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third thermocouple

Discussion in '348/355' started by darrenliu, Jul 5, 2015.

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  1. darrenliu

    darrenliu Formula Junior

    Jun 24, 2008
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    Melbourne Australia
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    Darren Liu
    #1 darrenliu, Jul 5, 2015
    My 355 spider is a 1997 model, UK spec car. It does not have the thermocouple on the bypass valve. Any know the reason for this? The advantage of this is i can keep the bypass valve open without any check engine light.
     
  2. gothspeed

    gothspeed F1 World Champ

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    #2 gothspeed, Jul 5, 2015
    You can wire it open but there is really no performance benefit ...... having the valve closed and letting the ECU trigger the actuation is best on the 5.2 motronic 355 .... it allows proper exhaust flow over the o2 sensors during low RPM operation ... :)
     
  3. darrenliu

    darrenliu Formula Junior

    Jun 24, 2008
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    #3 darrenliu, Jul 5, 2015
    goth, a while ago, i had the LHS cat burn out completely, leaving a hollow shell. I didn't get a cel, but the slow down light came hard on and the car shut down. in that situation, should the 02 sensors have thrown a cel? I replaced the cats with hyper flows and haven't had a slow down light since, but 'm wondering if i should change the 02 sensors. The tailpipes are very black with soot (like its running rich).
     
  4. gothspeed

    gothspeed F1 World Champ

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    #4 gothspeed, Jul 5, 2015
    At this age .... the only thing most factory cats are good for now .... is to have them re-cored with metallic elements or as you did .... replace them altogether.

    Yes, the ECU should have triggered an o2 sensor code ...... but if the ECU is getting a low coolant signal ... the engine will run rich without throwing a code ...... have you participated in the poll listed in my signature below? a great many of our thermostats are living a short life and not keeping the coolant at a high enough temperature ...... check out the poll and let me know what 'number' you are getting .... :)
     
  5. darrenliu

    darrenliu Formula Junior

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    #5 darrenliu, Jul 6, 2015
    goth, I replaced the thermostat last year after noticing very high fuel consumption and the temp gauge not getting off 0 when cruising on a cold day. I replaced the thermostat and all good now.

    How do know when its time to replace 02 sensors (if there is no check engine light)?
     
  6. gothspeed

    gothspeed F1 World Champ

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    #6 gothspeed, Jul 6, 2015
    There is no harm in replacing them if you suspect they are bad ...... but you can take them out (do not mix pre and post cat positions) and visibly make sure the vanes are not clogged with soot ..... if you had/have soot in the exhaust pipes then you may have some soot on the o2 sensors ....... let me know what you see or post pics of the o2 sensor tips ... :)
     
  7. darrenliu

    darrenliu Formula Junior

    Jun 24, 2008
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    #7 darrenliu, Jul 6, 2015
    ok Goth, thanks very much for your help.
     
  8. taz355

    taz355 F1 Veteran
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    Feb 18, 2008
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    #8 taz355, Jul 6, 2015
    Goth would wiring the bypass open not actually reduce the bottom end torque and not help the top end sine the valve is open on top end anyway?
     
  9. gothspeed

    gothspeed F1 World Champ

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    #9 gothspeed, Jul 6, 2015
    You are correct! .....thats why I mentioned the lack of benefit ...... but depending on which post cat exhaust one is running .... there can be a slight low single digit HP gain .... but generally speaking wiring the bypass open does a lot more harm than any potential HP gain .... :eek:
     

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