My approach to running Sunnyvale Foreign Car Service is that when customers leave the shop, they have learned something about their vehicles.
It’s rewarding to hear from readers and find out what they – and we – have learned. I feel that many of you have a good grasp of various automotive systems.
As I was writing last month’s column, it dawned on me that new readers might be lost about what oxygen sensors and manifold absolute pressure (MAP) sensors do. Two days after the column was published, a friend of mine who just started reading my columns told me that she liked the flow of the piece but did not understand the technical side of it. So this time around, I will explain the oxygen sensor and the MAP sensor in depth.
The true driving force for both sensors is the environment and fuel savings. In the early 1970s, there was a big push to cut air pollution and lower our dependency on oil. Robert Bosch LLC started developing the first oxygen sensors as early as 1974 but did not put them in a car until 1976. The Volvo 240 series cars were the first to adopt the technology.
Oxygen sensors
Oxygen sensors are approximately 3 inches long and 1 inch in diameter. They sit in the exhaust manifold before and after the catalytic converter. The purpose of this sensor is to measure the amount of oxygen in the exhaust. Fire or combustion needs to have the right amount of fuel and the right amount of oxygen. If a fire has plenty of fuel but no oxygen, it will not burn. If a fire has oxygen but no fuel, it will not burn. The term for perfect combustion is stoichiometric. The stoichiometric calculation is 14.7 parts of air to one part of fuel.
The example I use to explain this is lighting a match. The beginning and end of a match’s life is problematic, but when the match is burning in the middle of its life, it burns perfectly. The oxygen sensor’s job is to keep the match, or the combustion chamber, in a perfect stoichiometric state. If the engine has a vacuum leak (air leak), the oxygen sensor will see this in the exhaust and command the engine control module (ECM) to add more fuel. Transversely, if an injector were to stick and spray too much fuel, the oxygen sensor would know that there is not enough air and command the ECM to subtract fuel.
Oxygen sensors have evolved immensely since the 1970s and are considered one of the great advancements in automotive emissions.
MAP sensors
I do not have as much information on the origin of the first MAP sensor, but I know it was created in Silicon Valley. The MAP sensor’s job is to control the amount of air and fuel coming into the engine. The MAP sensor is roughly the size of a golf ball, with a small hole or tube on one side. It sits on or near the throttle body/intake manifold, and the tube is placed facing down on the manifold.
Inside the MAP sensor is a small cavity with a small piece of silicon chip placed over the hole. The MAP sensors cavity is precalibrated with vacuum. The silicon chip is attached to the sensor. When the car accelerates, the pressure drops in the intake and the chip pulls into the MAP sensor, commanding the ECM to add more fuel. When the car returns to idle, the chip returns to its set position and slows down the flow of fuel.
Just like the oxygen sensor, the MAP sensor controls the air fuel mixture. These two sensors are not the only things controlling fuel, but they are two of the most important ones.
Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call 960-6988, email [email protected] or visit sunnyvaleforeigncar.com.
