An array of computerized sensors closely regulates the engines of modern cars. These include the computer brain, the ECM (Engine Control Module), which monitors airflow via either a MAF (Mass Air Flow) or MAP (Manifold Absolute Pressure) sensor.
Naturally-aspirated power plants will typically use one of these sensors to monitor the engine’s performance. Vehicles with turbocharged engines will often use both.
The MAP sensor, then, is a vital link in the chain. If it fails or begins playing up, the ECM cannot function properly, and neither will the engine. To ensure your engine works at peak performance, it is wise always to keep your MAP sensor in tip-top condition.
How The MAP Sensor Works
Usually, the map sensor is mounted onto the intake manifold, but this is not always the case. In some instances, the MAP sensor may be connected to the intake manifold via a hose.
The MAP sensor sends critical data to the ECM to enable it to work out its engine management calculations. The information sent will include, for example, the load on the engine, spark advance, and what the fuel injector pulse is doing.
When it is not gathering this data, the MAP sensor measures the atmospheric pressure at sea level. It uses this reading as a baseline as atmospheric pressure varies with altitude and weather conditions. The ECM uses this baseline to calculate a ‘zero-point’ immediately before the engine starts up. As a result, the ECM can fine-tune the spark and fuel-injection mapping for optimum efficiency.
At idle, the intake pressure will typically be below atmospheric pressure; the air is rammed into the intake. Likewise, when the driver uses engine braking, air pressure can drop even further. However, when accelerating, an open throttle will let air get in much faster, increasing the intake manifold pressure.
When wide open, the throttle will almost equalize the intake manifold pressure to close to atmospheric pressure.
Telltale Symptoms Indicating a Bad MAP Sensor
The first sign of a bad MAP sensor is the generation of a diagnostic trouble code and the check engine light illuminating. Often, MAP sensors malfunction due to becoming clogged, damaged or contaminated.
Another common culprit is under bonnet engine heat cooking the MAP sensor’s chips or cracking its vacuum lines. If the MAP sensor decides to quit, it’s bad news for the ECM. Without the information stream from the MAP sensor, the ECM cannot accurately reckon the engine load. This data loss will result in the management of the air to fuel ratio going to pieces. Either it will run rich (too much fuel) or lean (too little fuel).
So how do you know when a MAP sensor is on the way out? Here are a few of the common indicators.
Decreased Fuel Economy
With no information from the MAP sensor to work with, the ECM will assume the engine is under a high load. A failing MAP sensor tricks the ECM into believing the reading is low or there’s no vacuum. Consequently, the ECM fires in extra fuel and advances the spark timing to compensate. This massively increases the fuel consumption.
Down On Power
Conversely, if the ECM thinks there is a high vacuum reading, it not unreasonably concludes the engine load is low. It then cuts the fuel injection levels and retards the spark. While better fuel consumption may seem like a bonus, you will notice a decrease in power and acceleration.
Emissions Test Failure
Another outcome of a dud MAP sensor is an increase in the release of harmful climate-killing emissions. This is because fuel injection settings don’t correspond directly to the engine load. Burning excessive amounts of fuel produces higher levels of hydrocarbons and carbon monoxide. An engine running lean will jack up the level of nitrogen oxide emissions.
How your engine performs at idle is indicative of a MAP sensor failure. Too lean a fuel mix, and the engine becomes starved of fuel, leading to uneven idling and even cylinder misfires.
Reluctant to start
An air to fuel ratio that’s out of kilter will make the engine harder to start. If it will only fire up with the accelerator depressed, then there’s a good chance a broken MAP sensor is to blame.
Stalling and hesitation
You’re likely to feel down on power when overtaking or moving off from a stop. Likely a faulty MAP is feeding the ECM garbage data.
Check Engine Light is On
This dashboard light is likely to illuminate when the MAP sensor is acting weird. Diagnostic trouble codes could range from simple circuit faults to something more sinister. A MAP sensor that has died won’t send any readings to the ECM. One that’s on the way out will give the ECM contradictory information, with this nonsense data showing up as rough idling.
Troubleshooting a MAP Sensor
An inexpensive but useful tool for troubleshooting a range of engine problems, including dying MAP sensors, is a Bluetooth OBD2 scanner.
As a fully functional MAP sensor is a crucial engine management component, it’s necessary to ensure it is working reliably. If you suspect a faulty MAP sensor is in play, check these things out first.
Check the MAP sensor’s wiring and connector. It should be connected securely, and the pins free of dirt and undamaged. Bent or corroded pins will also adversely affect the sensor signals. The wiring from the ECM to the MAP sensor should also show no signs of damage. Wiring that has chaffed can cause short circuits, while breaks can result in open circuits.
If your MAP sensor is linked to the intake via a hose, check it is not damaged. Check, too, that the port is clean and unblocked as this can lead to unreliable MAP readings.
If the sensor checks out okay, the next step is to try a scan tool or voltmeter and vacuum gun to gauge the MAP sensor’s output. Check the voltages you need to see for full and no vacuum. If you don’t get the expected readings, you can conclude the sensor needs replacing.