Okay so the 0211 bank 1 and bank 2 fluctuates but the 0212 bank 1 and 2 is steady. Is one bad and one good?

 

I don't know the answer to your question, but I'd sure like to know what app you are using. I'm using Torque, and looking for something better.

 

I'm using obd car Doctor. I'm only using the free version right now.

 

Thank you.

Somewhere around here is a post where CCKen and TJWalker gave me some general info on the O2 voltages, but I can't find it at the moment. I'll look for it after work today - maybe there's some useful info for you there.

What year is your beast?

 

1999 4.0

 

O2 voltage readings should fluctuate constantly, not be steady. General O2 sensors operate on 0.0 - 1.0 volts. The closer it gets to 1 volt, the more rich the mixture is. Closer to 0 volts and the leaner the mixture is. It fluctuates between rich and lean as the ecu constantly tries to keep the right air-fuel ratio.

If the one O2 sensor is steady then yes it's bad (stuck).

 

The rear o2 sensor should be constant as all it is doing is monitoring cat performance.

 

I disagree. The downstream O2S will cycle just as the upstream O2S does, but not every 2 seconds like the upstream.

 

Thanks very much, gentlemen. This is very helpful.

Maybe we should have a section of the forum dedicated to this kind of thing, or at least a sticky that lists the various major readings and what they should look like.

 

Yes - when the catalyst is warmed up the downstream sensor should resemble a delayed muted version of the upstream. Getting flat output without a 100% perfect burn from the engine is not possible (at least on a stock XJ, maybe some fancy stuff with a wideband could)

 

Watching my sensors again tonight, and have a question:

What conditions would cause the O2 sensor to fluctuate between 0 and .2 volts, never getting above .2, and mostly reading either .1. or .2?

That's what I'm getting when it's running badly. Fuel Trim 1 is mostly at 38.2, which I'm assuming is pegged, as it's never above that.

I'm questioning the vacuum leak diagnosis because it comes on like flipping a switch. Sometimes it goes away the same way.

It does seem to be heat related, that is, it's fine when cold, but it has to have been running a good while to kick in the problem behavior. Just getting up to normal operating temp doesn't do it. That would suggest a heat soak issue, perhaps somewhere other than the engine itself. The O2 sensor? The wiring?

 

With that radically high + fuel trim it could be an exhaust restriction such as a clogged catalytic converter, or a MAP sensor gone nuts.

If you hook up a vacuum gauge to the intake manifold check the idle vacuum when it is acting up compared to when it is not.

Here's some vacuum diagnostics charts. Look for the one that shows a restricted exhaust.





Here's a MAP sensor test:

 

Thanks Ken. Hmmmm. One of the readings I can get is "manifold pressure", expressed KPa. Is that the manifold vacuum? It's a little hard to read the images - are those readings inches of Mercury?

If I can get some logs out of this thing, I can toss them into Excel and convert the KPa to in/Hg if that's really manifold vacuum I'm seeing.

Clogged cat? It often feels like something is clogged. If I stomp on it, especially going up a hill, things clear up suddenly (with a jerk) and it's fine for a few seconds.

Hmmmm.

 

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Okay, I'll do that. Do you know what that reading is?

I'm thinking I might temporarily remove my cat just for testing. I can replace it with some flex exhaust pipe for a quick test drive. I can't think of any other way to test for a clogged cat.

Any other suggestions?