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Thread: 3.0 throttle body issue

  1. #1
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    3.0 throttle body issue

    My S-Type failed to start recently due to a gummed-in throttle body butterfly valve. This was obvious on inspection as there was a ring of oily, sooty deposits (normal) around the seating area of the valve, but also what looked like lumps of brown sugar (normal ?), some kind of residue I suspect from the fumes which are directed through there from the cam covers. Luckily a sharp tap with a file handle freed it, but I couldn't risk leaving it like that for it to stick again, most likely at the most inconvenient time. Now I know the mantra about not cleaning TBs, it even says so in the manual, but I gave it a go anyway. I removed all the deposits, some of which were very firmly glued in there, using a plastic spoon, then very gently applied some fine wire wool to get rid of the last marks. A spray lubricant was applied to the pivot areas and some WD40 and a good wipe over finished it off. Yes, I know about the special coating which was now most likely completely eradicated, but I had to give it a go. It started first time and runs lovely now. Well almost ...

    It seems now that the throttle sort of sticks very slightly when it should be fully closed. So when the accelerator is released, such as slowing for traffic or a junction, the car seems to want to continue a little, like a mild cruise control. It's only slight, but it's annoying as I have to brake more than I should, plus slow-moving traffic is a pain as I am having to dab the brakes way more than normal. It still settles to idle when at rest, but it takes a few seconds longer than it should. The worst thing is that it seems to confuse the auto box into late downshifting, like it drops into first with a bit of a jump just before you stop completely. I should point out the 'box was perfect before I cleaned the TB.

    So I suspect the TB now leaks a little air past the butterfly when it is shut due to the absence of the magic coating. This unwanted airflow possibly makes the fuel system apply a little power when it is not required. Am I into a replacement TB or can it be re-sealed ? I thought about applying a ring of non-setting sealant such as Wellseal where the valve seats, or is that courting disaster ? What is the original coating anyway, could a coat of clear lacquer do it I wonder ? I have some for brake calipers which is certainly temp-rated enough for a TB. Anyone had this issue ?
    '01 S-Type 3.0, Topaz/Cashmere
    Previous Jags : '01 S-Type 3.0, Sapphire Blue/Cashmere, '77 XJ6C, Fern Grey/Moss Green

  2. #2
    Senior Member Jim_S-V6_2004's Avatar
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    Forget the coating and don't try to put anything on there either.

    Did you clean the Venturi (Throttle body where the valve sits)?

    I used Isopropyl alcohol to clean all the gunge off, and a plastic scouring pad, the one without sponge.

    Just have another go and get all the residue off, mine had a solid ridge of gum.

    On my old Vectra there is a tiny air bypass hole inside the Venturi and that was blocked causing the engine to stall when slowing down.

    I don't think the Jag has one but have a good look just in case.

    The ECU should settle in a couple of hundred miles through its continual learning process.

    Make sure you haven't left off a vacuum tube!

    .

  3. #3
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    I'd start by looking at fuel trims to see if you have an air leak or "just" some other problem that appeared at the same time. OK, it'll be a leak but it's worth a check.

    And then get a used TB.

    Don't use things to attempt a seal because if they get into the engine you could be in for much worse.

    The TB is a high precision item best cleaned MUCH more gently if done at all.

  4. #4
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    I had cleaned the venturi thoroughly with a green pad, the butterfly disc was actually quite clean. The whole thing was spotless when it went back in after completing the timing chains replacement. I've done a few hundred miles since and it's not getting better. I recall I didn't use a new TB gasket, plus the TB had already been off a couple of times before, to gain access to hoses when I did the DCCV. So that gasket is well-used now. Maybe a quick coat of Wellseal on both sides could rule that one out at least. I'll check and seal the vacuum pipes too, could be as simple as that.
    '01 S-Type 3.0, Topaz/Cashmere
    Previous Jags : '01 S-Type 3.0, Sapphire Blue/Cashmere, '77 XJ6C, Fern Grey/Moss Green

  5. #5
    Senior Member Jim_S-V6_2004's Avatar
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    Use an ELM327 or other code reader to read the Long Term & Short Term Fuel Trims, those will show any lean mixture, indicating an air leak. Then as you waggle the hoses you'll see when you get the right one if it's one of them.

    The best way though is inject smoke.

    You can also spray contact cleaner, carb cleaner or brake cleaner or other stuff slowly round the top end and TB with the engine running to find it.

    Yep it is annoying.

    .
    Last edited by Jim_S-V6_2004; 11-02-19 at 20:40.

  6. #6
    Senior Member Lost it's Avatar
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    This is a useful video to watch if you are interested in checking fuel trims, explains it well. As Jim has suggested, a slow return to idle is usually an air leak after the TB...

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