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MAF vs. MAP Sensors

Both mass airflow (MAF) sensors and manifold absolute pressure (MAP) sensors supply the computer with information about air entering the engine. But they do it in different ways.

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MAF Sensors

Information from the MAF sensor is necessary for the engine control unit to balance and deliver the correct fuel mass to the engine. Air changes its density with temperature, and pressure. In automotive applications, air density varies with the ambient temperature, altitude and the use of forced induction, which means that MAF sensors are more appropriate than volumetric-flow sensors for determining quantity of intake air in each cylinder.

There are two common types of airflow sensors used in automotive engines: the vane-meter sensor and the hot-wire sensor.

The vane-type MAF was the first one out there and was used on import vehicles from the 1970s and 1980s. It didn’t have many actual problems. However, many of them were replaced, because back then the vehicles didn’t have onboard diagnostic capabilities. Usually after mechanics and DIYers had replaced all the other ignition parts and sensors, the MAF sensor was the last-ditch effort.

The old vane-type MAF sensors don’t provide many sales opportunities by themselves. However, it can be a good “know-how-to-ask” conversation starter. Ask the customer if the vehicle has been tuned up lately. Something that few mechanics and DIYers consider is that ignition tune-up items such as spark plugs, spark plug wires, the distributor cap and the distributor rotor can flash-back high-energy voltage through the ignition coil and damage computers and sensors. The reason for that is when the gap widens too much from normal use, it causes resistance in the secondary ignition system. That high-energy voltage is looking for somewhere to go. As the ignition coil houses both secondary and primary wiring, the high energy jumps in the primary circuits and it heads for the computer and other electronics. 

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The other type of mass airflow sensor is the “hot-wire” type of sensor. 

With this one you can breathe a little easier. This type of mass airflow sensor uses a heated wire in the middle of the intake air duct that’s heated by the computer. As air flows over it, that cools the hot wire and the computer can calculate the amount of fuel to supply the engine with. 

There are a lot of sales opportunities with the late-model or “hot-wire” MAF sensors. You can recommend mass airflow cleaners and air filters as maintenance items before spending the money on the sensor. These items may prevent the sensor from being returned as defective. 

MAP Sensors

The MAP sensor is taped into the manifold and watches the changes in vacuum pressure to let the computer know how much fuel to give the engine. When selling a MAP sensor, it’s good practice to have the conversation concerning the overall condition of the engine and exhaust system. Anything that affects the breathing of the engine will affect the manifold pressure and, consequently, the MAP sensor. 

One time I had a vehicle come into my shop running poorly with a MAP-sensor code. As I had always done in the past, with the engine running I slipped off the vacuum line of the sensor and used my fingertip to see if it was getting vacuum. I replaced the sensor and, to my dismay, the problem returned after a test drive.  It was time to “stop, look and listen.” I raised the vehicle up on the hoist and started following the vacuum line with my eye. I noticed that where the vacuum line passed through a little bracket it took a funny angle. I removed it and found a tiny crack in it. When the engine was twisting a little on the road, it would open that little crack up. Line replaced; problem solved. 

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When your customers have repeat failures, have a long conversation that includes “stop, look and listen.” 

Charles Dumont is an ASE-certified counter professional with NAPA Auto Parts in Shelton, Washington. A regular contributor to Counterman, Dumont is the 2020 NAPA/ASE Parts Specialist of the Year.

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