LazersGoPEWPEW

So I was poking around the FSM trying to figure my O2 sensors out and gosh darnit I think I am one of those unlucky guys with a CA Equipped LEV XJ.

Anyone have to deal with this? Can I change it to a normal system? It looks like it has 5000000 O2 sensors on it.

5-90

I do not believe the XJ ever came out as an LEV, TLEV, or any other "Low Emissions Vehicle" variant - the engine is too large.

Here's a general breakdown:

1987-1990, the RENIX years. These have a signle HEGO sensor in the exhaust collector. Note that this is not a standard type of sensor! Rather than generating a low-current 0-5VDC signal, it is a variable resistance used to return a signal which amounts to part of the reference voltage supplied by the ECU (you don't have to know how voltage dividers work, just know that it does - and that this is an atypical sensor.)

1991-1995, OBD-I. These tend to have a single sensor as well, in the same location, but it is now a standard type (voltage-generating, 0 to +5VDC at very low current. The HEGO is an air battery that works off of the differential between oxygen content of exhaust and of atmospheric/ambient air. This type is used in Jeep from 1991 forward, although there is still some sensor variation.)

1996-1999(ish), early OBD-II. These probably have two sensors - one in the exhaust collector (feedback loop for fuel metering trim) and one after the catalytic converter (in place to monitor catalyst efficienty.) The feedback HEGO in the exhaust manifold is referred to as the "upstream" sensor, and the one after the cat is the "downstream" sensor (for DTC purposes, the first one is Bank 1 Sensor 0 and the second one is Bank 1 Sensor 1.)

1999(ish)-2001, late OBD-II. You now have the exhaust split into two halves (cylinders 1-3, and cylinders 4-6) with two "precats" in the manifold proper. And four HEGO sensors - two for fuel metering trim (Bank 1/2 Sensor 0) and two for catalyst efficiency (Bank 1/2 Sensor 1.) This is where things can get ugly.

Note that the "downstream" HEGO sensors have nothing to do with fuel economy or fuel trim - they are there principally to monitor catalyst efficiency and make sure the damned things are working. If you get a downstream HEGO code and the sensor itself tests good, buy a new catalytic converter. It is the "upstream" HEGO that provides the feedback loop for fuel metering trim, and losing the upstream sensor(s) will force the ECU into "Open Loop" mode (which invariably runs rich for engine safety) and will cause a sharp drop in your fuel economy - between that and the CEL/MIL being lit and the DTC being set, those are the warnings you get of a failed/failing upstream HEGO (the only notice you get of a downstream HEGO going west on you is that you get a CEL/MIL, and the code reads for a downstream HEGO. However, a downstream HEGO fault could be set by a failed/failing HEGO sensor or a failed/failing catalytic converter, so be sure to test.)

You can see that things get nuts rather quickly, so it would help to have more information about your rig...

LazersGoPEWPEW

It's definitely got the two precats. I've been monitoring my mileage since I bought it in march and average maybe around 13.5-14 MPG. I've changed the air filter, oil, and just about everything else I can think of that would improve mileage.

I think the Jeep might be running rich because it smells as if it runs rich after I run it for awhile. I also notice blackening around the exhaust tip.

I've never replaced an O2 sensor. I don't have a check engine light at all.

Could it be something in my fuel system possibly? Is there a big downside to having the precat system? I pulled the LEV bit off the label under the hood of the jeep.

5-90

You mean, apart from parts cost?

Not much. I'm sure the precats add some restriction to the OEM exhaust system, but I'm also pretty sure it's negligible (it's unavoidable, but they'll get rid of as much restriction as they can.)

13-14mpg sounds a bit low - if you haven't changed any HEGO sensors yet, you might think about doing so. How many miles on the clock, and how many are yours? Spec life for a HEGO sensor is 60Kmiles, but how far they can really be pushed depends on a few things (most notably combustion efficiency - the sensor doesn't "go bad" - the porous ceramic sensing element clogs up with particulate carbon; and the control system, later controls are more picky about signals. I've pushed RENIX HEGO sensors out to 200Kmiles, and I just replaced an OBD-II Honda sensor a couple of weeks ago at 130Kmiles.)

LazersGoPEWPEW

I've got about 91200 miles on the XJ with about 4300 of them mine.

5-90

So the HEGO is rather likely OEM, and probably wanting replacing (the signal will degrade somewhat before OBD throws a code at you.) Save your nickels for a replacement, and do it sometime fairly soon. It will probably pull your fuel economy back up and pay for itself anyhow.

zjrog

I'll disagree with this. I don't know whether the XJ was ever LEV or not, but the argument of too large doesn't apply. My 2002 F150 Supercrew with the 5.4L V8 is LEV (got a nice window sticker claiming it to be...)...