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Thread: 3.0 CDTi fuel consumption & gearbox.

  1. #1
    Regular Member RhayaderSax's Avatar
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    Default 3.0 CDTi fuel consumption & gearbox.

    06 plate facelift 3.0 CDTi Signum 6 speed auto is just coming up to 50k miles on the clock & is due a service shortly. Last years was done by a competent independent. I'm considering taking to Vx for next service due to one or two problems:


    1. Very occasionally it only fires on 5 at tickover. Fires on all 6 when moving. This fixes itself quickly & then reoccurs randomly - almost as if there's a bit of grit in an injector that blocks the hole then rattles itself free.
    2. Auto gearbox seems to like to hang on to the revs when coasting, not dropping out of gear. This is almost like my old Vec B auto behaved with the sport button pressed.
    3. Also changes down when I brake going downhill.
    4. Occasionally hangs in 1st up to 3000+ revs before changing up to 2nd at low speed with very little throttle - usually around Somerfield carpark which is level (ie: not a 1:6 hill!!).
    5. Fuel consumption. I used to get 38 to 45 mpg on a run. I'm now getting 30 to 33 mpg on the same journeys & a shocking 18 to the gallon towing a horsebox.

    There's no loss of power. The gearbox is still smooth (except for problem #4). There are no unusual noises or rattles apart from "feeling" it missing when problem #1 happens.

    Does the gearbox have some sort of driving style memory? I've done a bit (but not a lot) of towing over the last 12 months but other than that my driving style hasn't changed & my right foot has the same same amount of lead in it as before!

    As some of the problems are intermittent I'm inclined to take it Vx for service & have them plug it in to the computer.

    Help & advice appreciated on what I should be asking them look for & do.

    PS: engine & tuning are factory standard. There are no mods/tuning boxes. All fluids & filters were changed 11 months (6000 miles) ago.
    Last edited by RhayaderSax; 18th July 2010 at 15:47.

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    ECU may have picked up on the fact that sometimes the car seems to weigh a lot more on some occasions! This will alter the way that it fuels the car (injection timing etc).

    I'm not surprised that it also seems to be hanging onto gears if you have used it for towing! Even after you have not done any towing for a while, the ECU will take some time to settle back into 'normal' mode.

    Easy remedy is to disconnect the battery for half an hour, then reconnect it, and then 'drive like your Granny' for a while.

    Edit: Not sure about the 'missing'. Never happened on mine.
    Last edited by diesel dog; 18th July 2010 at 16:18.

  3. #3
    Regular Member RhayaderSax's Avatar
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    Thanks for your reply diesel dog. I too had presumed problem #4 was the ECU still thinking it had a trailer attached. I don't do THAT much towing though & problems #2 & #3 are permanent - which (of course) won't help with the fuel consumption.

    I don't do many miles a year so losing a few mpg isn't costing me a lot but it's worrying that all is not right.

    The missing on tickover is completely random & even when it does happen, it fires fine when revved. Fuel filter was changed last service but it did it before then as well. I've never run it with less than 1/4 full showing on the fuel gauge.

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    I think that #2 and #3 may be because of #4, even if #4 only happens occasionally.

    I would still have a go at the battery disconnection thing. It can't do any harm, and it's only at the cost of a few minutes spanner work.

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    Regular Member Derek Mc's Avatar
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    Quote Originally Posted by diesel dog View Post
    I think that #2 and #3 may be because of #4, even if #4 only happens occasionally.

    I would still have a go at the battery disconnection thing. It can't do any harm, and it's only at the cost of a few minutes spanner work.
    Would second that action as it is a cheap fix doing a "hard reset" levels the ecu back to stock setting and will reveal if 2 & 3 are going to recur on their own and should get round 4 by the trans going back to basic settings.

    In my Saab days this process was done and then a good hard drive was undertaken to re-write the rules again though I never went that far, I preferred to let my car develop a gentle re-write.

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    Quote Originally Posted by Derek Mc View Post
    Would second that action as it is a cheap fix doing a "hard reset" levels the ecu back to stock setting and will reveal if 2 & 3 are going to recur on their own and should get round 4 by the trans going back to basic settings.

    In my Saab days this process was done and then a good hard drive was undertaken to re-write the rules again though I never went that far, I preferred to let my car develop a gentle re-write.
    Best to do a spot of 'Driving Miss Daisy' if you've had trouble with the tranny hanging on to gears.

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    Regular Member RhayaderSax's Avatar
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    Quote Originally Posted by diesel dog View Post
    Best to do a spot of 'Driving Miss Daisy'...
    Easier said than done with my right foot in a 3 litre lump!

    Quote Originally Posted by diesel dog View Post
    ...if you've had trouble with the tranny hanging on to gears.
    It can be really troublesome when his wig gets stuck in the synchromesh!

    I'll give the battery suggestion a go. Wonder what's causing the intermittent miss on tickover?

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    Quote Originally Posted by RhayaderSax View Post
    Wonder what's causing the intermittent miss on tickover?
    With a bit of luck, resetting the ECU will clear that up too (but I wouldn't count on it).

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    Regular Member RhayaderSax's Avatar
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    Hard reset done. Lost trip computer data, auto windows (know how to sort) but kept radio settings & sat-nav memory. Presume an hour of disconned battery is enough.

    60 miles of embarrassment (I was overtaken by a Ford Fiesta & wanted to commit suicide) driving round the elan valley with a featherlight foot at 40 (ish) & straight A roads at constant 60 ish in 5th/6th (forcing it to change up at < 2500 prm) returned an average 37 mpg. It's hilly over here in mid Wales so it doesn't constitute a "steady flat run."

    It still stuck on to a low gear once or twice but the gradients around here are deceptive. The surrounding mountains make you think the road isn't as steep as it really is sometimes. Better overall. Fingers crossed eh?

    Still unsure of the "missing on tick-over" problem. It's only done it half a dozen times over the last 18 months. Therefore it's that random that it's impossible to "make" it do it. Any suggestions on that problem are greatly appreciated.

    I'll keep you posted on the gearbox/mpg behaviour.

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    Just looking through this thread, my 2007 3.0 vectra has a miss on tick over, it doesnt doit all the time, but i have realised that when it does miss on tickover the DPF cleaning cycle starts when i get it onto a 70 road, just wondering if it is a DPF related problem?

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