In this article, I want to introduce you to the seldom-used diagnostic power of the Manifold Absolute Pressure (MAP) sensor. A tech asked me to come by and look at a Dodge 1500 pickup with a P0305 code that indicated a cylinder 5 misfire. The tech’s primary tool was a code reader. His question was, is there a quick way to determine which direction to take in the diagnostic procedure?
I hooked up my scan tool and saw the data in Figure 1 on the screen. The idle was surging at about 200 RPM. Normal idle should be steady with no more than a 50 RPM variation. This was the first clue.
The MAP voltage readings should range between 0.9 to 1.5 volts. This vehicle is high at times, indicating pressure pulses in the intake.
Erratic RPM and MAP sensor readings normally indicate an internal misfire condition due to improper sealing of the cylinder. The next step in the process is a cranking MAP sensor test. To get the best effect of using this test, select only the MAP sensor and possibly the RPM readings for a quicker screen update on your scan tool.
To perform the test, pull the fuse or relay for the fuel pump and run the engine until it dies. Have the throttle closed when cranking the engine.
In Figure 2, notice the pulses in the signal as the engine is cranking.
The initial pull is close to three inches (2.65”) of mercury, going from 29.23-26.58 inHg. As we continued to crank the engine, the pressure would build at the MAP sensor and in the intake manifold.
The two tests indicate a mechanical problem with the engine. In the case of this Dodge V8, it turned out to be a broken valve spring.
Figure 3 is an example of a good reading on a MAP cranking test — good cranking speed with only 1 psi pressure change. To get the inches of vacuum, multiply the MAP sensor pressure number by two. This example has 2 inches of vacuum. A good engine will pull 3 to 5 inches of vacuum while cranking.
On this example, it would be a good idea to plug off the idle air bypass and check again, if necessary.
Vacuum Gauge vs. MAP Sensor
Why not use a vacuum gauge to perform the same scan tool test with the MAP? First, you can’t record the readings with a normal vacuum gauge. Second, the vacuum gauge measures vacuum pressure while the MAP is measuring barometric pressure.
Barometric pressure is the atmospheric pressure pressing down on the earth. It can be influenced by elevation and changes in air density. Old barometers used an open container of mercury (element Hg on the periodic table) with a glass tube in the center. As the atmospheric pressure changed, the level in the glass tube would change. This movement is measured in inches, and why the data PID for the MAP is inches of mercury or inHg. Modern MAP sensors use a capacitive pressure sensor that senses pressure changes on a small diaphragm.
Barometric pressure is influenced by altitude and weather conditions. You can check with the local airport to get the local readings and altitude. If the MAP indicates the BARO, we can use it as a pressure sensor for the intake manifold. On some vehicles, there is a separate BARO sensor mounted in the engine control module.
With the engine not running and the key on, the MAP sensor should read around 28-inHg. When the engine is started and idling, the vacuum of the engine should reduce atmospheric barometric pressure by 20-inHg. The reading on the scan tool scan tool should be approximately 8- to 9-inHg. If you add 20 inHg to the reading on the scan tool, it should equal the outside barometric pressure, or the reading with the engine off.
Altitude readings are different, so check the key on/engine off readings where you are. Some OEMs have a chart that relays the altitude, barometric pressure and voltages in the service information. See Figure 4.
Running the engine with a closed throttle acts as a restriction and creates a negative pressure in the intake manifold. The opening and closing of the valves create positive pressure, but it is lower than atmospheric pressure.
Volumetric Efficiency and MAP Sensors
Add the BARO sensor to your recordings to get another diagnostic indicator. I recently had a Dodge Grand Caravan in with a low-power complaint. A road test at a wide-open throttle gave me the readings in Figure 5.
The BARO updates at wide-open throttle. On this vehicle it decreased, indicating an intake restriction. In this case, a dirty air filter was not replaced for a long time. A normal reading will be the same as the key on/engine off readings, or go slightly higher.
Snapping the throttle should show a vacuum increase after the throttle closes. If no increase is seen, then a restricted exhaust, EGR, or valve timing issues could be the cause.
The MAP sensor can also be used as a transducer when you use a scope and set it to AC voltage. I hooked this to the vehicle using a 9-volt battery as my voltage source. Now we can see the pulses of pressure in the intake manifold. See Figure 6.
I have provided several ways to use the MAP/BARO sensor to give you a quick view of engine condition. Use these steps to determine if you need to do further testing. I use them as a Triage tool, before bringing out my other, more costly tools. Give it a try; it just might save you some time.