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Discussion Starter #1
Does anybody know what are the differences between "open loop"
and "closed loop", especially for a 4.9 liter enigne?

For example, how does the computer control a 4.9 liter engine
in open loop with what kind of sensors? How does it do the same
thing in closed loop? What kind of sensors are used and not
used in open loop and in closed loop, respectively? What kind of
devices are contorlled and not controlled in open loop and in closed
loop, respectively?

I know O2 sensors play important role in closed loop and they don't
in open loop, but that's pretty much all I know about it.

Thank you very much in advance for any info!
 

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You pretty much covered it! ;)
The terms "open" and "closed" loop refer to the O2 sensors feeding back info to the computer and thus "closing the loop". Open loop is just the pre-programmed (slightly rich) engine control routine that is used prior to the O2 sensors coming online. This is primarily used while the sensors warm-up after initial start-up but is also used for a "predetermined" amount of time after each start-up.
 

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Discussion Starter #3
Thank you for your reply!

So, only difference is the amount of fuel being injected, right?

In open loop, the amount of fuel is determined without any
feed back from the O2 sensor. On the other hand, in closed
loop, the amount of fuel is determined with feedback from the
O2 sensor, am I right? What about timing? Does it stay the
same all the time?

I thought when rich (open loop), timing could be automatically
advanced. By the way, why fuel concentration is set slightly
rich for open loop operation? Does it warm up the engine faster?
Or just to have a little bit of margine for knocking? (then GM
wouldn't advanse timing in open loop...)

Katshot said:
You pretty much covered it! ;)
The terms "open" and "closed" loop refer to the O2 sensors feeding back info to the computer and thus "closing the loop". Open loop is just the pre-programmed (slightly rich) engine control routine that is used prior to the O2 sensors coming online. This is primarily used while the sensors warm-up after initial start-up but is also used for a "predetermined" amount of time after each start-up.
 

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Yes, the timing is generally "fixed" in open loop. The computer adds fuel during open loop for the same reason that you need a choke on a carburated engine. Fuel tends to stick to cold metal and thereby artificially create a lean condition in the combustion chamber. this is one of the main reasons why you see so many composite and aluminum intake manifolds, they warm-up quicker.
 

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Discussion Starter #5
Yeah, you are right, there is no choke! I didn't realize until
you mentioned. It does make sense.

The reason I asked this question was when the timing was
accidentally too advanced, I had knocking problem on my car.
But when the engine was cold or right after I started the car
while the engine was warm, there was no knocking. I came
to the conclusion that the car runs fine with too-advanced
timing when in open loop

I fixed the problem by adjusting the timing, but it made me
think what was going on.

Well, theoretically, does this mean if you can somehow richen air
fuel mixture or just retard the base timing (maybe both need to
be done simultaneously in cordinated manner), the car may be
able to run with regular gasoline without knocking problem?

Anyway, thank you very much for answering my question.
I really appreciated.

Katshot said:
Yes, the timing is generally "fixed" in open loop. The computer adds fuel during open loop for the same reason that you need a choke on a carburated engine. Fuel tends to stick to cold metal and thereby artificially create a lean condition in the combustion chamber. this is one of the main reasons why you see so many composite and aluminum intake manifolds, they warm-up quicker.
 
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