What is the difference between the two?

When should the engine be in each state?

What causes the engine to go from one to another and what is the detrimental effect?

Thanks for the education

 

The short and sweet definition is that Open loop is when the car's oxygen sensor is cold and is ignored for engine operation. When the O2 sensor warms up the computer will go into Closed Loop and then use the values from the O2 sensor for engine operation.

 

And to add to what Scorp said, WOT will also put you into open-loop (for lack of a better word) where it looks at the tables in the chip for air/fuel.

Mike

 

The ECM begins in open loop at startup; after a set time interval, if coolant temp is high enough and the O2 sensor is functioning the ECM goes into closed loop mode. The advantage of open loop is that the stochiometric A/F ratio (14.7/1) is maintained by the ECM for optimum performance and economy.

When the ECM senses WOT it effectively changes to open loop, where it reads O2 sensor values but does NOT trim the A/F ratio based on these readings. The A/F ratio taken from stored tables is based only on engine parameters like throttle position and air flow and used directly to obtain an A/F of close to 12.6/1, since the stock O2 sensor is not linear in. The disadvantage is that the A/F is now an open loop parameter with no feedback to assure that it is indeed at the optimum power ratio. Ideally the ECM should use a wide band O2 sensor in each bank of cylinders and maintain the best A/F for any driving conditions, including WOT.

Since the A/F ratio is an open loop parameter, after we make air flow mods we must either modify the fuel tables in the EPROM (or EEPROM on later ECMs) or bump fuel pressure to try and maintain the A/F for best power at WOT. This is where "tuning" comes into play.

For completeness, it should be mentioned that when the ECM detects a problem with certain elements of the system it will go into Limp Home mode where it fixes timing and takes A/F only from the stored tables to protect the engine.

 

As has already been said, closed loop comes on when the O2 sensor reaches a set temperature of 350 C, and when the ECM's timers have gone off.

Heated O2s reach 200 C before they send reliable signal to the ECM.

 

So ....... after the engine warms up enough to go into closed loop it is important that the 02 sensor(s) (mine is a 95) are working properly otherwise you will get a mixture that is too rich hurting fuel economy or too lean which would hurt acceleration performance or could damage the engine. Correct?

How does one know when the car is in open or closed loop?

How does one test the 02 sensor(s)? and must this be done at warmed up operating temp? ouch!!

Thanks!!

 

On my car it idles a little better in open loop than closed so I can tell when it changes also the idle rpm may change just a little bit.To check the O2's you need a scanner like datamaster to see what numbers the O2's send out.In datamaster it should be around 800 cts and be somewhat equal between both sides.The O2's will continously pulse high and low so you need to look at the average.

Usually if an O2 is bad you will get a code but if one is flakey or a little off you will not always get one.

 

If you would like a very detailed explanation of Open and Closed Loop operation, read the section called:

INTRODUCTION - AIR/FUEL MANAGEMENT MEASUREMENTS:

at:

http://members.aol.com/InjuneerZZ/Sc....htm#step%2013

Very informative stuff......

 

Thank you that is good stuff.