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  1. #1
    Hank
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    WHY do Throttle Bodies go bad?

    I still experience intermittent surging problems that may or may not be the TB. The reason I think possibly not is because I now notice that bumps on the highway can cause a brief loss of thrust. Is this also the TB?

    I don't know but I would like a technical description as to how the throttle body functions and WHY certain years are prone to TB malfunctions. Does anybody know the route cause of the malfunction?


  2. #2
    Thierry_1500
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    Re: WHY do Throttle Bodies go bad?

    The malfunction for which there is a recall is due to the build -up of gum in the TB. This gum can cause the TB to think it has reached an end-position marker and no more to open completely.


    The cure is to reprogram the TB eeprom with a new S/W that will not allow this malfunction.

    Some later TB need to be exchanged as they do not have an eeprom but a prom.
    (proms are cheaper and Jaguar certianly thought from a given point that the design was good enough to be frozen. Bad bet...)

    Other problems of the TB usually involve a malfunctionning potentiometer.


  3. #3
    richyp
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    Re: WHY do Throttle Bodies go bad?

    Hank,

    This is a long shot, and a real guess. Nearly 2 yrs ago I closed the bonnet (hood) on a roll off kitchen paper and twisted the bracket that holds the plastic intake to the TB. Under full load the engine would "hang up" and then surge. The answer was obvious when I opened the bonnet! Perhaps there is someone as stupid as me, but he didn't tell you! If the Intake is not snug on the TB something is amiss (Yep, they wobble a bit)

    Rich

  4. #4
    Hank
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    Re: WHY do Throttle Bodies go bad?

    I doubt that is the problem Rich since I had the TB cleaned by a mechanic, which I had hoped would solve the problem, but it didn't. It just made things worse and the car started to go into a full stall at highway speeds, rather than just pause and surge. I assume he would have noticed if the intake was loose. That's when I had it hauled down to a Jag Dealer for the reprogramming called for in the recall. It doesn't stall anymore but I'm back to square one with the peridodic pause/surge crap. Happens about 50% of the time but nearly always during the first few miles the car is driven from a completely cold start. Once it gets warmed up and has run a bit the problem seems to resolve itself. I also notice occasional hesitation when I hit a rut in the road at highway speeds. Can this be related to a TB problem? Who the hell knows.

    What I do know is I'm tired of chasing the problem with my wallet. I have this sneaking suspicion that if I replaced the TB (and I have a used one) the problem would still be there. Maybe it's just paranoia but I would never trust this car on a long road trip. I was going to drive it the 300 miles to visit my Parents next month but there is no way in hell I'm doing that. I'll just let it do it's thing around town until next spring. If I'm lucky it will break definitively so the flaw becomes clear because this intermittent crap does nothing but soak up money and time. If I can't get the situation resolved by next spring, then I sell the damn thing and move to something less aggravating.

    It's like I tell everyone, it's the most beautiful car you'll ever see going down the road on the back of a flat bed truck!

  5. #5
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    Re: sounds electrical to me

    Hank,
    A stab in the dark, but if you're getting an intermittant fault such as a periodic pause/surge, and hesitation after going over a bump.... surely a potentially loose connection on the electrics would deserve some investigation.
    waddyafink?
    AndyD

  6. #6
    Hank
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    Re: sounds electrical to me

    Sounds reasonable. The question is where to start and what to check? I've heard that a faulty brake sensor can also cause this but how do you check that? The brake lights don't stick on and when the car surges I check the brake peddle with my left foot to see if it is sagging at all but it doesn't seem to be.

  7. #7
    MarkyM-UK
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    What about a loose lambda sensor?

    Just a thought.

    M

  8. #8
    Hank
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    Re: What about a loose lambda sensor?

    What's a lambda sensor and where do I find it?

  9. #9
    MarkyM-UK
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    Re: What about a loose lambda sensor?

    It attaches to the front of the exhaust, specifically the catalytic convertor which is part of the downpipe.

    As you'll appreciate hard to diagnose a problem remotely....have you been to a main dealer (or another with WDS) and checked any fault codes. From memory (and someone else can correct if wrong) it was only the first batch of XK's (96-98) that had the TB recall.

    Things I would check (unless they've been checked or replaced recently) are:

    Temp/lambda sensor (cheap to replace)
    Injector performance (V expensive from jaguar if faulty) can be cleaned/calibrated for around $150
    coil condition (regularly on ebay)
    spark plug condition (typically changed every 60k miles)
    electrical grounds
    air filter condition
    any leaks in airflow

    But I am no mechanic, when I've had faults in the past I've always checked the lowest ticket items first.

    Mark



  10. #10
    Hank
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    Re: What about a loose lambda sensor?

    No fault codes come up. Like I said, it has been to a Dealer for the recall issue. Plugs, filter have been checked and/or replaced. It also passed emissions, so that leads me to believe the oxygen sensors are good or it would have failed. Of course, the emissions test was about 1 week prior to the first time it acted up.

    Looks like the closest mechanic I can find, next to a Dealership, that is competent with Jags is in RI. I guess I should have checked on the availability of a Jag mechanic within close proximity first before buying the car!

  11. #11
    Hank
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    Re: What about a loose lambda sensor?

    Finally found a local Mechanic who MAY have a clue when it comes to Jags. He was recommended to me by another Mechanic that specializes in Volvo's.

