Please explain Closed loop and Open Loop and how this applies to our ECU and F/I and any timing boxes used and FMU's used with F/I units

 

"Open loop" and "closed loop" refer to control systems in general.

Looking at cruise control first:

In an open loop control system, the controller will respond to it's inputs in a pre-determined fashion. So if we had an open-loop cruise control, the input might be engine load (might be an indirect measurement, like measuring engine vacuum or something) and the output might be throttle position. If the engine load increases, (say when driving uphill) the system would increase my output by a certain amount. Period. It would never take a measurement of the speed of the vehicle, it'd just know that the engine performs in a certain way and if the load changes, adjust the throttle a pre-determined amount. This will probably feel like crap, and I'm not sure if it would even work the way I describe, but you get the idea.

In a closed loop control system, we would actually use vehicle speed as feedback to the controller. If the vehicle speed decreases, the throttle output will increase. As the vehicle speed approaches the setpoint, the throttle will also decrease until the setpoint is maintained. This makes so much sense that you probably wonder why I ever described the open-loop method. Your engine is a perfect example why:

In the EFI world, you're attempting to control engine's A/F ratio. The feedback to the engine computer is the data read from the O2 sensor(s). If the O2 sensors are removed from the system or the computer thinks the data is unreliable, the system should revert to open-loop control, making precise adjustments to the amount of fuel entering the cylinder based on it's inputs, but never getting any feedback on the actual performance. The alternative would be that your engine would not run at all unless all of the sensors used for feedback were good.

I'm fairly sure that my '94 Nissan pickup has been running in open-loop mode for the past two years.

 

Wow. Nice second post