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open loop versus closed loop

What does open loop and closed loop mean?

Open loop and closed loop are terms used in computerized auto controls

On Board Diagnostics (OBDI & OBDII) is a term used to describe computerized engine controls to reduce emissions. OBD systems are either in open loop or closed loop depending on engine temperature and which sensors the computer is relying on for data

Your car is in open loop when you start it

When you turn the key to start a cold engine, the computer looks at just a few sensors to determine how to calculate the air/fuel mixture. To calculate the right amount of fuel to start a cold engine the computer needs to know at least the engine temperature and outside air temperature. So it looks at the engine coolant temperature

coolant temp sensor

Engine coolant temperature sensor

sensor and the intake air temperature sensor. Some computerized systems are equipped with a manifold absolute pressure sensor that map sensorcompares atmospheric pressure to intake manifold vacuum. The difference between those two readings allows the computer to calculate barometric pressure. With engine, air and baro readings, the computer can calculate air density and arrive at the right amount of fuel for a cold engine. All of those calculations are based on programming from the factory.

Engine stays in open loop for a few minutes

In OBD I vehicles, the computer watches the engine coolant temp sensor to determine when the engine is reaching full operating temperature. At that point, the oxygen sensor usually reaches a temperature where it can accurately report back to the computer on how good a job it did in calculating a good air/fuel ratio.

However, OBDII regulations tightened up the amount of oxygen sensoremissions a car can emit during warm-up. To get faster oxygen sensor readings, car makers added heaters to the sensors. With the heaters, an oxygen sensor can start reporting live readings in just one or two minutes after start up.

Then it switches into closed loop

The closed loop term is used because it’s the point where the computer is basing it’s air/fuel mixture calculations on readings from all the sensors. In other words, it’s “closing the loop.” In an OBDII vehicle in closed loop, the computer is no longer running off default factory programming.  It’s adjusting air/fuel mixtures based on the real time results it’s getting from the oxygen sensors and adjusting its algorithms to account for actual conditions. In other words, it’s self learning.

Self learning is called Fuel trim

If the computer is in closed loop and it’s not getting the expected readings from the oxygen sensors, it will self adjust and add or subtract more or less fuel to achieve the expected emissions readings. Fuel trim readings can be short term or long term.

Short term fuel trim

Fuel trim readings are constantly changing. Every time you move the accelerator pedal, the computer must recalculate the air/fuel mixture. If the computer notices that it must constantly add fuel in all accelerator positions, it compensates for that by changing Long Term Fuel trim readings.

What’s the difference between long term and short term fuel trim?

Let’s say your fuel injectors have accumulated carbon buildup and are no longer delivering the same amount of fuel as the day they were installed. If the computer learns that it has to keep the fuel injector open 5% longer to achieve the proper air/fuel mixture, the Short Term Fuel trim reading will be +5%. If the computer has to add that 5% at all times, it will deduct the 5% from short term fuel trim and add it to long term fuel trim. In other words, the computer starts its air/fuel calculations 5% higher than it normally would if the injector were delivering the right amount of fuel.

Fuel trim maximums

Fuel trim maximums vary by car maker, but in most cases the computer maxes out at 25% additional fuel. Once it reaches that point, the computer sets a trouble code and lights the check engine light.

How are fuel trims used to diagnose a problem?

If a vacuum line is disconnected, the oxygen sensor will detect a constantly lean condition in the exhaust. To compensate, the computer will add fuel. This is a perfect example of how a technician would diagnose a customer’s symptom of “my car is using more gas,” or “my car lacks power.” If the technician plugs in a scan tool and sees fuel trims in the 15% -25% range, they’ll know right away that the computer is compensating for a vacuum leak or clogged fuel injector or bad fuel pump.

©, 2017 Rick Muscoplat


Posted on by Rick Muscoplat

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