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Gerald 03-25-2008 03:39 PM

Oxygen Sensors
Need a bit of info, how many O2 sensors are there on a 98 4.0. My gas milage has been shot to hell recently. Do you need any special tools to renew them?
Thanks in advance

jpdocdave 03-25-2008 03:54 PM

should be two, but they are pricey, and will usually give check engine light when bad. make sure state of tune is good. plugs, wires, fuel filter etc.
they make an oxygen sensor socket, but a 7/8 or 22mm (same size) wrench will work just fine

Jerry Bransford 03-25-2008 05:23 PM

There are two. The Upstream 02 sensor is located just below the exhaust header and the Downstream 02 sensor is screwed into the rear of the catalytic converter.

It's only the Upstream sensor that affects fuel economy. When it goes bad, the engine computer switches the air-fuel mixture to full rich. Changing it is like changing a spark plug, possibly even a bit easier. They make replacement 02 sensors that will plug directly into your '98 TJ's wiring harness so don't buy a universal model that you'll have to splice in.

jpdocdave 03-25-2008 05:59 PM

if the ecu is switching to full rich, it will put on check engine light. if it is getting bad readings from sensor, it could affect it, but they pick up problems with o2's pretty well due to strict emissions regulations. definately don't do universal, get direct fit.

Gerald 03-26-2008 02:52 PM

Has anyone had any experience with using the ones supplied by Crown? The crown parts manual seems to list only one sensor on the 98 model. Very confused now!!

Jerry Bransford 03-26-2008 04:23 PM

IIRC from when I replaced mine, the front and rear 02 sensors were the same which makes sense to me since they both do the same thing.

4point 03-26-2008 04:29 PM

Not sure if its the same on the jeep, But I just ordered one for my chevy and the upper one has a heater in it and the lower does not.

Jerry Bransford 03-26-2008 04:57 PM


Originally Posted by 4point (Post 211692)
Not sure if its the same on the jeep, But I just ordered one for my chevy and the upper one has a heater in it and the lower does not.

Both of his TJ's 02 sensors have heaters in them. You can also tell because they both have four wires coming out of them. :)

jpdocdave 03-26-2008 05:30 PM

doesn't anyone think this is an expensive guess, or is this just that common w/ tj's? i didn't think so.

1BLKJP 03-26-2008 05:49 PM


Originally Posted by jpdocdave (Post 211722)
doesn't anyone think this is an expensive guess, or is this just that common w/ tj's? i didn't think so.

Yeah they do go out and don't always throw a code. If it were me I'd put it on the snap on scanner and see how it was all runnin, but I understand that not everyone has access to something like that.

Jerry Bransford 03-26-2008 06:06 PM

Just to be clear, I would only change the upstream 02 sensor since the downstream 02 sensor has no effect on power or economy. And as old as his '98 is, it's about time he changed the upstream 02 sensor anyway, they do always need replacing at some point.

jpdocdave 03-26-2008 09:40 PM

oxygen sensors go bad all the time, i replace a lot of them. graphing it on the scanner is the best, but not logical for most. vehicle's ecm picks up even lazy o2's pretty quick. pre obd 2 cars this was more common cause of poor mileage, with no code, i'd do some diagnosis before replacing. what are they goin for? more than $100 at discount store most likely, some cars are closer to $200.

Jerry Bransford 03-26-2008 09:53 PM

I paid $60-70 or so for my plug-in direct replacement 02 sensor at my local auto parts tore.

sailsurf7713 03-27-2008 01:32 AM

Summit has NTKs for a little under 50 and Bosch for a little over 50. They both work well. I've used them both. O2 sensors lose their effectiveness over time so replacing them every 3 years or so is a good idea, but definitely keep in mind that only the upstream affects the performance. The downstream monitors catalytic converter efficiency and cannot have an affect on Air/Fuel Ratio.

Check the typical stuff first: Plugs, Wires, Filters, Cap, Rotor. Knock on the cat and see if it rattles. The stock cats are freakin terrible. Also check your tire pressure and look at your alignment.

You'll find the cause, its there somewhere.

Gerald 03-27-2008 04:44 PM

Thanks for all the info. Problem I have is this - rough idle and a hesitation on take off, tis has since progressed to bad gas consumption. I have read the codes a number of times but nothing, the lamda values were not changing as quickly as expected. I have renewed the TPS, cleaned the IAC, renewed the plugs, rotor, cap, leads and reset the ECU which seems to make the jeep run better for a week or so then its back to its old tricks........ good thing I like the truck!

jpdocdave 03-27-2008 08:30 PM

you've done everything good so far, sounds like a tough one. good luck, and keep us informed

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