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Thread: Mot passed, lambda question

  1. #1
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    Mot passed, lambda question

    Hi all

    My car just passed the first mot in my ownership with just a mention that the discs are worn, I knew that as I got £350 off the price when I bought the car because of it.

    The question is a common one (according to the internet) the fast idle lambda value is right on the upper limit of 1.030 so it passed, according to the internet this means that the fuel mixture is a bit rich.

    The air filter is good so where should I look for a fix (in specific Jaguar terms) as even a bit more mpg would be nice!

    Ta
    S Type 4.2 SE Plus 2002 original spec: Xenon HID/SatNav/Voice/Cassette Player/CDC/TV/GSM Phone/Sun Roof/Rear Blind/CATS.
    Retro-fitted: CD Player/Bluetooth Phone/Aux Power Socket, USB & Volt Meter in Boot/Reversing Cam/Dash Cam & PSU/Dual USB sockets in Armrest/
    Wireless Phone Charger under Instrument Surround.
    Pictures of technical stuff

  2. #2
    Senior Member rchiv's Avatar
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    Hi Graham, You have 4 HO2 sensors so a code reader would be required to tell you which one is at fault.

    Roger
    HEY I THOUGHT I HAD RETIRED, Ex Auto Electrician
    2006 X Type 2.2d SE Manual Estate. Zurcon Blue Champagne/Mocca, F/R Parking sensors, ,Auto Lights, Auto Wipers, Headlight Washers, Sat Nav 2012 update, 17 in Wheels, 116K Mods:- Electric Autofold Mirrors on Locking
    Simple Autofold Mirrors on Locking,http://www.jaguarforum.com/showthrea...irrors+locking
    UPDATE YOUR SIGNATURE http://www.jaguarforum.com/showthread.php?t=91945

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  4. #3
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    Thanks Roger

    Sounds about right, I do have an iCarsoft LR v2.0 code reader and....... unfortunately I have no codes at all, just checked.

    What are these fuel trims I keep reading about ?
    Last edited by graham-r; 07-11-18 at 21:31.
    S Type 4.2 SE Plus 2002 original spec: Xenon HID/SatNav/Voice/Cassette Player/CDC/TV/GSM Phone/Sun Roof/Rear Blind/CATS.
    Retro-fitted: CD Player/Bluetooth Phone/Aux Power Socket, USB & Volt Meter in Boot/Reversing Cam/Dash Cam & PSU/Dual USB sockets in Armrest/
    Wireless Phone Charger under Instrument Surround.
    Pictures of technical stuff

  5. #4
    Senior Member rchiv's Avatar
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    I have no experience of checking fuel trims. But I am sure someone will be along to advise.

    Roger
    HEY I THOUGHT I HAD RETIRED, Ex Auto Electrician
    2006 X Type 2.2d SE Manual Estate. Zurcon Blue Champagne/Mocca, F/R Parking sensors, ,Auto Lights, Auto Wipers, Headlight Washers, Sat Nav 2012 update, 17 in Wheels, 116K Mods:- Electric Autofold Mirrors on Locking
    Simple Autofold Mirrors on Locking,http://www.jaguarforum.com/showthrea...irrors+locking
    UPDATE YOUR SIGNATURE http://www.jaguarforum.com/showthread.php?t=91945

  6. #5
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    The first question you should ask is whether when doing the fast idle test the car is actually going into closed loop mode i.e. whether the output from the lambda probe(s) is being used to adjust the fuelling, or whether the car is remaining in open loop mode, where the ECU simply uses a set of baseline map figures to determine the appropriate fuelling.

    Closed loop mode is usually entered only after about 20 seconds of steady constant throttle, hence why the fast idle test runs for 30+ seconds. (Inside 20 seconds, and whilst accelerating/decelerating the ECU will simply use its mapped values)

    You should be able to check open/closed status via your diagnostic device assuming that it has a screen showing the fuelling status, lambda probe outputs etc.

    When considering fuelling, the two lambda probes of interest are the upstream pair; the downstream pair serve only to confirm that the catalytic converter is doing its job.

    If your diagnostic device shows the lambda probe outputs in real time you should see both of them displaying a sawtooth waveform switching between lean and rich.

    If either one is not doing so then there’s a clue to a fault source. Similarly you may find that one is slooow to switch.

    Regarding the fuel trims; they are adjustment values stored and iterated by the ECU as it learns the characteristics of your specific engine. The trim values represent a delta from the baseline map loaded at the factory, and should only be of the order of a few percent at most (worst).

    You should be able to reset the trims via your diagnostic device.

    Your diagnostic device should show the trim values, the size of which will again give a clue to a problem.

    Finally, despite all the electrickery, it can only work on what it is told, so before delving too deeply do check that there are no tiny leaks in the inlet tract or in the exhaust, for either can deceive the sensors.

  7. #6
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    Quote Originally Posted by Zulu 10 View Post
    The first question you should ask is whether when doing the fast idle test the car is actually going into closed loop mode i.e. whether the output from the lambda probe(s) is being used to adjust the fuelling, or whether the car is remaining in open loop mode, where the ECU simply uses a set of baseline map figures to determine the appropriate fuelling.

    Closed loop mode is usually entered only after about 20 seconds of steady constant throttle, hence why the fast idle test runs for 30+ seconds. (Inside 20 seconds, and whilst accelerating/decelerating the ECU will simply use its mapped values)

    You should be able to check open/closed status via your diagnostic device assuming that it has a screen showing the fuelling status, lambda probe outputs etc.
    The Lambda value was arrived at during the MoT so I have no idea how long they ran the engine, looking at the value being exactly the upper limit did/does make me a bit suspicious as to what is actually going on, especially when the other params are all very good - i.e nowhere near the upper limit.

    Quote Originally Posted by Zulu 10 View Post
    When considering fuelling, the two lambda probes of interest are the upstream pair; the downstream pair serve only to confirm that the catalytic converter is doing its job.

    If your diagnostic device shows the lambda probe outputs in real time you should see both of them displaying a sawtooth waveform switching between lean and rich.
    I have an iCarsoft LR V2, I think it should do fuel trims, I won't be able to try until Monday......

    Quote Originally Posted by Zulu 10 View Post
    If either one is not doing so then there’s a clue to a fault source. Similarly you may find that one is slooow to switch.

    Regarding the fuel trims; they are adjustment values stored and iterated by the ECU as it learns the characteristics of your specific engine. The trim values represent a delta from the baseline map loaded at the factory, and should only be of the order of a few percent at most (worst).

    You should be able to reset the trims via your diagnostic device.

    Your diagnostic device should show the trim values, the size of which will again give a clue to a problem.
    Thanks for the reply, this may take a little investigation.
    S Type 4.2 SE Plus 2002 original spec: Xenon HID/SatNav/Voice/Cassette Player/CDC/TV/GSM Phone/Sun Roof/Rear Blind/CATS.
    Retro-fitted: CD Player/Bluetooth Phone/Aux Power Socket, USB & Volt Meter in Boot/Reversing Cam/Dash Cam & PSU/Dual USB sockets in Armrest/
    Wireless Phone Charger under Instrument Surround.
    Pictures of technical stuff

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