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Thread: How use VAGCOM/VCDS to test for boost & vac leaks

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    How use VAGCOM/VCDS to test for boost & vac leaks

    Looking for help on how to use VAGCOM/VCDS to test for boost & vac leaks?

    Thanks
    stuart_tate41 likes this.

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    Welly's Avatar
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    Easiest way is to check block 032.

    There are two fields in the block, and both belong to fuel trims.

    Left hand one is short term adaption, right hand one is long term adaption. Left hand one is the one to look at in this instance.

    Make a note of these two figures. Reset the fault codes (even if there are none), as this will clear the fuel trim data back to 0% for both blocks.

    Then start the motor and log block 032. If the value in the left hand block steadily climbs to or past the 3-4% area, it's normally a good indication of a leak.
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    Thanks Welly, you the man

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    what should it be if there are no leaks welly mate?

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    Quote Originally Posted by DaveA3 View Post
    what should it be if there are no leaks welly mate?
    As close to 0% as possible..... It means it's neither adding, nor taking away fuel at idle.
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    Will be trying this in the morning!

    Leggy suggested that at 3000rpm under no load the real time MAF reading should be exactly 10g/s on a car with no leaks....
    -------------------------------------------------------------------------------------------------

    Prawns TFSI Turbo'd 1.8T Track Car

    But I know nothing so ignore me.

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    Quote Originally Posted by Prawn View Post
    Will be trying this in the morning!

    Leggy suggested that at 3000rpm under no load the real time MAF reading should be exactly 10g/s on a car with no leaks....
    Not tried this, but did see it. Would be interested to know if it works from someone who's tried it....
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    well, it works in mine. holding the revs as accurately at 3k as I can I see bang on 10g/s, and to my knowledge, I don't have any leaks.

    GOing to check the fuel trims in a mo see what that gives me to confirm
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    I did try this too with my mates liquid gauge, held the revs at 3000rpm and I was only showing 8g/s...

    Went to Jabba yesterday and they smoke tested the system and I do have a boost leak by the inlet manifold, going to put the new pipe on Monday as Jabba didn't have the part in stock, once I've done the fix I'll try the liquid gauge again,

    Let you know how I get on...
    revo'd

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    Thanks thats a good bit of info

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    Does the 3k rev method and block 032 apply to 1.8t A3's too?

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    Quote Originally Posted by nick4501 View Post
    Does the 3k rev method and block 032 apply to 1.8t A3's too?
    Prawns is an A3 1.8T and it seems to work on his...
    revo'd

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    Quote Originally Posted by leggy View Post
    Prawns is an A3 1.8T and it seems to work on his...
    Good point! - Cheers, will give it a try.

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    Cheers, will try this too.

    Sticky?

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    what should my boost gauge be reading on idle with a stage two map and forge mbc set to 1.5 bar 1999 aoudi s3 apy?
    its reading -0.7 bar is this correct?

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    Just tried the fuel trims test on mine, before resetting it was -0.4% short term, and -4% long term.

    Reset them and the short term didn't move from 0 at all for 5 mins idling. drove 30 miles home, and short term is 0 still, and long term is -3.9%

    Does this sound fairly normal to you guys?

    If it's pulling out 4% fuel long term, does that imply the map is running rich as standard, and it's possibly got a smallish leak somewhere? or being a generic map, will they all adjust a few % up or down to suit individual cars?
    -------------------------------------------------------------------------------------------------

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    But I know nothing so ignore me.

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    Quote Originally Posted by Prawn View Post
    Just tried the fuel trims test on mine, before resetting it was -0.4% short term, and -4% long term.

    Reset them and the short term didn't move from 0 at all for 5 mins idling. drove 30 miles home, and short term is 0 still, and long term is -3.9%

    Does this sound fairly normal to you guys?

    If it's pulling out 4% fuel long term, does that imply the map is running rich as standard, and it's possibly got a smallish leak somewhere? or being a generic map, will they all adjust a few % up or down to suit individual cars?
    Short term trim sounds spot on dude.

    Long term trim's acceptable tolerances are between +/-10%, and you are well within that. Bottom line is that it's a generic map, and every engine is different...

    The ECU won't throw the EML for fuelling until +/-25% is breached.
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    I thought the short term adaptation is considered fine -+ 10%?

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    Quote Originally Posted by dannycawsey View Post
    what should my boost gauge be reading on idle with a stage two map and forge mbc set to 1.5 bar 1999 aoudi s3 apy?
    its reading -0.7 bar is this correct?
    mine reads on idle -20 which i -1.3 bar so you might have leak some where
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    Quote Originally Posted by finesse View Post
    I thought the short term adaptation is considered fine -+ 10%?
    Yeah.....? I don't understand what you're getting at here dude....
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    Did both the methods and my results are:

    Rpm at 3000rpm is between 10.3-10.9
    Short term fuel is at 0.8 and long term is at 0 after 5 mins

    Does this look like I have a leak?

    Also, how would my boost gauge behave if there is a leak?
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    Sekenzo,

    Not sure about the fuel trims but the theory of holding 3k revs and your G/s maf reading would suggest that your car is fine, I was told that the tolerance is 0.9, as your seeing 10.9 would suggest your car is fine...
    revo'd

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    i might sound alittle special here but do you hold 3000rpm stationary or whilst moving(3rd, 4th, 5th Gear)?

    thanks

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    At a standstill.

