Understanding: Open & Closed Loop Operation

[Luke]

OPEN & CLOSED LOOP OPERATION

The PCM operates in two different fuel control modes:

Open Loop and Closed Loop.

Whenever the vehicle is first started, the PCM operates in "Open Loop" fuel control. When the Powertrain Control Module (PCM) determines that the engine coolant temperature (ECT) sensor has reached at least 20 degrees C (68 F), and the oxigen sensor has reached operating temperature of 318 degrees C (600 F), it will then go into Closed Loop fuel control operation.
Closed Loop fuel control operation will be maintained as long as certain parameters are met.
In "Closed Loop" fuel control, the PCM varies the fuel to the engine according to signals received from the oxygen sensor, located in the exhaust manifold. The oxygen sensor varies a voltage signal to the PCM, indicating the oxygen content of the exhaust gases. If the oxygen sensor signals that the air/fuel mixture is lean, the PCM will increase the amount of fuel to the engine. If the oxygen sensor signals that the air/fuel mixture is rich, the PCM will decrease the amount of fuel to the engine.
During "Closed Loop" fuel contol operation, the PCM is constantly adjusting the amount of fuel to the engine, according to signals received from the oxygen sensor, to try and obtain a 14.7:1 air fuel ratio.
If there are any oxygen sensor related PCM/EC diagnostic trouble codes, or if the oxygen sensor does not switch between rich and lean, or if the ECT does not obtain 20 degrees (68 F), the PCM will not go into "Closed Loop" Fuel control operation.


Source: Service Manual
Engine Electronic Controls (ENEC)
Page 23

[wolfman]

AND, the PCM WILL SET an O2 sensor code (Either SLOW/RICH/LEAN) If there is NO CODE for the O2 sensor, it is FINE. (Argueably even if there IS a "rich or lean" O2 sensor code, it is probably NOT the O2 sensor, but is related to another sensor fault or problem in the fuel system.) Replacing an O2 sensor is RARELY the "cure" as it is almost always the bearer of bad news and NOT the cause of it.

[robh456]

That depends on the particular vehicle/year. On vehicles with OBD-II systems ('96 and later), then yes, it's probably true that if there is no code, the O2 sensor is fine. Older systems were generally much "dumber", though, and the sensor has to be almost completely dead to set a code.

[meseville]

I know that some non-Saturn cars drop out of closed loop durning full throttle acceleration to richen the air-fuel ratio for better acceleration. Do Saturns do this also?

[wolfman]

Originally posted by meseville
I know that some non-Saturn cars drop out of closed loop durning full throttle acceleration to richen the air-fuel ratio for better acceleration. Do Saturns do this also?

YUP, at anything above 80% throttle or BEFORE the engine reaches it's "normal operating temp" threashold for the PCM, she's on factory preset open loop operation.