In this article, I'm gonna' show you a simple but very accurate way of testing the manifold absolute pressure (MAP) sensor on your 4.3L, 5.0L, or 5.7L GM car or pickup or SUV.
If your specific GM vehicle is OBD II equipped, and the check engine light (CEL) is lit, you'll see one of the following codes: P0106, P0107, P0108.
If you're driving a pre-1995 vehicle with OBD I, the check engine light is lit by the MAP sensor diagnostic trouble code: 33 (Manifold Absolute Pressure Sensor Circuit High Signal) or 34 (Manifold Absolute Pressure Sensor Circuit Low Signal).
Before we get started, let me tell you that this tutorial will show you how to do a MAP sensor bench test. So you'll need a vacuum pump, that you can rent at your local auto parts store (AutoZone, O'Reilly Auto Parts) and can be accomplished in a few minutes. If you don't have access to a vacuum pump, no big deal, you can just just the 'good ole lungs' (more about this in the tests).
Now, in case you're wondering, “Why a bench test?”, this is due to the fact that so many different conditions can trick the fuel injection computer (PCM = Powertrain Control Module) into thinking the MAP sensor is bad when it isn't. So, the best way to find out if the MAP sensor is truly fried or not is by testing it off of the engine.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar El Sensor MAP (GM 4.3L, 5.0L, 5.7L) (at: autotecnico-online.com).
Symptoms Of A Bad MAP Sensor
When the manifold absolute pressure (MAP) sensor fails, the check engine light will turn on and you'll see one of the following diagnostic trouble codes:
- P0106: MAP System Performance.
- P0107: MAP Sensor Circuit Low Voltage.
- P0108: MAP Sensor Circuit High Voltage.
Besides the diagnostic trouble codes, your GM pickup, van or SUV will experience one or several of the following symptoms:
- Rough idle.
- Black smoke coming out of the tail-pipe.
- Engine stalls under load.
- Engine dies as soon as it starts.
- Lack of power when you accelerate the vehicle down the road.
MAP sensor go bad all of the time but sometimes some other condition (affecting engine performance) fools the PCM (Powertrain Control Module = Fuel Injection Computer) into thinking that the MAP sensor has failed.
To be absolutely sure the MAP sensor is bad or not (without replacing it to find out), you need to bench test it and the MAP sensor test that I'm gonna' show you is a MAP sensor bench test.
If after doing the MAP sensor test and the MAP sensor is good, then take a look at the section: MAP Sensor Code Won't Go Away.
How The MAP Sensor Works
The MAP sensor's job is to measure the amount of vacuum the engine is creating and report it to the PCM.
If the fuel system on your particular 4.3L, 5.0L, or 5.7L is a Throttle Body Injection (TBI) or Central Port Injection (CPI) type, both of these have one thing in common: No mass air flow (MAF) sensor.
This means:
- Your fuel system is a 'Speed Density' system.
- This also means that the PCM uses the MAP sensor and engine RPMs to calculate how much air is entering the engine.
- With both the MAP sensor signal info and crank sensor signal info, the PCM now calculates the approximate amount of air entering the engine and:
- Adjusts fuel injection pulse width.
- Adjusts the ignition timing.
If your specific 4.3L, 5.0L or 5.7L has the Central Sequential Fuel Injection (CSFI), then your vehicle has a mass air flow (MAF) sensor.
The PCM uses the MAF sensor to precisely calculate the amount of air entering the engine and the MAP sensor doesn't play as crucial a role as in the 'speed density' type.
This is what happens when you turn the key and start the engine:
- The PCM feeds 5 Volts to the MAP sensor thru' the wire labeled with the number 1.
- Ground is provided, by the PCM too, thru' the wire labeled with the number 3.
- Once the MAP sensor gets power and Ground, it now starts to measure the amount of vacuum the engine is producing and shoots the info to the PCM thru' the wire labeled with the number 2.
Whether your vehicle is the 'Speed Density' or the 'Mass Air Flow' type, the MAP sensor plays a role in the performance and efficient operation of the engine in your pickup (van, SUV).
