What does this mean, go into a closed loop?

Are you saying after warming up the O2 sensor becomes active? Does it need to reach a certian temperature to begin sending data?

Thanks,

This one I can answer... I'm not an expert mechanic, but I know computers and I've written control loops. This is going to be a bit long, but I'll try to keep it as clear as possible.

There is a "control loop" that the engine computer uses to run the engine fuel system. Basicly that means there is a piece of software in the computer that controls the engine. The computer is calibrated (i.e. it is designed to accept data within certain limits) to take input data from the various engine sensors (e.g. O2, Crank position indicator, etc...).

The control loop can run in two modes - open loop or closed loop. The difference is how the data is interpereted and how the control loop reacts.

In an "open loop" mode the control loop uses some of the data available, but generally operates based on a set of default "guesses" about how to run the engine. It is sort of like saying there is a certain sequence that will always work to turn over the engine, so that is what the computer does. In open loop mode the data from a lot of the sensors is not used by the computer.

When we say "closed loop" we are refering to the same piece of software in the engine computer. However, in a "closed loop" mode the data from the various engine sensors is being used by the computer to figure things out the best way to run the engine. For example:
The computer uses the O2 sensor value to figure out how much oxygen is being drawn into the intake. It also knows how hard your pushing on the gas pedal. It can combine these two pieces of information and determine how much fuel to squirt into the cylinder by adjusting the amount of time the fuel injector is open. When the cylinder fires it can use the other sensors (like the knock sensor) to determine when the cylinder fired. Then, when the next cylinder is ready to fire, the computer can feeback (i.e. control or adjust) the amount of fuel that comes from the injector to optimize fuel usage and engine power.

So, the reason it is called "closed loop" is the computer is using the results of the last operation it performed to optimize the next operation... The last operation is feeback the computer uses to control the loop...

Now, the catch is that when you start a cold engine the system runs in an open loop mode until the engine comes up to temperature. The reason is the values the sensors report have to be within certain limits for the computer to use them. So, what Sundar and I were talking about was that if the engine (and consequently the engine sensors, fluids, etc...) is up to temperature, but the engine isn't running closed loop, then maybe the problem is a sensor (like the O2 sensor) isn't working, so the computer has to run open loop...

Get it?

JW

I figured the engine would run... When I tried to get the closed loop reading the engine was hot - I drove it to work and before I shut it off I plugged in the sensor and let it idle for a few mintes. The temp gage was right in the middle (I think 210 deg?). Unfortunatly, the reading I get is from when the engine is cold (coolant tem is reported at about 55 deg).

The engine, even when cold, runs smoothly. I don't have any notable power problems getting on the highway when it is hot or cold. The only thing I can point to is the OBD data and when I'm sitting at idle I can hear when I think may be an occasional miss (when the engine is hot or cold).

I can think of 2 possibilities -

1) The misfire only happens when the engine is cold. If this is the case, the data I have makes sense (i.e. I can't get a reading when the engine is hot). However, I can't figure out why it soulds like there is an occasionaly miss.

2) The OBD tool I've got sucks, and doesn't read well. If this is the case, I need a better tool that is more suited to this.

I am hesitant to assume it is the O2 sensor because there doesn't seem to be an impact on driveability...

I'll see if I can get a better scan tool...

JW

for starters that was great right up <img src="/forums/images/graemlins/kewl.gif" alt="" /> , it shows that you got engine controls down. one quick fix. the O2 sensor senses Oxygen in the exhasut. when you are rich there is no oxygen, when you are lean there is an execess of O2 and the o2 sensor sends a variable signal from .2-.8V volts. to tell if it is goo or not get yourself a good multimeter(i perfer a good analgo for this because you can see the needle move realtime much better than the update rate of a digital.) and watch to see if it is constantly switching some where around .2-.8V (basically a high then low then high...) about 8 times in 10 seconds. if it doesn't roam at idle or it seams slow... then just replace it.

cant help you with the scan tool though.... maybe pay the 20 bucks or whatever to have a shop do it for ya.


if it does it when its cold only it sounds like it could be a dirty injector. some shops will properly clean an injector and you might be able to get a kit, if you want i can elabortae mroe.

Thanks Sundar... By profession I'm a software engineer, and I've written control loops for robot motor controllers and radio transmit power controllers. They aren't cars, but they feedback loop control theory is the same. <img src="/forums/images/graemlins/cheers.gif" alt="" />

Most of my experience with cars has been basic maintenance and mechanical work - replacing seals, alternators, starters, half-shafts, brakes, etc... My '98 XJ is the first major rebuild/modification I've tried.

Anyway, tomorrow I'll see if I can track down an analog multi-meter and check the 02 sensor voltage. I think they may still carry them at Radio Shack (which is conveniently 2 minutes from my house). I already have a digital one, but I agree the analog one would be better for this. If that doesn't seem to turn up anything I'll see about getting another OBD tool.

My DrawTite 2" front reciver came in today, so I'm going to have to mount that up tomorrow too... Going to be a busy weeked! <img src="/forums/images/graemlins/smile.gif" alt="" />

I'll let you guys know if I find out anything new!

JW