CAN SOMEONE EXPLAIN OPEN AND CLOSE LOOP? HOW DOES IT AFFECT DAILY DRIVING VS RACING? HOW MUCH MORE HP OR ET CAN BE EXPECTED. THANKS

 

OK, forst of all turn capslockk off.
Second of all, this isnt an advanced tech question, but since i cant move it, I will answer it....

Closed loop is a term for when your cars computer is changing the air / fuel mixture and keeping it at 14.7 to 1.
In order to reach closed loop your car has to be warmed up. about 180 probibly.

Open loop just means you car isnt in closed loop. You O2 sensors are still giving readings to the computer, but the computer isnt changing anything.

Somtimes it feels like the car is a little stronger in open loop because you are running a little richer, but not much. and only at part throttle. As soon as you go to wide open throttle it drops the air fuel raito to about 13:1 anyway. So, it effectively goes into open loop at wide open throttle every time. Since your fuel economy is better in closed loop and power should be about the same, thats where your car will usually run. There are ways to make the car stay in open loop ( I think pulling the air pump fuse will do it) but there is no reason to, you will just wear out your cat (s) really fast and get worse gas milage.

FYI

Different air fuel raitos are better for different things.
about 17:1 is actually the best for fuel economy but on a high powered engine like ours it would pre-ignite at that raito. Only some of the new hybrids actually run at super high raitos like that.

14.7:1 to one is the best for emissions its also a pretty good balance for fuel economy and power.

12.7-13.1:1 is best for power, it varies a little from motor to motor.

 

As Dave pointed out, this post belongs in "Computer Programming....." forum, and I will move it there.

Additionally, some of what is in Dave's response is not quite correct..... .

Moving.....

 

closed loop is when the car is warmed up to opporating temps and after a certain predetermined time period of something around 3 minutes has passed. basically its the point where the oxygen sensors are hot enough to work accruately and begin feeding information back to the PCM. in closed loop the PCM is attempting to constantly maintain an AFR of 14.7:1 which if you are familiar with chemistry is the stoicheometric ratio where all the fuel and oxygen are consumed in the reaction. this of course will yeild the best fuel economy b/c you arent wasting gas. the oxygen sensors are what the PCM uses to measure the amount of oxygen left over in the reaction so the PCM knows whether to add or subtract fuel to maintain the target closed loop AFR of 14.7:1.

open loop is simply when the car is cold, the oxygen sensors are cold. because the sensors are cold they cannot read accurately so the PCM just ignores their output until that certain temp and time period has been reached like i mentioned before.

 

so, uh what did I say that was not right?

 

The PCM is getting feedback from the O2 sensors, and using that info to adjust the short term fuel trims to achieve 14.7:1. Intentionally over- and under-shoots slightly to maximize catalytic convertor effciency.

1) O2 sensors must reach 600degF and produce a usable signal. 2) Coolant temp must reach a certain minimum, and that is a variable value, between 120-140degF, 3) timer from startup must time out... 2-3 minutes.

Fine point, but GM only uses the term "open loop" to refer to the startup condition, before the 3 conditions noted above are achieved. When you go WOT( and that is a term defined by a table in the PCM, and can be as low as 28% TPP at 5,800rpm) the PCM ignores the O2 sensors, but GM calls that "power enrichment" (PE) mode. Semantics.

Not sure I follow this at all. When the engine is cold, it is usually in an enriched state, based on coolant temp... much like a "choke" on a carb. Otherwise, the PCM is still trying to achieve 14.7:1.

Target A/F ratio in PE mode is a variable based on a couple of sensor inputs. Typically it is 11.7:1, hence the comment "LT1's run rich".

Fuel economy is better because it is using a leaner target A/F ratio. Nor sure what you mean by "power should be about the same."

There is no real evidence that lack of AIR will cause premature failure of cats. It will just prevent them from reaching operating temperature as fast. If for some reason your car runs ridiculously rich in open loop, it could result in combustion of unburned fuel on the cats, which will cremate them. I'm also not positive that the SES for AIR will kick things into open loop.... wouldn't make any sense, since the AIR system doesn't operate except for a few minutes at startup. Pull the wire off the O2 sensor = open loop.

There is a simplified explanation of all this in the "Air/Fuel Management" section of my ScanMaster writeup.

 

Thanks Fred, I learnd a couple things from that reply...

I didnt know it could reach closed loop as low as 120 degrees.

I had never heard of the (PE) mode but I knew it wasn't exactly the same as open loop.

"Not sure I follow this at all. When the engine is cold, it is usually in an enriched state, based on coolant temp... much like a "choke" on a carb. Otherwise, the PCM is still trying to achieve 14.7:1. "

like you said it is in an enriched state. running slightly richer than 14.7:1 should in theory mean slightly better power.


I had no idea that the Stock tuning dropped the A/F raito as low as 11.7 to one. I thought anything below 12.5 was counterproductive.

"There is no real evidence that lack of AIR will cause premature failure of cats. It will just prevent them from reaching operating temperature as fast."

If you have ever watched a 5 gas analyzer there is a huge difference in HCs in open loop vs closed loop. The excess HCs could cause pre-mature cat failure.....And I'm told if you pull the air pump fuse the car will stay in open loop. I've never tested this though. A friend with a 95 camaro said he was having problems and the car would die as soon as it tried to go into open loop. And he said pulling the air pump fuse keeps the car in open loop, till he had time to fix it.

Thanks for the corrections...