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Unread 02-16-2008, 01:13 PM   #1
1999JCS
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O2 Sensor Replacement

1999 Jeep Cherokee 4.0 L Auto Transmission with 81,000 miles.

Gas mileage has been mediocre at best 12-13 mpg (winter time here in Indiana with 10-20 deg during driving). Got under jeep and checked out the pre-cat O2 Sensor and it was coated with grease/oil. Photos show an after shot of it being wiped down. There are small holes on the side of the sensor. Do these function to allow air into the sensor? If so they were clogged with grease/oil. There is still some in there considering I just wiped it off with a rag. If these holes are clogged, does sensor maybe not work properly? No apparent oil leaks can be seen, so it might just be 9 years of grime and junk on it.

I'd prefer not to replace if it is not necessary. Part is $66 with tax and it looks like I'll need a special socket to get it out which will run another $10-15 maybe. $80 isn't a lot of money, but don't want to throw money at the heep if it isn't necessary as that cash can go to something else. BTW, after cat sensor was clean. Both are originals. If replacing pre-cat part number seems to be OE Bosch #13100 (can anyone confirm that). Haven't seen any pictures of the O2 on the forum, so here are a few of mine.

Sensor


Plug


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Unread 02-16-2008, 03:27 PM   #2
tjwalker
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"Generally speaking", most guys get at least 100k from their original oxygen sensors. I have 124k on my originals. You don't mention that you have any check engine light; if your sensor was really on the fritz, it would set a code. Yea, they do get sluggish over time and therefore have trouble optimizing fuel/air ratio and some guys do replace them as a preventative thing, but I wouldn't expect any gas mileage miracles if you replace it. And should you decide to do that, just replace the upstream as the downstream after cat sensor just measures efficiency of your catalytic converter.

Just as or more important, I'd make sure the engine had a fresh distributor cap, rotor, plugs, wires, and air filter.
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Unread 02-16-2008, 05:43 PM   #3
CharBroiled
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those holes are for it to compare the O2 from the exhaust to the O2 outside the exhaust (thats how it figures O2 levels out.. its a ratio) and they are NOT supposed to be clogged. so its probably reading wrong. although Im not sure how to unclog them..

this is from the 2000 FSM:

Quote:
An O2 sensor must have a source of oxygen from
outside of the exhaust stream for comparison. Current
O2 sensors receive their fresh oxygen (outside
air) supply through the wire harness. This is why it
is important to never solder an O2 sensor connector,
or pack the connector with grease.
my 2000 has 111,000 and original O2 sensors. an O2 sensor wont necessarily set a code unless its way off or open/shorted. so its possible just to be off enough to cause bad mileage but not set a code.

more info from the 2000 fsm:

Quote:
Upstream Sensor (Non-California Emissions):
The upstream O2S sensor (1/1 sensor) is located in
the exhaust downpipe before the catalytic convertor.
It provides an input voltage to the PCM. The input
tells the PCM the oxygen content of the exhaust gas.
The PCM uses this information to fine tune fuel
delivery to maintain the correct oxygen content at
the downstream oxygen sensor. The PCM will change
the air/fuel ratio until the upstream sensor inputs a
voltage that the PCM has determined will make the
downstream sensor output (oxygen content) correct.
The upstream oxygen sensor also provides an input
to determine catalyst efficiency.

Downstream Sensor (Non-California Emissions):
The downstream heated oxygen sensor (1/2
sensor) is located near the outlet end of the catalytic
convertor. The downstream sensor is also used to
determine the correct air fuel ratio. As the oxygen
content changes at the downstream the PCM calculates
how much air fuel ratio change is required. The
PCM then looks at the upstream oxygen sensor voltage
and changes fuel delivery until the upstream
sensor voltage changes enough to correct the downstream
sensor voltage (oxygen content).
The downstream oxygen sensor also provides an
input to determine catalyst efficiency.
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Unread 02-16-2008, 05:56 PM   #4
XJohnnyC
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And you don't need a "special socket". An open-end wrench or a Crescent wrench will work fine.
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Unread 02-16-2008, 06:22 PM   #5
1996xj
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The hole is there so the sensor can compare the exhaust air to air outside the vehicle, if the hole clogged up the sensor wont be able to make proper comparison.
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Quote:
Originally Posted by toates89
i am just gunna drive around in 4lo on dry streets, and peel out
Quote:
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I once got a ticket for jerking while driving it. The cop wouldnt let me use his pen to sign the ticket.
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Unread 02-18-2008, 08:32 PM   #6
lazermule
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180K on my orininal O2's

I have 180K on my original O2 sensors in my 2001 Cherokee. My mileage is suffering a bit at about 16mpg where I previously got a little better than 20 on the highway. The vehicle seems to be running fine otherwise. I think I will pick up a pre-cat sensor and another set of Bosch platinum spark plugs as I think these plugs have close to 100K on them. I'm thinking this may restore the lost mileage.

