O2 Sensors?

Ive been having a problem where I have had a pretty substantial drop in gas mileage...went from around 20 on the highway to maybe 14 in the matter of a few weeks. First thought is 02 sensors, though I try to get codes pulled and there was nothing. Thinking maybe AIS motor, though I dont think that could cause that substantial of a loss of mpg. My question is would bad 02 sensors cause this bad of a loss in mpg and if so just how many are on the 4.0. I can only see two when I look under but someone told me there is 4?

Just would like to figure out what is going on with the horrible mileage as of late..

RoadTrippin said:

Ive been having a problem where I have had a pretty substantial drop in gas mileage...went from around 20 on the highway to maybe 14 in the matter of a few weeks. First thought is 02 sensors, though I try to get codes pulled and there was nothing. Thinking maybe AIS motor, though I dont think that could cause that substantial of a loss of mpg. My question is would bad 02 sensors cause this bad of a loss in mpg and if so just how many are on the 4.0. I can only see two when I look under but someone told me there is 4?

Just would like to figure out what is going on with the horrible mileage as of late..

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A decline in fuel efficiency can be caused by any number of things - some of them prosaic. When was your last tuneup, and how extensive?

How many oxygen sensors depends largely on what model year you have - it could be one, two, or four.

I have 154k on the clock right now

Its a '00 XJ 4x4 with the 4.0

Last tune up was at 120k replaced plugs, all filters, new serp belt, and did the brakes while I was at it.

RoadTrippin said:

I have 154k on the clock right now

Its a '00 XJ 4x4 with the 4.0

Last tune up was at 120k replaced plugs, all filters, new serp belt, and did the brakes while I was at it.

Click to expand...

2000? I think your header is split into two banks of three cylinders - and you'll have four. Two before the catalytic converters (for fuel trim) and two after (to monitor catalyst efficiency.)

You may be losing your HEGO sensors - spec life for them is, as I recall, 80Kmiles. Worry more about the "upstream" HEGO (before the converters,) since they have a greater effect on fuel trim and fuel economy. The ones after are simply there to monitor the catalyst and make sure it's working.

Cat's... They can have simular simptoms

Do we still have "winter fuel" here? Bad mileage is pretty common when that change happens; could be as simple as a bad tank of gas too unless this is multiple tanks.

Jim www.yuccaman.com

I just replaced the upstream o2 sensor on my 2000.. there was just one right after the downpipe.:dunno:

I had been using 1/2+ a tank to travel 175 miles... after the o2 change it went to about 3/8 of a tank.

So, since mine is originally a FED XJ it should only have 2 sensors....and replacing the one sensor before the cat should bring it back to normal right?

I should add that I can only find 2 sensors after looking under the XJ and under the hood.

Is there any way of identifying for sure if it was a FED XJ vs a Cali XJ? Im just assuming that since it was originally built for an IL dealer it is probably a FED but here is my build sheet just in case...I dont see anything that says its a FED vs a Cali..

Sorry for the stupidity here I just dont wanna spend $$$ on multiple sensors if I only really need to replace one.

Thank you for your email to DaimlerChrysler Motors Corporation regarding
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B06-INTAKE MANIFOLD DEBRIS SHIELD

Check your emissions tag underhood - it will tell you either "Federal Emissions regs" or "California Emissions regs" in the blurb at the bottom. It is quite common for vehicle built for California to be sold elsewhere, tho - so do check.

And, the "before/after" HEGO sensors isn't a California thing - it's an OBD-II thing. The specs on the parts may be a bit different, but the basic idea still applies - OBD-II wants to monitor catalyst function and efficiency, thus the extra sensors.

i would have the coolant sensor tested. it may be reading cold and therefore making a rich condition. is the thermostat working properly too?

Alright, so I was poking around today and I do only have 2 sensors just to clear things up there is no other sensors that are in the exhaust except for these two..checked the cat, its fine, not sure how you would go about testing the CTS or CPS (heard that can cause a drop too) though I havent even come close to over heating ever, temp is consistently 210 so I think that may rule out the CTS, leads me to think its the 02 sensor....for $50 ill jus replace it and see what happens.

Don't confuse the CTS with the coolant temp sending unit. Your CTS is mounted on the thermostat housing (I believe for this model year) and leans out the mixture when you reach normal running temps. The other comes off the head and is a closed loop with the guage or light. An overheat condition or guage readings are not any indication of your CTS condition. Your manual will spell out the specific tolerances and you will have to take one reading when the engine is cold and another when it is hot. A general rule of thumb for sudden drops in gas mileage is to look at the CTS first.

Hoss

Well I went ahead and decided to replace the O2 sensors after all I have 150k on the XJ and I am almost certain they are the factory originals....Question I have though is I pulled off the O2 sensor that is before the cat and noticed that there are small holes in the tip. In the new Bosch sensor (part 13100) there are no holes in the tip....did I buy the wrong one? or is this one the rigth one for the upsteam flow....thats what it is listed as anyways