. Pull the sensor up and turn it over. You will see three wires going into the sensor. Stick a paper clip into the "Signal" wire opening, which should be green.
Remove the vacuum line from the sensor nipple, and the small gasket on the nipple. Hook up the hose on a hand-pump vacuum gauge to the sensor nipple, and set it aside. Hook up the red lead from a multimeter to the paper clip end stuck in the hole. Hook up the black lead of the multimeter to a good metal ground source. Set the multimeter to DC volts. Turn the ignition key to the "On" position. Not the start-run position.
Read the voltage on the multimeter. It should measure about 5 volts, no more or less. Pump up the hand-pump vacuum tool so it reaches 5 inches Hg. Look at the multimeter; it should now drop 1 volt and rest on 4 volts. Pump the vacuum tool again to reach 10 inches of vacuum and watch for another 1 volt drop, which will now read 3 volts. Pump the tool again until you have 15 inches of vacuum. The volt reading should now be 2 volts. Pump the vacuum tool to 20 inches of vacuum and look for a last reading of 1 volt.
Keep the vacuum gauge connected and pumped to 20 inches and 1 volt. Watch it for several minutes to see if the vacuum reading bleeds down. If it gradually starts to lose vacuum, you have an intake manifold leak. If the voltage readings appear lower or higher during the vacuum pump testing intervals, the fault lies with the MAP sensor and it must be replaced.