Spark Plugs and O2 Sensors -
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Unread 12-01-2012, 11:56 PM   #1
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1999 WJ 
Join Date: Dec 2011
Location: Duluth, MN
Posts: 255
Wj Spark Plugs and O2 Sensors

'99 4.7L V8, 160k, seems to be doing fine right now, no codes. Since I know the spark plugs are at least 5-7 years old, and may be original, I am going to replace them. There are no codes, but I was also thinking about replacing the O2 sensor(s) (Federal Emissions). A few questions;

1) I have the plugs (yes, I have the right plugs) in hand and was going to do it tomorrow. Does it matter at all if I do the plugs first and then the sensor(s) in a month or two? (I am concerned about a less than perfect O2 sensors causing excessive fouling on the new plugs.)

2) If I wait until I have senor(s) and plugs both in hand, does it matter which one I do first? (I don't want to do both at the same time in case there is a problem and it will probably be about a month in between since my wrenching time is VERY limited.)

3) Am I throwing away money by replacing the O2 sensor(s) when there are no codes?

4) If/when I replace the O2 sensors should I replace both, or just the upstream sensor?

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Unread 12-02-2012, 06:34 AM   #2
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2004 WJ 
Join Date: Nov 2012
Location: Windsor, Ontario
Posts: 627
Do the spark plugs. You will be so pissed off after doing the passenger rear one you won't want to do the O2 sensors.
Be sure to put anti seize on the plug threads, and if the current ones have been in that long, they will most likely very stuck so be careful and hope non break. Be sure the engine is Cold , do not attempt on a warm engine, you could destroy the plug threads.

Now, the O2 sensors, the most important ones are the up streams, they control the air fuel ratio, the down streams are just for cat checking. Spray them with a good penetrating lube, several times this week, and next week you should be able to get them to come out. O2 may not throw a code, but the do get slower with age adjusting, so they probably need changing.
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Unread 12-02-2012, 04:49 PM   #3
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2002 WJ 
Join Date: Aug 2008
Location: Georgia
Posts: 214

Blow all the debris from around the plug area before you start. There is nothing like getting STUFF in the threads ! Champion copper. Use Dielectric grease on the coil leads (keeps out moisture). If you can find a copper based anti seize, you'll be happy the next time you do a tune up. The cheap stuff dries out. After 50 years of working on cars, I found out the hard way. We are on our 3rd JEEP ! Plugs are a PITA. The PCM adjust the voltage as they age and they last a long time. Take your time as the 2 on both sides nearest the firewall are a rear character builder on the 4.7L !
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Unread 12-03-2012, 12:42 AM   #4
Registered User
1999 WJ 
Join Date: Dec 2011
Location: Duluth, MN
Posts: 255
Thanks for the input. Yes, I have Champion copper plugs. GTG.

I got my stuff out to start the job today and got pulled away 30" in. At least I got the oil changed. Giving Rotella T6 5aw-40 with an oversized Motorcraft filter a shot. I will be sending it in at 3k/3 no and will report back with info later.

So, now to try to make sure I get those plugs done next weekend!
Thanks sgain.
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o2 sensors , spark plug , wj

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