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Does anyone have a definitive answer on the Downstream 02 sensor debate?

IMO, if you can successfully fool the CA-emissions ROM downstream O2 sensors, its better to run the CA-emissions ROM because this ROM will provide better fuel trim control due to using one upstream O2 sensor to set closed loop AFR for three cylinders and another upstream O2 sensor for the other three cylinders.
 
I just may end up doing that maxbraketorque. I plan on having this jeep until the doors fall off I may even order a new motor for it but Being how I can't find a federal wiring harness even online. The last ditch effort I have is reading through the fsm and trying to see how the 2 harnesses are different and modify the California one into a federal one.

Did you try Car-Parts Inc. ? I saw two in Washinton, but it didn't say Fed or not. I'll see if I can determine the diff from my 00 to 01. The exhaust is a bit different.
 
Yeah so I'm just going to go ahead and weld in some bungs and go ahead and try the spark plug extensions for the downstream o2 sensors.

I gotta finish pulling this motor and installing the replacement but..

I'll update on here and do a mpg comparison and all that when I finish.
 
I did look into the wiring differences. My '01 came as a basket case. Motor was blown, no head. Wiring had some cut pieces. It looks like the difference is that wiring piece that runs from the EMC/PCM to the Injectors, TB etc.
So, the possibility is to swap in an '00 FED of this part of the engine wiring harness, an '00 FED down or crossover exhausts pipe and possibly the '00 FED ECM/PCM. I will try this when I get to that point on my project.
 
In my travels I came across an '00 with Federal Emissions in the boneyard.
I own a '00 with CA Emissions.
From researching this best I can tell is what 75SV1 said.
You need one of the 2 engine bay wiring harnesses, the one with the fuel injectors on it, and the computer.
So I grabbed them. Figure never seen one up close and personal before and probably never will again.
Maybe at some point in time I will try what 75SV1 wants to do.
Now I put a brand new Walker, dual cat back, exhaust on awhile ago.
There is a bung hole for an O2 sensor in the rear cat. Also one in the down pipe.
I even bought the Walker Y pipe that replaces the dual cats in a Federal Emissions Jeep.
My thinking is if I am going to do this I am going to wait.
Cause in my state once a vehicle is 25 years old or more it is exempt from Emissions testing so...... LOL.
 
To the Computer there are three connectors. One comes from the main wiring harness. The main one goes to the block that goes to the Inside or Cab wiring harness. The other two connectors go to the Injectors, TPS, etc. There are two other wiring harnesses in the engine bay. The headlight harness and the starter relay harness. Or at least that is what I know. Knowing that, I'll be looking for an '00 Fed "Injector' Harness. I might already have one. I will need another for the MJ. I plan to got to '00 Fed for it too. It's less cluttered on the Starter side.
 
Yeah thats what I was noticing too 75SV1. I could not even find the last time they sold a used one of those harnesses so I just could not compare them my under standing is that the federal one is on a relay with maybe an extra wire to the pcm and the other one is not.

If you you guys have the harness's that's so much easier than trying to rewire it from the fsm, I was finding federal pcms online but absolutely nothing for harnesses maybe someone can extract the differences from the two and find a way to do a writeup, even pictures of then side by would be enlightening to the the internet.
 

Thank you I actually ended up ordering as they looked more precise than the foulers and got them in the mail today, they seem of high quality and should do the trick for the downstream sensors.


UPDATE to this thread. Sorry iv been busy working on the jeep. I have one motor stripped down ready to be pulled and the next is almost ready to be pulled and painted. (Iv been saving pictures to start a separate thread)

I may have found a solution that may deserve a writeup later down the line, I'm waiting for tomorrow to call this custom exhaust place and another member from another forum that did the same thing to respond before I give give it my effort.

it's simply welding the upstream bungs into only cylinder 1 and cylinder 6.

It doesn't involve buying an expensive header and cancels out having to buy a header that has the 02 sensors welded into each bank. But iv yet to hear back on what exactly it affects I just know iv seen a build thread where it didn't throw any codes. Hell the dude didn't even use 02 simulators he just put the downstream 02 sensors farther back and it still didn't throw a cod until one of them went bad.

I want to know if it affects fuel trim or is just setup that way to diagnose problems easier. If it's the second option I'm going to do it.

I thought it was all figured out and i could move on but when I started looking for a header that would fit a 97 4.0 I started to find that the only ones that had bungs or enough space past where the 3 tubes convert into one were either way too much or way too cheap/low quality.

The problem is nobody makes decent headers for a mid price point that would work for swapping pre 99 engines into 2000+ xjs so I was finding that'll I either had to buy a pre 99 or a post 99 header.

The difference is pre 99 is only one downpipe tube

2000+ headers have a split downpipe and smaller diameter head to header inlet than the 97-99 engine I was swapping in.

The entire point of this whole thing is I don't want the excessive heat the precats generate and to eliminate chokepoints on my exhaust system

that lead me realizing spending 500$ on a header was not worth investing into until maybe after I was done with other modifications that were more important to me. But....... that still remains to be seen.

My plan for now is to simply use the exhaust manifold from the 97 and weld in the 02 sensors because the way the tubes are setup is exactly as the 2000+ are and even includes a more header like design here is plenty of space to do so and give it the same setup as stock minus the precats.


but if I end up finding an answer to exactly what having it set up like I mentioned above with the o2 sensors on cylinder 1& 6 I may end up saving that money before I waste 50$ having a shop weld the bungs to the stock exhaust and go ahead and just have it done to a header.
 
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