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How to Tell if You Have a Bad MAP Sensor

Alex Palmeri
April 6, 2018

At its core, an internal combustion engine is a vacuum pump. The pumping motion of the pistons causes a constant change of pressure in the intake manifold. The manifold absolute pressure (MAP) sensor monitors this and reports the information to the engine’s computer. 

If the MAP sensor starts to fail, these are some of the most common symptoms:

1. Your engine accelerates poorly: If a MAP sensor fails, it can cause the engine’s computer to miscalculate the amount of fuel and ignition timing needed for proper engine performance. This will cause the engine to run poorly and lose power. Turbo and supercharged engines will suffer even more due to their reliance on the MAP sensor for a boost in fueling and timing needs.

2. Your engine misfires and shakes: If a MAP sensor reports a false high pressure reading, the engine’s computer will signal for more fuel. This results in a rich mixture, which can foul the spark plugs and cause a cylinder not to fire. A misfiring engine will shake and transmit that motion into the cabin of the vehicle. 

3. Your engine pings: In some cases, a failed MAP sensor can cause the engine to run lean or the spark plugs to fire sooner than they should. This will result in detonation, which creates a pinging sound from the engine when you accelerate. In some cases, detonation can cause catastrophic engine failure, meaning the engine would have to be rebuilt or replaced.

4. You start getting worse fuel economy: In some cars, a failed MAP sensor will cause the engine’s computer to default to a safer, rich fuel mixture. This will decrease your fuel economy.

5. Your car surges and dies out: A faulty MAP sensor can cause engine RPM to fluctuate or surge, primarily at idle or low speeds. If you turn on the air conditioning or use the power steering when the happens, the engine can die out. It will usually restart, but this condition will worsen and can be dangerous.

6. Your car fails to start: The engine’s computer uses the MAP sensor to gauge atmospheric pressure before the engine is started. A false reading could cause too much or too little fuel to be delivered to the engine. Both conditions could keep it from firing up.

7. Your check engine light is on: The engine’s computer can monitor MAP sensor function by comparing its data with other engine sensors. For example, if the throttle position sensor reads 100% open and engine RPM is climbing, but the MAP sensor is reporting high vacuum, the ECU knows there is an issue with the MAP sensor, as the vacuum is always greatest at idle or low cruising speed. This will set a fault code in the computer and illuminate the check engine light. 

8. Your car goes into limp-home mode: Most modern vehicles can implement a safety feature called limp-home mode. This happens when the engine detects an issue with a sensor that can cause damage to the engine or drivetrain. In this situation, you will experience reduced engine power, and you may be stuck in one or two gears. This will get you off the highway and allow you to drive a few miles to a repair shop, but you shouldn’t drive any more than needed if this happens.

Get it diagnosed by a professional
 

How a MAP sensor works

MAP sensors are used in both gasoline and diesel engines, and are attached either directly to the intake manifold or by a vacuum hose. 

When you turn the key (or press the start button) in your car, the engine’s computer — also called the engine control unit or ECU — sends a voltage reference to the MAP sensor. The MAP sensor contains a pressure-sensitive electrical circuit that can monitor the movement of an internal rubber diaphragm and change the electrical output signal back to the ECU. 

As the throttle is opened further and pressure increases inside the intake manifold, the diaphragm moves, and a new voltage signal is sent back to the ECU to indicate engine load. The ECU uses this data, along with information from other engine sensors, to determine the proper amount fuel and ignition timing needed to run the engine efficiently.

MAP sensors are also used on some newer vehicles to monitor exhaust gas recirculation (EGR) valve function, which plays a crucial role in the vehicle emissions system.

» MORE: Get an accurate estimate for your car repair

MAP sensor repair advice

A MAP sensor is not a maintenance item and does not have specific service interval. Since it does have a moving rubber diaphragm, it can fail but, it will typically last the life of the engine.

If a MAP sensor is suspect, a professional repair facility will perform electrical testing on the sensor. A scan tool can also display live data showing how the sensor is performing and sometimes determine if it’s faulty.

The cost of replacing a MAP sensor can vary. Some of them are easy to find and replace, and some are located in a spot that will require a lot more labor. 

The MAP sensor is specific to each make and model, and high-quality replacement parts from the original manufacturer are highly recommended. If you use aftermarket parts, make sure they are of equal quality This is a very important sensor on your engine, and you shouldn’t take a chance with a cheaper part that may fail quickly or without warning.

If you do it yourself

Replacing a MAP sensor can be straightforward, especially if it’s mounted in an accessible area on the intake manifold or an air intake tube. The hard part of the procedure is the diagnosis. If you don’t have the proper diagnostic equipment, or aren’t familiar with engine management systems and electrical circuit diagnosis, it will likely be cheaper and easier to let a professional work on it. 

The MAP sensor is about the size of your key fob. It has an electrical connector attached to it that normally houses three wires. On most engines, there are two smaller screws or bolts that secure it to its mounting location. Sensors mounted on the intake or on an intake tube will have a seal on the bottom side, and remotely mounted sensors will have a nipple that a hose or tube goes over. Both need to be airtight to function properly.

The MAP sensor is secured using small screws that can break easily. Pay close attention to their torque requirements and never overtighten the screws.

After the part is replaced, you’ll need to reset the ECU to clear out the diagnostic trouble code, and you should take test drive to verify the issue has been fixed. 

 

Alex Palmeri

About the Author

Alex Palmeri worked nine years as a master technician at Mercedes-Benz of Chicago and is currently the foreman at a large fleet garage. He writes about automotive news, maintenance and racing, and runs a YouTube channel called Legit Street Cars.

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