Anyone have any experience in just replacing the pre-cat sensor and not the post-cat sensor? Any issues with this? Should they be replaced in pairs?

LAZER

Last edited by lazermule; 01-26-2009 at 09:26 AM..
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Unread 02-18-2008, 08:39 PM   #7
Kieffer87
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I just checked my engine code today and it showed up as " o2 sensor bank 1 sensor 2". Is this the same sensor before the cat? Also what would you guys recommend replacing it with? I remember back in high school when I replaced the o2 on my Honda I bought a Bosch sensor from autozone and my mechanic suggested that I don't install it as they are kind of universal and may perform poorly and don't last all that long. I ended up replacing it with another oem one off a header I had laying around. Any input on this?
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Unread 02-18-2008, 08:46 PM   #8
McWood
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i believe sensor two is the after cat one, but i have been wrong before. as far a brand do the bosch. Ive always liked their stuff and eventhing i found bout replacing 02 sensor said bosch part no. 13100 was the best to get.
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Unread 02-18-2008, 09:01 PM   #9
McWood
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Eureka!
Oxygen Sensors (more info than you probably want)
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Unread 04-04-2008, 11:21 AM   #10
pdjeep
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So my Check Engine light is on, and testing shows "Bank 1 Sensor 1" and "Bank 2 Sensor 1" fail. I have two new O2 sensors, and a diagram showing where they are supposed to go... but um, I can't make ends or tails (almost literally) from this diagram. If it were the sensors at Bank 1 Sensor 2 or Sensor 3 (the ones pre and post the the Catalytic Converter) then this would be a no brainier...

But it appears the two I want are in the so very "space efficient" engine compartment.

does anyone have a photo or info on finding these two sensors? I would drive to the shop to have someone show me, but I kind of wanted the engine to be cool when I go reaching around to replace these to minimize the burns.
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Unread 04-04-2008, 01:51 PM   #11
1996xj
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Quote:
Originally Posted by pdjeep View Post
So my Check Engine light is on, and testing shows "Bank 1 Sensor 1" and "Bank 2 Sensor 1" fail. I have two new O2 sensors, and a diagram showing where they are supposed to go... but um, I can't make ends or tails (almost literally) from this diagram. If it were the sensors at Bank 1 Sensor 2 or Sensor 3 (the ones pre and post the the Catalytic Converter) then this would be a no brainier...

But it appears the two I want are in the so very "space efficient" engine compartment.

does anyone have a photo or info on finding these two sensors? I would drive to the shop to have someone show me, but I kind of wanted the engine to be cool when I go reaching around to replace these to minimize the burns.
You need to fill out your jeep info.

You only have one "bank" unless yours has a GM 2.8 V6.

What year and engine do you have?

Welcome to the forum
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3" Rough Country Lift, 3in shackles, 3 RC shocks, 31" BFG ATs, 2 KCs, Lighted Roofrack, Soft 8s, TC drop, Custom oil leak deflector.
If only i could get a set of tires to last me 10,000Miles
Quote:
Originally Posted by toates89
i am just gunna drive around in 4lo on dry streets, and peel out
Quote:
Originally Posted by 4.0H.O.XJ
I once got a ticket for jerking while driving it. The cop wouldnt let me use his pen to sign the ticket.
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Unread 04-04-2008, 06:27 PM   #12
pdjeep
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Quote:
Originally Posted by 1996xj View Post
You need to fill out your jeep info.

You only have one "bank" unless yours has a GM 2.8 V6.

What year and engine do you have?

Welcome to the forum
Hey thanks for the welcome, I didn't even realize the extra fields were there! Great forum!

This is a Grand Cherokee Limited (1999) with the 4.0L V8
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Unread 04-04-2008, 06:43 PM   #13
XJohnnyC
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Just FYI...not flaming.

But your Grand Cherokee is not an "FSJ", as stated in your profile. It's a WJ. And this is the regular 'not so grand' Cherokee forum.

And the 4.0 is an I6...not a V8.
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