    At any rate, the Chief Mechanic & Owner was out of town for a car show, so I spoke with his Wife who was also very knowledgeable. It sounds as if this guy has dealt with a lot of Jags and mainly deals with European and exotic cars. In fact, his Wife tells me he is booked for the next month and I consider that a good sign. Sort like seeing the best Dr. You always have to wait unless it is an emergency.

    Anyway, this guys Wife tells me that even if the problem is my TB it might still have to go to the Dealer to get reflashed. Of course, the final say on this will come from her Husband who is the true expert. She said they had to truck a number of Jags down to the Dealership to get reflashed because the TB software is proprietary and can only be done at a Dealership. In my naivety I was unaware that the TB itself carried computer chips. I thought the signals came from a separate computer module.

    Bottom line, I couldn't replace the TB myself and make it run right even if I wanted to. It still has to go to the Dealer to get programmed.

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    Re: sounds electrical to me

    Replace the TB. I replaced mine, and never had another problem. I know it seems odd that the TB can cause those weird intermittent engine jerks and stalls, but it does. Another area of concern is the motor mounts. If they are loose, your motor can torque and stretch the wire loom causing sensor input errors. Spend the 2K! Join the TB relpacement club! : )

  13. #13
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    TB flash programming depends on MY

    MY 1997 TB replacement did NOT require any reprogramming. It was a true bolt on fix! MY 1997.

  14. #14
    Hank
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    Re: TB flash programming depends on MY

    If this is true, then why did both the Jag Dealer AND Jag Corporate tell me I had to get the TB reflashed due to a recall? The Dealer did it for free and even gave me a loaner for the day, so they didn't make a dime off me for doing this, and I burned a half tank of gas on them in the process driving their loaner all over CT.

  15. #15
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    Re: TB flash programming depends on MY

    my car was a 1997. What year is yours? The Model Year (MY) makes a huge difference.

  16. #16
    Hank
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    Re: TB flash programming depends on MY

    Mine is an early '97.

  17. #17
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    Re: WHY do Throttle Bodies go bad?

    All, in general, I firmly believe that the best way to understand how thing go wrong is to understand how they work when the're right, and look for differences. That being said, here's what I know about throttle bodies, throttle position sensors, and the failure code P0121 which leads to the performance inhibit/ABS/failsafe/limp home mode.

    The throttle scheme on our XK8's is a fly by wire scheme, where your right foot drives the accelerator pedal, which pulls a cable connected to a Throttle input Position Sensor. (Actually, there are two, "A" circuit and "B" circuit, each connected to a different part of the ECU.) To be clear, your foot does not open the throttle, just moves a pair of variable resistors.

    The ECU takes these two inputs representing instructions by your foot, and provides a signal to a motor on the throttle spindle which opens or closes the throttle according to your foot input as well as pre-programmed responses from the ECU.

    To complete the feedback, there are two throttle position sensors, again "A" and "B", also variable resistors, which allow the ECU to compare the desired throttle position, represented by the input TPS signals, to the actual throttle position, represented by the output TPS signals.

    The throttle body itself is a chunk of die cast zinc alloy, with an abrasion resistant coating at the surface the throttle plate contacts when fully closed. It is extremely unlikely to fail. Attempts to fix TPS related issues by cleaning amount to a snake oil remedy in my opinion. Not that a clean throttle body is a bad thing, but it won't address the root cause of our issues, which is the sensors themselves. Other problems can contribute, such as a throttle motor failure, poor connections, etc., and these can be easily sorted out, but the main cause is sensors varying over time.
    These sensors are carbon track and wiper setups, which are manufactured by Nippondenso, and are known to change in value over a lifetime of opening and closing. The ECU is expecting agreement between the input and output sensors within a particular range, and when the values are outside the expected range, the dreaded P0121 is invoked. Re-flashing is prescribed for some model years, but it is not the throttle body which is reflashed, since there are only passive components there (2 input sensors, one motor, two output sensors, plus AAV). The re-flashing is done to the ECU, to change the values of acceptable deviation between input and output throttle position sensors before the fault code is triggerred.
    If you start having the problem, start looking for a new throttle body, since it will only get worse, though it may be intermittent for a while since the variable resistors can change with temperature (probably wiper pressure)and other factors such as internal debris. Jaguar sells throttle bodies only as complete assemblies, and there doesn't seem to be a reliable sensor repair/replacement scheme available yet. It seems like a nice cottacge industry opportunity.
    Well, that's the extent of my knowledge on this subject, I welcome additions and clarifications.

    Happy Motoring,
    Jerry Lippmann
    2000 XK8 (waiting on arrival of new throttle body)

  18. #18
    SanDiegoMac
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    Outstanding Post!!! Thank you Jerry


  19. #19
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    Re: WHY do Throttle Bodies go bad?

    Jerry: Your analysis is essentially correct. However, I believe that the computer compares the signals from both the pedal position sensors to each other to monitor failure. Also the throttle plate position sensors to each other. But it does not compare the pedal sensors directly to the throttle position sensors. Of course it could compare them to monitor when the throttle plate position matches the "Request" signal from the pedal position sensor(s).

    Another interesting feature is the "Fail Safe" Throttle coupling. Normally the pedal mechanism "lags" the position of the throttle plate by some number of degrees (maybee 30 degrees). If the throttle motor fails, the throttle is spring loaded to the closed position. Then the pedal mechanism rotates 30 deg before mechanically engaging the throttle plate shaft for manual throttle operation. Because of the 30 deg "lag", the mechanical operation of the throttle plate can not open fully, only to 30 deg short of full opening. Hence the "Restricted Performance" capablity.

    2000 XKR BRG/Cashmere

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