I noticed something that seems a bit strange to me. Many times when I let off the gas the ECU drops out of closed loop into open loop for about a second.
You can see on the graph that this happens both when coming off a rev, and when coming off a steady, high rpm.
This is on a 98 Mazda 626 2 liter.
Why does this happen?
This is normal behavior. Closed loop is present during steady-speed operation, else it defaults to open loop.
This site offers a well-summarized explanation:
Closed Loop
An average ECU using a narrowband sensor will generally only use the lambda sensor's output during two specific conditions:
during idle, ie. when the engine is under no load apart from keeping itself running
during part-load conditions (which we usually term 'cruising speed') where the engine is keeping the car at a constant speed. These two conditions account for the majority of the car's operating time
Open Loop
The ECU will generally ignore the lambda sensor's output during three further conditions:
when the car is accelerating - the ECU will spent much of its time deliberately enriching the mixture to avoid hesitation and to provide extra power
when decelerating or 'engine braking' (which we usually term 'on the over-run'), when most ECU's will shut off the fuel completely to aid economy
that of cold starting, where the ECU deliberately runs the engine rich all the time to promote quick starting and ease cold-running. Once everything is up to temperature, normal operation will resume
The early OBDII engine control systems did this often. They show open loop on decel because they turn off the injectors for a bit on snap throttle close. Newer designs, usually with auto throttles, remain in fuel control. The auto throttle closes the throttle more slowly, keep the injectors on and the system remains in fuel control.
There is no rule in ODB that requires one method over the other, it is up to the engineering design team and they are juggling issues of engine design, emission regs and the marketing dept.
OBDII regulations change often, major changes were made in 2004 and 2010. OEM's controller software varies from engine to engine and year to year, they are highly varied and in all cases proprietary. The only way to understand normal sensor and actuator behavior is to scan and scope normal systems.