TEST 1: Verifying The MAP Signal
To get this show on the road, the very first thing that you'll do, is to see (with a multimeter) if the MAP sensor is able to create a good MAP signal. This is done using a vacuum pump while the MAP sensor is still connected electrically to the vehicle.
There's a good chance you may not have a vacuum pump, in this case you have two options: 1.) Rent one from AutoZone or O'Reilly Auto Parts or 2.) Use your mouth to suck on the MAP sensor via a vacuum hose (using the good ole' lungs is not the most accurate way, but it does work).
At the bottom of the article I have included the three different possible results you'll obtain from this test and how to interpret them.
NOTE: If you don't have a multimeter or need to upgrade yours, check out my recommendation here: Buying A Digital Multimeter For Automotive Diagnostic Testing (at: easyautodiagnostics.com).
NOTE: The test instructions assume you're using a vacuum pump to test the sensor. You can also use your mouth to apply a vacuum to the MAP sensor. When using your mouth, the important thing to notice is the voltage decreasing/increasing as you apply/release vacuum.
OK, to get this show on the road, this is what you need to do:
- 1
Remove the MAP sensor from the engine.
IMPORTANT: If you needed to disconnect the MAP sensor's electrical connector to remove it or its vacuum hose, reconnect it now since this test requires that the MAP sensor be connected to its 3-wire electrical connector. - 2
Connect the MAP sensor to the vacuum pump with a piece of vacuum hose.
NOTE: The vacuum hose should fit tightly around both the vacuum pump and MAP sensor's inlet vacuum nipple. - 3
Place your multimeter in Volts DC mode.
- 4
Connect the red multimeter test lead to the wire labeled with the number 2 (see the image above).
NOTE: The MAP sensor harness connector needs to be connected to the MAP sensor, so you'll need to either back-probe the connector or use a wire piercing probe to get to the signal inside the wire (to see what a wire piercing probe looks like: Wire Piercing Probe Tool). - 5
Connect the black multimeter test lead directly to the battery negative (-) terminal.
- 6
Turn the key to the ON position but don't start the engine.
This will enable the manifold absolute pressure (MAP) sensor to get power in the form of 5 Volts and Ground from the PCM (Powertrain Control Module= Fuel Injection Computer). - 7
Your multimeter should register 4.7 Volts DC at this point with the Key On Engine Off and without any vacuum applied to the MAP sensor.
- 8
Apply vacuum to the MAP sensor.
As you apply vacuum to the MAP sensor, your multimeter should register the following voltages at the following vacuum values (they may differ a little on your specific GM car or pickup or SUV or van or mini-van):
1.) 0 in. Hg ...... 4.7 Volts.
2.) 5 in. Hg ...... 3.9 Volts.
3.) 10 in. Hg .... 3.0 Volts.
4.) 20 in. Hg .... 1.1 Volts.
Repeat this test step several times and each time, you should see the same values on your multimeter.
OK, let's take a look at what your vacuum pump test results mean:
CASE 1: Your multimeter displayed a decreasing voltage signal as you pumped up the vacuum gauge. Your multimeter test results confirm that the manifold absolute pressure (MAP) sensor on your GM van (mini-van, car, pick up, SUV) is OK and not the cause of the problem or issue. No further testing is required.
Now, if your vehicle still has the MAP sensor code lighting up the check engine light (CEL) on your instrument cluster, take a look at the section: MAP Code Won't Go Away for more info.
CASE 2: Your multimeter DID NOT display a decreasing voltage signal as you pumped up the vacuum gauge. This confirms that the manifold absolute pressure (MAP) sensor on your GM van (mini-van, car, pick up, SUV) is bad and needs to be replaced. Replacing the MAP sensor will solve the issue.
CASE 3: Your multimeter DID NOT register any voltage. This usually means that the MAP sensor is fried. To be absolutely sure, I suggest confirming that the MAP sensor has power and Ground. If both (power and Ground) are present, the MAP sensor is bad. To test for power, go to: TEST 2: Verifying The MAP Sensor Has Power.