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    do you think it would work on a AGU (no MAP sensor)?

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    plus or minus 10% is what the ecu works on.
    if its outside these perameters the management light is supposed to come on showing up fuel trim faults.
    whether these faults are due to air leaks/injector faults/dodgy air mass's is anyones guesses!

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    Hello everyone this is my first post ive been lurking around the site gathering info to try and resolve 2 faults on a 02 Audi A3 1.8 T AUM engine
    error codes are
    2 Faults Found:

    17608 - Boost Pressure Control Valve (N249): Mechanical Malfunction

    P1200 - 35-00 - -

    17544 - Fuel Trim: Bank 1 (Add): System too Lean

    P1136 - 35-00 -

    up to now i have replaced 2 breather hoses 06A103247 & 06A103213AK
    ordered a Recirculating closed loop Dump Valve which i hope will clear the Boost Pressure Control Valve (N249): Mechanical Malfunction
    just had a look at Block 032 whilst stationary idling at the moment i have a reading of:
    7.9 Idle O2 Adapt & -3.1 Run O2 Adapt
    because the first reading is high could this be because the DV is faulty & creating another vac leak as well as giving no boost pressure ?
    cel comes on after about 15 miles 23 mpg is not good at all even when driving like miss daisy

    update
    Just found another split pipe under inlet manifold it comes out of a T piece to the underside of the inlet manifold I'm not sure about the part number i think its
    06A 133 783 AT could anyone confirm this will fit A3 1.8 T AUM engine Ive cobbled something together for now to see if it cures:
    17544 - Fuel Trim: Bank 1 (Add): System too Lean fault
    Last edited by Garber; 31st December 2010 at 17:08.

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    Boost Fixed

    Fingers crossed think Ive sorted faults:
    17608 - Boost Pressure Control Valve (N249): Mechanical Malfunction
    P1200 - 35-00 - -
    17544 - Fuel Trim: Bank 1 (Add): System too Lean
    P1136 - 35-00 -
    Replaced split vac pipes & old DV
    bought a piston type from ebay see if it lasts longer than a 710N
    Link to before and after boost charts could anyone tell me if 2nd chart looks about right for a standard 1.8T
    Last edited by Garber; 23rd January 2011 at 20:13.

  30. #29
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    Tried this method out yesterday while doing some tinkering with a couple of other guys from here and its a good quick way to check to see if you have any leaks that need investigating furthur.

    My car from the results is looking pretty leak free and held 10g/s at 3k rpm so all is good cheers for the guide chaps
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  31. #30
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    Hi Guys

    I tried the Block 032 test last night to see if if there were any leaks, but would really appreciate some advice to clarify the results

    I ran the car for a short while to allow it to warm up and then I ran the test.

    -1.3% on the Left - Short Term and - 6.3% on the right - Long term

    I then cleared the faults as suggested and these results differed to the below

    -0.4% rising to -1.7% Short term & 0.0% on the long term

    I already have concerns that the car is running lean, but I do not understand these results enough to be able to make an eductaed decision?

    Sadly it was too late at night to be holding the engine a 3k to test the other way

    Thanks in advance

    Quote Originally Posted by Welly View Post
    Easiest way is to check block 032.

    There are two fields in the block, and both belong to fuel trims.

    Left hand one is short term adaption, right hand one is long term adaption. Left hand one is the one to look at in this instance.

    Make a note of these two figures. Reset the fault codes (even if there are none), as this will clear the fuel trim data back to 0% for both blocks.

    Then start the motor and log block 032. If the value in the left hand block steadily climbs to or past the 3-4% area, it's normally a good indication of a leak.
    Worried that one day I may have to buy something sensible, but until then!

  32. #31
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    -6.3% LTFT is more likely to indicate you have a leak under boost... a vacuum leak will show as a positive figure in STFT but may well adapt to a negative value in LTFT... LTFT's will take a little while to adapt...

    If you suspect you have a fuel issue then you need to log blocks 001, 031 (as you have a wideband ECU on your AMK S3) as this will show lambda control values (001) and requested AFR vs actual AFR (031)...

    <tuffty/>
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    Thanks Tuffty I will try this again tonight, as I balls up logging Blocks 001 & 031 last night. (driving and clicking go on the laptop!)

    Cheers Ian
    Worried that one day I may have to buy something sensible, but until then!

  34. #33
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    Quote Originally Posted by Wingnut Express View Post
    Thanks Tuffty I will try this again tonight, as I balls up logging Blocks 001 & 031 last night. (driving and clicking go on the laptop!)

    Cheers Ian
    Click go before driving off then just look at the run rather than all the surrounding data... safer.. also... hover the mouse over the marker button and tap the tounch pad prior to a run... you can also use the 'right arrow' cursor key too...

    <tuffty/>
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  35. #34
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    Sorry to drag this up, if I have a positive reading in the left box. The number went from 0 to 3% in a few minutes after resetting, what is that telling me?
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  36. #35
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    Probably got a leak, A smoke test would be the next step to find the leak, the 1.8t is well known for leaks.

 

 

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