Bad MAP Sensor – Symptoms and Solutions

Bad MAP Sensor – Symptoms and Solutions

If your engine is running poorly, it can have a serious effect on your wallet. Pinging and poor acceleration can destroy your fuel economy, and long-term damage can make services far more expensive. One of the common causes of a poorly running engine is a failed sensor.

In this article, we will be talking about the MAP sensor. We’ll start by discussing the symptoms, then talk about sensors and computers and what they do. At the bottom of the page, we’ll walk through the steps to replace a MAP sensor.

Symptoms of a Bad MAP Sensor

A bad MAP sensor will cause a host of issues. It’s an integral part of the engine’s fuel management calculations, so most of the issues have to do with the fuel and air mixture. Before we start, there are a few terms that we need to define.

Glossary

  • Rich: If a car is running rich, it means that there is too much fuel being added
  • Lean: If a car is running lean, it means that there isn’t enough fuel being added
  • Pinging: Pinging is a hollow, rhythmic and metallic sound that you will hear as the motor runs
  • Misfire: If the fuel and air mixture in a cylinder burns at the incorrect time or fails to burn, it’s known as a misfire.
  • Computer: The engine control unit (ECU) or engine control module (ECM).

Issues Caused by a Bad MAP Sensor

Unfortunately, many of these symptoms have multiple causes. There are many reasons that a car can be running lean or rich, and there are other sensors that can fail and cause the same result. If you have these symptoms, though, checking the manifold absolute pressure (MAP) or mass air-flow (MAF) sensor is a good first step.

The Check Engine Light Is Illuminated

The first clue that an airflow sensor is going bad will be that the check engine light will come on. In many cars, the engine light has two different modes: constant and blinking. A blinking check engine light tells you that you need to stop driving and that something is seriously wrong, while a constant light means that you should check for problems when you are able.

Generally, unless your car is running very poorly and misfiring, you will get a solid check engine light that will gently prompt you to get your engine looked at.

Your Engine Runs Poorly

A common issue caused by a bad air sensor is that the engine will run poorly. When engines don’t get the correct amount of fuel, they have trouble running efficiently. That leads to misfires, lack of power, and surging. It will fail to hold idle (the RPMs will climb and fall randomly while sitting still), and it may start taking much longer to accelerate to highway speeds.

The Gas Mileage Suffers

An engine running poorly can also cause your fuel economy to suffer greatly. Especially if the failed sensor is causing the car to run rich. You’ll notice that it smells like unburned fuel when you walk around the car. That unburned fuel making its way through your exhaust is part of the reason your gas mileage is dropping.

You Notice Funny Noises

Unusual, rhythmic engine noises can occur from lean fuel mixtures. Pinging is the result of the combustion happening too soon due to excessive cylinder temperatures. It’s not harmful unless it occurs for long periods.

If your engine is pinging, the first thing you should try is running a higher octane gas. Running lower octane gas in an engine built for higher octane is a surefire way to cause pre-detonation, and it will affect your fuel economy to the point that you won’t be saving any money.

The Car Takes Longer Than Usual to Start

If the engine takes a while to start, it could be because the sensor cannot transmit the correct data for the startup. Usually, a cold engine needs more fuel to start. If it doesn’t get that extra fuel, it can make starting difficult.

What Is a MAP Sensor?

The MAP sensor, which stands for manifold absolute pressure, is a small electronic device that gets attached to the intake manifold and measures the volume of air that the engine is taking in. It does that by comparing the atmospheric pressure to the pressure inside the intake manifold.

The ECM

ECMs go by a lot of names. The most common is simply “the computer,” and often gets abbreviated ECU or PCM depending on the manufacturer. Technically, a PCM includes more than just an ECU, but they function the same for the purpose of this article. It stands for engine control module (engine control unit,  powertrain control module) and what it does is calculate things. Things like when the spark plugs should fire and how much fuel should be added.

Modern cars rely heavily on the computer to make most of the decisions that happen under the hood, and the computers rely heavily on sensors. In order to calculate how much fuel to add, for instance, the computer needs to be able to measure the amount of air going into the motor. That way, it can maintain the correct fuel to air ratio.

MAPs and MAFs

To measure the air going into a motor, cars have one or both of the following sensors: a manifold absolute pressure, MAP, sensor, and/or a mass airflow sensor, MAF. The main difference between the two sensors is where they are placed. A MAP sensor is in the intake manifold. A MAF sensor is directly behind the air filter to measure the air going into the manifold.

A bad MAF sensor’s symptoms are identical to a bad MAP sensor’s symptoms since they fill the same role on different engines.

Replacing a MAP Sensor

upper and lower intake on a four cylinder engine

On most vehicles equipped with a MAP sensor, it is fairly simple to replace the sensor itself. It’s a project that most people are capable of tackling in a few hours using only hand tools.

Doing It Yourself

The following steps are all that are necessary for replacing a MAP sensor.

Determine if Your Vehicle Has One

The first step is to determine if your vehicle has a MAF sensor or a MAP sensor. Some cars may have both. The easiest way is to do a quick google search for your make and model of car and airflow sensor. Chances are good that the first pages will be titled with either “… MAP sensor …” or “… MAF sensor ….”

Purchase a New Sensor

MAP sensors cost between $10 and $200. Your car’s make and model, combined with whether or not you have to special order the sensor, will greatly affect the price. Vehicles like the new Subaru Outback have a very specialized sensor that costs over 200 dollars and they must be ordered from Subaru.

Meanwhile, GM products tend to have very inexpensive parts that are available anywhere. Cheap sensors that are available on Amazon like this one for older GM vehicles are perfectly fine. The best thing to do is to simply google the part you are looking for. The first entries will give you a good idea of what you can expect to pay.

Locate the Sensor

Even if you don’t own a service manual that can simply tell you the location, the sensor is easy to find. First, find your air filter. Then trace back the tubing from the air filter to the intake manifold. On most cars, it looks like a bunch of metal or plastic tubes going into the motor. Somewhere on that intake manifold will be a few wires going into a black plastic box that’s about the size of a 9v battery. That’s the sensor.

Carefully Disconnect the Sensor

Undo the connection to the sensor. You may need to gently wiggle the connector back and forth while depressing the clip with a screwdriver. Be careful with these plastic connectors, especially on older vehicles. They are very easy to break since the heat of the engine can make plastic very brittle over time.

Clean the Area

Spraying a gentle degreaser and wiping up around the area can help prevent debris from falling into the engine. It will also make things like bolts and clips easier to find. Cleaning the area you are working on in an engine bay is just a good habit no matter what you are doing.

Remove the Sensor

Some sensors are simply held in-place by friction or a small spring, while others may have a bolt or two holding them in place. Try to avoid getting dirt or oil in the hole where the sensor used to be. Anything dropped into the hole will go straight into the motor since you are beyond all the filters.

Place the New Sensor and Plug It In

When you push the new sensor in-place, make sure it is seated down fully. On vehicles that have both a MAF and MAP sensor, you may need to drive around for a while before the computer “learns” to adapt to the new sensor.

Paying a Mechanic

If you don’t feel comfortable or don’t have time to do it yourself, a mechanic should be able to do it in less than an hour. Hourly rates where you live might be wildly different, but you can expect to pay between 100 and 300 dollars in general. It will probably be about 50-250 dollars for the new sensor since they will mark it up, and between 75 and 150 dollars for an hour of the technician’s time.

Enjoy Peak Efficiency

MAP sensors are an engineering marvel, and they are integral parts of your car’s motor. If they fail, the lack of efficiency can be both annoying and dangerous. We hope this guide helps you solve the issue so that you can know that your car is breathing as well as it can.