Magnum Swap, which 02 sensor to use, pre-cat or post-cat

Magnum Engine Swaps

  1. robcuda

    robcuda FABO Gold Member FABO Gold Member

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    Hey guys,
    On a swap using a 97 Magnum 360 with computer. Donor has two 02 sensors. Which 02 sensor is used to make the computer happy? Pre, Post, or both? I will not have a catalytic converter and running dual exhaust. Thanks!!! Rob
     
    Last edited: Sep 30, 2020
  2. leechris81

    leechris81 Well-Known Member

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    Pre-cat O2 sensors are used to control air/fuel mixture in OE applications. Post-cat sensors are nothing more than a catalyst efficiency monitor, meaning it tells the computer whether the converter is functioning properly. If you have a check engine light and no catalytic converter, you will have to fool the computer in some manner to keep the light off.
     
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    • dano

      dano Evil Handy Man

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      Depends. If the truck/van had one cat then one o2 was pre-cat and the other was post. If they are both missing then you will get a code. If dual exhaust then they are both pre-cat. The post-cat o2 needs to be defeated somehow as it is looking for converter efficiency.
       
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      • robcuda

        robcuda FABO Gold Member FABO Gold Member

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        Would it make sense then to use the pre-cat 02 sensor in order to get the engine to run best?
         
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        • leechris81

          leechris81 Well-Known Member

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          If you're using the fuel injection from the '97, yes. Without the feedback from the O2 sensor, your fuel mixture would never be entirely right.
           
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          • robcuda

            robcuda FABO Gold Member FABO Gold Member

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            Yes, that makes sense! I will use precat, and shouldn't care if I get a code from NO postcat sensor.
             
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            • leechris81

              leechris81 Well-Known Member

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              Only issue would be if the check engine light is on all the time, you won’t know if another code pops up. That may not matter to you though unless drivability was affected, which you would notice regardless.
               
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              • dano

                dano Evil Handy Man

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                And it goes into limp home mode which takes a lot of the table.
                 
              • robcuda

                robcuda FABO Gold Member FABO Gold Member

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                I'm
                I believe 98 and up will go into limp mode, with a 97 I'm hoping this is not the case...but will surely find out.
                 
              • CudaFish1968

                CudaFish1968 Active Member

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                I did a magnum swap in my Barracuda in 2012, ran factory pcm, no cats. The way I tricked the pcm was to use a non foul sparkplug adapter in down stream O2 hole, then screw down stream O2 cell in. What happens is the down stream O2 cell will report a change but since it is not in the exhaust stream it will not read correct. The pcm sees a signal and never knows the cat is gone. I also did this when my cat on my 2000 dodge dakota went bad, removed cat straight pipe, used this trick and presto no codes.
                 
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                • TT5.9mag

                  TT5.9mag Two atmospheres are better than one

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                  • dano

                    dano Evil Handy Man

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                    Yep. A pretty common work around if ECU tuning is not available, not yet done or if the ECU still has fits. Some are really sensitive.
                     
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                    • robcuda

                      robcuda FABO Gold Member FABO Gold Member

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                    • robcuda

                      robcuda FABO Gold Member FABO Gold Member

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                      Appreciate you sharing your technique! Just curious how you ran your cuda without a PreCat? Even though your tricking it, it's not really going to make any adjustments for air/fuel, right?
                       
                    • CudaFish1968

                      CudaFish1968 Active Member

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                      The O2 cell after the cat tells the pcm that the cat is working that is it's only job. The up stream O2 cell is responsible to tell the pcm if rich or lean that is key! The magnum swap I did was from a 1996 truck. It had 1 up steam O2 cell and 1 down stream cell. I used a H pipe for crossover and in the H pipe I welded a bung and that was were the non foul adapter and down stream O2 cell was located. My engine was a little fat at idle due to camshaft change of duration and overlap, other than that it ran fine. I did not have any codes.
                       
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                      • robcuda

                        robcuda FABO Gold Member FABO Gold Member

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                        Thanks for clarifying, I appreciate it!
                         
                      • jbc426

                        jbc426 Well-Known Member

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                        I went through this when I set up my factory EFI on my 1999 OBD2 system, and used the pre-cat sensor for the closed loop metering.

                        I also hooked up an electronic cat simulator to feed an artificial post-cat signal to the computer to not have it throw a code and go into limp mode. However, I'm not sure not having that signal would have thrown it into limp mode. It may have just turned on the check engine light and continued to run as programmed.

                        You can always call SOLO PCMs in Florida, which is where I got my ECM with no security and set-up for a manual trans car, as I run a 2004R. Another great resource for SCT Tuner custom programming is Chris at Performance Injection Equipment.

                        https://www.performanceinjectionequipment.com/
                         
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                        • robcuda

                          robcuda FABO Gold Member FABO Gold Member

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                          Hey thanks for adding input as I have followed a lot of your stuff! The engine is up and running with just the pre-cat. Smooth as silk!! I owe a TON to this site and a guy here who went OUT OF HIS WAY to coach me through this. I have not driven it yet, but hope that my 97 does not go into limp mode, will find out soon, from what I have read it looks like starting in 98 security features were added so crossing fingers! I can deal with a check engine light for the time being but nice that you shared resource in Florida if I need it!!!!
                           
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                          • jbc426

                            jbc426 Well-Known Member

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                            Another insight I found is that routing the plug wires correctly eliminated an otherwise imperceptible misfire that showed up on a Matco Scanner I was able to borrow to sort my set-up out.

                            Here's the link to the TSB from Chrysler.

                            5.2/5.9 Misfire TSB 18-48-98
                             
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                            • dusted

                              dusted FABO Gold Member FABO Gold Member

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                              Never seen this happen on these engines
                               
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                              • jbc426

                                jbc426 Well-Known Member

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                                When I was first getting my EFI set-up running, the car would go into limp mode when it transitioned from open loop to closed loop after the first minute of running.

                                It turned out that I had an open ground circuit on my O2 sensor. A quick bit of solder, wrap of tape and some heat shrink solved the issue. The Matco Scan tool I had borrowed from a buddy allowed me to find the issue. Without a good scanner, it would have been nearly impossible for me to diagnose. You can see where I had to ground the wire in this picture.

                                ac1 (Medium).png
                                 
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                                • dusted

                                  dusted FABO Gold Member FABO Gold Member

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                                  I could understand it doing this, a lot of times it will default to pig rich.
                                   
                                • robcuda

                                  robcuda FABO Gold Member FABO Gold Member

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                                  Good to know! Both about using a great scan tool AND watching for ground issues.
                                   
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                                  • C130 Chief

                                    C130 Chief Mechanical Genius

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                                    The JTEC systems won't go into limp mode, but will throw a code/light if the signal from the downstream O2 sensor is out or the converter fails. At least on my 2005 Jeep 4.0.
                                     
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