Can someone explain this? I should look it up in the helms manual, but I am sure that someone here can explain it better...

 

Not really. Closed loop is when the engine and O2 reach operating temp, and the ECM adjusts the fuel mixture to satisfy the O2. Open loop is when the ECM regulates the engine off of pre-programmed tables burned into it. For a better, more detailed explanation, see your Helms.

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Thats basically it, the ECM pays attention to the O2s in closed loop operation, otherwise it runs off of preprogrammed A/F mixture tables.

Once certain timers have gone off, and the O2 reaches 350 C, the ECM begins reading it.

 

That about answers my question. For me, it just seemed that closed loop would be when the ECM is running pre-stored values, and open loop would be reading data from the 02s (open as in accepting new data). I just had it backwards. Thanks for clearing it up.

 

Closing the "loop" is like closing an electrical circuit. Like with electricity, if the loop or circuit is "open" it isn't a loop or a circuit. Close it, and everything runs around the loop and works.

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Helps to think of closed loop as a closed loop involving only the PCM and O2 sensors for air/fuel ratio. Think of open loop as the PCM opening a book to lookup what it needs with regard to air/fuel ratio. In closed loop the book is closed. Sort of like an exam in school: open loop = open book (lookup the tables); closed loop = closed book (brain only: i.e. pcm and sensors only).

Also keep in mind that the MAP sensor is of vital importance too, even on a MAF equipped car. Even on a MAF car MAP input is used to determine spark advance, rpm vs. MAP is how the PCM knows how much load is on the engine and is what it bases its spark advance decisions on.

 

There isn't a MAP on A MAF car. One or the other is used to measure air flow, but not both.

 

These higher level explanations are not quite right.

Basically, a closed loop system uses the output of the system as an input to the system.

This is not like an electrical circuit, because an electrical circuit that is not closed doesn't do anything ("circuit that is not a loop" is not possible by definition.)

It's also not an either-or, where there is no lookup done in closed loop mode. There are lots of constants that still have to be looked up in closed loop mode.

Google "closed loop control" if you want to learn more.

 

My vette has both.

 

My 86 doesn't.

Jake

 

My 86 doesn't. Unless you meant to type MAT, not MAP.

Jake

 

All of the later (94 and up) MAF strategy cars are also equipped with a MAP sensor. This is so that in case of MAF failure the PCM has better SD (Speed Density) control. Plus the MAP sensor in later cars is also used in the tranny controls to determine engine loading.

In controls system, a individual control loop will have certain elements to control a giving loop. For instance, a level switch or transmitter, and a control valve.

Open loop means that the control is done without any feedback from a measurement element.

For example an open loop control loop can be a single valve on a timer, it opens and closes based on just a preprogrammed timing event.

An example of closed loop control can be a tank level loop consisting of a valve, and a level transmitter. The level transmitter tells the controller that the level is below setpoint, the controller then opens the valve to fill the tank, once the level meets the controllers setpoint the valve closes. You toilet bowl with its float and valve is basically closed loop control with porportional gain only. The level drops below the float valves setpoint, which cause the valve to open porportionally to the amount of the error from setpoint, which fills the tank to its preset level and then the float valve closes.

In your cars PCM, the term closed loop means that the PCM is now performing its fueling strategy based on the feedback from the O2 sensors. Based on how long the sensors stay above and below stoich (14.7 AFR) the PCM will adjust its fuel trims based on what the O2 sensors input. If they stay above 450mV then the PCM decides that its rich and it adjusts its long and short term tims to remove some fuel, if the opposite happens then the PCM adjusts the fuel trims to add some fuel.

The long term fuel trims is sort of a predictive control in that the PCM is predicting that its either rich or lean based on the O2 sensor feedback and therefore its making a fuel strategy adjustment for the long term, some control guys may even refer to it as adaptive control. In a sense the PCM is adpating to a situation that it sees based on the O2 sensors. The short term trims are immediate changes to the fuel trims. These trims are constantly changing for the immediate adjustments to maintain stoich condition.

I can go on and on about predictive, adaptive, modeling and PID controls. But I think you get the idea whats happening in your PCM or ECM. Most of these strategies have been developed way back in the 80s when closed loop control first came about in cars and trucks.

 

Yes there is. my '95 has one and using TunerCat and a laptop to look at the various tables in the PCM there are spark advance tables where spark advance is correlated to a certain momentary MAP value vs. momentary RPM. MAP vs. RPM tables. I replaced my MAP sensor. It sits on top of the intake manifold just behind the throttle body. Same for all 1992 - 1996 C4's.

 

Ah HA!! I knew the MAP had an effect on shifts. Now I know why. I had a problem with the 3-4 shift being kind of slow and sometimes tending to bog the engine. Someone on the forum suggested try changing MAP sensor. I swapped in a new MAP sensor and it seemed to correct the problem plus I could feel the car driveability improve.

 

You're making my brain hurt.

 

if you have a MAP senser and you block/plug it,, does it Lean the mixture???

 

I was not saying the open and/or closed loops were the same as an electrical circuit. I was merely trying to help eschoendorff, who said he had been getting it backward. "("circuit that is not a loop" is not possible by definition.)" While this is correct, then an open loop is no more a "loop" when it is open, than a circuit is. Have you never heard of an, "open circuit"? An oxymoron, perhaps.

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