O2 Sensor Readings

You've got it backwards. Low numbers are lean, high are rich. .45 is midrange and appx the average it should be showing all the time. It kind of sounds like it's starting in open loop as normal, in which the sensor is still cold and ignored. Then it warms up, and it goes to closed loop. But an .8 average in closed loop is pretty rich, and being rich it probably would still seem to run ok. But I suspect it would fail an emissions test, and show high HC, CO, but low NOx. Once hot, it should be running about the midrange I would think. .45 volts is about

14.7 to 1 or a stoichiometric mixture. It may have a flaked out sensor, or very sluggish. That rich .8 v mix is not good for the cat. I would check the whole system out. All the sensors, etc.. But it likely does have a bad O2 sensor being as that is what the car's puter thinks it is. But you want to make sure it really is going to closed loop, and all the sensors are working, including the temp sensors. Open loop is pretty rich for a cold engine, and if it stayed in open loop, it would probably run too rich once it warmed up. Does the sensor show cross counts? IE: jumping back and forth from low to high?

No, no cross counts that are apparent to an idiot like me.

I tend to agree with the computer -- if it says "I hurt here" then that is what I fix. The car is OBD I, so there are not alot of things upstream, and the exhaust manifold is split -- two sets of three on an Inline6 -- with a single sensor where the tubes come together just in front of the CAT. There is no Bank 1 and Bank 2...

I have never done a voltage check on an O2 before, so I am not sure what the numbers should be or what they mean. I got distracted by a neighbor when the motor was cold, so I missed the initial numbers. The car ran for several minutes before I got back to it, and the readings were 0.8nn at idle at that time, and the motor was fully warmed up. I fussed around with moving meter leads and looking at different things, and got 0.4-ish on the restart, which became 0.8nn after a short period -- less than 30 seconds.

As I said, if I punch the gas and release, the numbers drop to the 0.2nn range then come back to 0.8nn. Except for the changes from hitting the gas and releasing, the reading only floats a little on the last two digits during idle. If I slowly increase the engine speed, the number stays at

0.8nn, with the nn digits floating around a little. To put a fine point on this, the number is 0.86n, mostly, and the 6 stays in a range of 5 to 7. At idle and on slow increases -- as if driving -- of the engine speed, the number remains very flat.

The book says to unplug the vac hose to the fuel pressure regulator to force a rich condition at idle, but I have not done that test because the vac connection is hidden behind a cover that I haven't taken the time to remove.

Because the sensor responds to rapid changes to the gas pedal, I think it that "sluggish" isn't one of the symptoms.

I've been driving this car for about 5 years, and it is a '94 with 140k miles. I don't know any of the previous history, so I have every reason to suspect the sensor is the original. Instead of beating myself up over this, I think I'll let the computer tell me what hurts and fix it.

That's pretty rich overall. Will whack out the cat if left that way. Too much unburnt fuel for the cat to deal with without melting down.

You could simulate a rich condition if you can feed propane to it. But you may not have that stuff laying around. You can make it run lean by unplugging a vacuum hose.

It can still be sluggish and appear to respond fairly fast. The cross count speeds will vary to the type of sensor. IE: an old one like yours is not as fast as say a new type for multi injection. According to what I can find, your type should be doing 2-3 cross counts per second. But it might be hard to notice those depending on the type of meter. I can see them when I'm using my usual meter, but it has a digital readout, with an LED bar graph below that. On mine, I can see the pulses on the bar graph.

Being as that type is usually good for 60k miles on average, it probably could use a new one, and looking at some info I think that's probably the problem if it's a heated type sensor. On a heated sensor, it's usually catalyst depletion that occurs, and when that happens it usually goes rich just like yours is doing. The sensor is reading leaner that actual, and the puter is metering rich because of the overly lean O2 output. So that kind of matches what yours is doing. So I'd probably change it out first thing, being as it probably was about due a new one anyway. If that doesn't cure the problem, you may have other issues. The reason I mention the temp sensor, is because that is the sensor that tells the puter when it's time to go to closed loop. If the temp sensor is bad, it will stay in open loop and run rich all the time.

A couple of links to general O2 sensor info.

BTW, sometimes those things can be a pain to get off. It's usually the easiest if you get the pipe good and hot before trying to remove the sensor. Also, you usually need an O2 sensor socket with a slit for the wire. You can get those at the auto parts if needed. Most come with anti seize paste already coated on the threads of the new one.

Your O2 sensor is bad. Change it for an OEM one, clear the codes, and you should be fine.

I've got the socket, and -- BONUS -- the sensor is in a location where I can get on it with an open-end wrench.

Can the car actually be running as it is supposed to, but the sensor is reporting a false rich condition? If this is the case, then there is nothing wrong but the sensor.

The car can be running well enough that the driver can't discern that the mixture is slightly rich or lean. The primary input for determining how much fuel is needed is the air flow meter/MAP/MAF sensor. The O2 sensor helps the computer fine tune the mixture. If you unplug the O2 sensor, the engine will still run fine.

I got that part.

My question was if the car could be running properly vis a vis the fuel mixture, but the sensor reports falsely that there is a problem. I agree with your earlier suggestion that I need a sensor. I guess that answers my question of the false report on an otherwise properly working system.

The O2 sensor itself doesn't report problems, it just gives a voltage. If it is sluggish and not keeping up with the pulses from the fuel injectors, the computer sees a difference between the frequency of the injector pulses and sensor voltage swings so it sets a trouble code. If the sensor voltage is out of range, the computer will set a trouble code.

I didn't read all of your posts that closely, but I suspect that you have a sluggish sensor.

In the case of a sensor that gives voltages that don't correspond with the actual conditions, you probably wouldn't notice any change in engine performance since the fine-tuning is only making difference is hundredths of a second or less in injector duration. There is a limit to how much the sensor controls the air fuel mixture.