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Hunting idle and poor fuel consumption

Discussion in 'A4/S4 forum(B5 Chassis)' started by gillbe, Dec 14, 2005.

  1. gillbe
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    gillbe New Member

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    Relates to a 75K 1996 A4 1.8SE
    This may help someone with similar problems - I have read a lot of posts on the subject, here is my (ongoing) saga.
    Fuel consumption was down to 26mpg from around 36mpg. Water temp was also down from 90 to 70C - fixed that with a new thermostat and hoped the hotter running would bring mpg back up.
    Not the case. Over the period of a week of careful "economic" driving, still seeing only 26mpg. Took a look at the fuel system and discovered that the inde garage had not refitted the air inlet to the throttle body correctly (rubber ring grommet twisted and no jubilee clip!) Replaced this and got it all tight. Would this cure it? Not the case.

    Still getting 26 mpg, but now with the added problems of some hunting around for idle, and on constant speed a slight lumpiness - almost like a misfire but not so pronounced. At idle, I can blip the throttle - revs show 1500 rpm peak, then drop down to 400 rpm, almost to a stall before recovering to 900, back down again, before finally settling at the normal 900 figure. This occurs more at normal operating temperature than cold. After reading the posts, took off the MAF, no obvious signs of any dirt. Replaced without attempting to clean it.

    This week, took it to an inde for a VAG-COM readout. Cleared a list of fault codes as long as your arm (not seen an Audi garage for 5 years - too expensive!), reran the car and two codes popped up:-
    17972
    16515

    One means an intermittant fault with the idle circuit - the other a low reading on the O2 sensor. The inde then proceeded to clean the throttle body (since a new one is £400 fitted) and discarded the O2 fault as an irrelevancy. No improvement. - this cost £140 for the privilege.

    Reading (lots) of material around this issue, I suspect the O2 sensor is more likely culprit - why? because the poor fuel consumption and lumpiness even at 50mph. So I measured it with a handheld scope meter. The reading at idle is 0.4V unchanging, completely flat line. Should be switching between 0.1V (lean) to 0.9V (rich) around once a second. So I've ordered a new sensor and awaiting its arrival - will let you know the outcome.
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  2. Broken Byzan
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    Broken Byzan Photographic Moderator Staff Member Moderator

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    Excellent, my SE isnt as good as it should be IMO, its around 30mpg ish max, i changed the MAF last.It mad a great difference in how the car ran, but no difference to economy. Maybe i expect too much of the car,lol
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  3. gillbe
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    gillbe New Member

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    One new NGK O2 sensor now fitted (severe pain getting old one out - needed a 3 foot hollow extension bar on the wrench in the end).
    Result:
    All sorted! No lumpiness, idles OK. Expect the mpg to return to normal, too early to tell.
    /ubbthreads/images/graemlins/beerchug.gif
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  4. Broken Byzan
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    Broken Byzan Photographic Moderator Staff Member Moderator

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    How much was the sensor??

    Glad u sorted it
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  5. joost
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    joost Active Member

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    I paid about £60 for one about a month ago. This one's on ebay though...

    linky
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  6. gillbe
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    gillbe New Member

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    Sensor was an NTK direct fit (i.e. no cutting and strapping of wires needed) part no. OZA447-E14 £65.80 inc VAT and 1-day recorded delivery
    see Lambda sensor homepage
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