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Thread: 2.5 AFB seems to lack boost.

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  1. 2.5 AFB seems to lack boost. 
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    My A6 2.5 tdi(AFB) has just been put back on the road after 1 & half years, and to me it seems down on power a fair bit. I cannot feel any boost hardly unlike I can on my Bora when it pushes me back in my seat.
    It will rev to the red line ok but it seems to take quite a while. I've taken it above the legal limit for a few seconds and it will quite happily sit there so I don't think its in limp mode like what happened to my Bora once many moons ago. There's no warning lights on.

    I've taken it to a local indi garage today for a diagnostic test. The only fault code is 00523 (and 00553 G70), Intake air temperature sensor G42 Open circuit/short with power intermittent. All vac pipes and vacuum to EGR and turbo actuator hold vacuum with no leaks.


    They've also printed me out a graph of the boost pressure (actual and desired) and there's some differences.


    Speed Actual Desired
    10mph 1300 1850
    20mph 1625 1900
    40mph 1300 1500
    55mph 1800 2000
    80mph 1999 2040
    90mph 1999 2040

    There are peaks on the graph much lower but I've just put the ones where he must have booted it. In his opinion he thinks that its the turbo, maybe clogged with soot/crap but I thought you guys might know more.


    There's also a printout of a graph for the air intake temps:

    At 10mph the temp is 60oF and as he gets up to 80mph the temp goes down to 53oF

    And Air Flow readings:

    Mass Air Flow (mg/S)

    10mph 650
    20mph 750
    30mph 400
    40mph 725
    50mph 875
    60mph 400
    70mph 550
    80mph 1001
    90mph 450


    This to me is mostly greek. If anyone help can help me figure out what's causing imo a lack of boost I'd greatly appreciate it Maybe I ought to whack a bigger turbo on


    One thing I'll add is the temp gauge sits all day at about 62oC and the oil temp seems to stay at about 64oC. I don't know if this will make any difference but thought I'd add the info.


    regards
    Mike
     
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  2. Re: 2.5 AFB seems to lack boost. 
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    Your requested and actual boost is low. I would be expecting 2300 - 2500mbar. Was that reading taken at wide open throttle, or cruising?

    The temperatures, do you mean 0C not 0F? Anyway the thermostats all seem to go on these engines after a few years, so they don't warm up in the winter.

    If all the vacuum side is ok, I'd be looking at a clogged VNT in the turbo. If you have VCDS, this can be cycled to test it, otherwise unbolt the actuator, (but don't mess with the yellow painted adjuster rod!) and see if the VNT is totally free, and runs fully to it's stops. Ps put rags all round the turbo, as when you drop the bolts you can recover them. If not full engine rebuild to ever see them again!!!

    There is lots of other stuff to check in the measuring blocks if you have VCDS - eg temperatures of coolant, fuel and intake air. MAP , MAF, throttle position, Injected quantity, timing, torque limiters, EGR valve etc etc etc........................
     
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  3. Re: 2.5 AFB seems to lack boost. 
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    Hi,
    I believe both codes are MAF codes, check the wiring and the connector to the MAF first, if nothing suspicious is found, I would have tried with a new one.
    It also seems you need a new thermostat
     
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  4. Re: 2.5 AFB seems to lack boost. 
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    Thanks for the replies.
    The temps are in Fahrenheit on the printouts for the intake air temp. From the printouts I think it may be a mix of WoT up to each gear change and cruising judging by the spikes on the graph.

    As to checking the actuator, I removed the vac pipe from it and connected my vacuum pump and the actuator moves freely when i use the pump and stays in position until I release the vacuum. Then there is a click when it returns to the opposite stop. The same goes for the EGR valve.

    I do have VCDS lite. Will that allow for checking the measuring blocks? If so How do I do this? I take it I have to plug in the lead to my laptop, start VCDS lite then start my car?

    Sorry if these are silly questions.

    [Edit]
    Just out of curiosity I fit my VanAken tuning box and it has bags more power. Seeing as it intercepts the signal to the fuel pump could my problem be that the pump need a timing check with VCDS even tho there are no fault codes?

    Are there any members with 25-30 miles of me with VagCom?
    Last edited by Mr Tickle; 04-12-2014 at 10:29 PM.
     
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