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N75 valve and Vacuum ball....

Final 3Green Land Jan 13, 2014

  1. Hi,

    I have an AFN engined Avant A4 and I have boost issues, i.e.the boost drops off quire severely after 3000rpm in 4th and 5th but lacks power going up hill in higher gears... but quite strong up until then (not as good as usual but ok)

    I was wondering, as the N75 is a bugger to get to. is there any reason why it cannot be relocated to a different area within the engine bay where access would be so much better should the need arise? (Had new silicone vac hoses installed in June and they have already showing signs of hardening and not creating a perfect vacuum so will be changed tomorrow.)

    Is there any reason why the N75 and Vax ball couldn't be moved to the bulkhead near the battery as there is plenty of room there. I know I will need longer hoses and do a little wiring extensions but apart from that, any reason why I cannot do this??

    Boost leak....

    Have had the codes read... Nothing at all...

    As for the boost leak, I am having a new set of:-

    IP seals fitted this week (Slight fuel leak),

    new fuel temp sensor fitted, (whilst the pump lid is off, just in case)

    having the timing checked and adjusted,

    new N75 and ALL vacuum hoses and one way valves,

    N18 and hoses,

    actuator IS moving freely,

    Turbo only 10,000 miles or less... (GT1749VA)

    All intercooler hoses checked, hose couplings all cleaned and jubilee's tightened...

    Will be doing a system pressure test tomorrow...

    Any other suggestions??

    Thanks. F3GL
    Last edited: Jan 13, 2014
  2. jbh

    jbh TDi lover

    ive seen some folk place them on the inner wing near the airbox but never as far away as the beside the battery.
  3. TeKnodriver

    TeKnodriver Biodiesel Purrrr

    sorry to hear about it...

    been working through my vac hoses around the N75 and replacing, one-way valve too.
    this solved problems on mine except for cold start (that's probably something else)

    as for actual boost leak - can you hear it under hard acceleration? narrow street helps as you'll hear the echo maybe
    I had a holed intercooler for a few weeks and didn't see it,
    did pressure test on the IC using old bike innertube cut into a length then jubilee-clipped to boost hoses checking them an IC one by one...

    post 3 on this thread has a pic of relocated N75
  4. jbh

    jbh TDi lover

  5. When I say boost leak, I should say the boost drops off dramatically in 4th and 5th over 3000 rpm hence thinking boost leak...

    Had seen the pic with the relocated unit on the inner wing but thought it looks a little gash, not criticising the man who did this but I just thought it might look a little tidier, for my taste, in the bulkhead by the battery.

    Wasn't sure if it had to receive heat from its physical location near the turbo in order for it to work correctly. It will be a lot colder in both alternate locations.

    As for the poor starting, I am thinking the fuel temp sensor might be at fault and indeed that might be causing the poor performance at the top end if it is not allowing the timing to alter to its optimum position in order to give best performance. Thought it wise to change the fuel temp sensor whilst the top was off... any other ideas what else to do whilst changing the gaskets??

    WIll get on with the relocation tomorrow and go from there...

    Thanks for your help so far...

    Oh... as for the street idea, have a rather good tunnel for this just a few miles away from me but cannot really hear anything untoward hence thinking more vacuum hose related or sensor orientated...

    Its a shame the old girl is a little off colour, normally it goes like a scalded cat with a red hot poker up its bum!! :jump:
    Last edited: Jan 13, 2014
  6. Further to my post above, I was thinking that placing the N75 and the spherical ball reservoir in a cooler position, ie by the battery, the vacuum effect might have been more efficient as the cooling effect of the colder air would increase the vacuums efficiency.

    Only a thought but was looking at plastic bottles with warm water inside, crush quite quickly under vacuum as the liquid cools... was thinking the same principle might apply to cooling the air and increasing the vacuums efficiency....
  7. TeKnodriver

    TeKnodriver Biodiesel Purrrr

    AFAIK the vacuum is being constantly generated by the engine - keeping the ball cooler with have a marginal effect if any at all.

    the idea of relocating N75 is just to have access and to check the condition of tubing

    have you changed the fuel filter recently? I was getting what felt like some restriction over 3.5K rpm - this was cleared-up with new fuel filter and was not boost related.

    check the fuel temperature using VCDS before cracking open the fuel pump
  8. I will fit a new filter tomorrow and see... It was only changed about 8 months ago when the new turbo was fitted but I will renew it anyhow and see if it makes a difference.

    The vacuum hose thing was only a thought. I changed them at the same time as the turbo change and already they have started showing signs of failing; hardening around the T piece allowing air in, hoses coming loose around the airbox etc... Relocating the N75 will make things easier it check the connection of the hoses and their fit in the future.

    The relocating thing to the battery bay was only an afterthought...

    The pump is having a new set of seals as it has a slight fuel leak so thought it would be worth doing the fuel temp sensor whilst the top was off... I couldn't get any codes from vagcom at all, apart from the dog set the alarm off a couple of times, so thought it might be prudent to change the sensor incase it went off and required the top to be taken off again...

    Will let you know what I find tomorrow...
    Last edited: Jan 14, 2014
  9. TeKnodriver

    TeKnodriver Biodiesel Purrrr

    fuel temp sensor will probably not throw up a fault code unless it is well outside the parameters the ECU expects or is shorting-out

    if you have VAGcom,
    connect it up, login using 12233
    go to '1' 'Engine'
    select group '7' [other groups are useful to check air/coolant sender readings]

    this should show you the instantaneous fuel temp. reading as being sent to ECU.

    on a cold engine [left overnight] mine was within 2 deg.C of the coolant temp. and intake air temp.

    hope you get it sorted!
  10. Sorry its been a while since posting... been nursing a broken foot and ankle followed by a dislocated knee!

    The poor starting was down to the fuel pump shaft shearing off but still mating with the broken 'area' and creating a drive albeit a few degrees out...

    The problem was cured by fitting a new pump....

    However, the boost still falters when on full throttle... Have a boost gauge to read now and this confirms that from 3000rpm upwards the boost drops from between 1.3 to 1.4 bar to 1 bar or so...

    When applying full throttle on the motorway the turbo will provide boost at +1.4ish then falter sometimes creeping up to +1.6 before dropping to the 1.3 to 1.4 area again... and after 3000 rpm it drops off...

    It doesn't seem to have a static level of boost... is this normal??

    Any suggestions would be great.

  11. Sorry for the long long delay in posting an update...

    I finally gave up and changed the turbo for a bigger BV43 accompanied by a set of .240 nozzles and an FMIC... and the problem almost disappeared.... Almost..... but then it came back again... sort of....

    Sorry to be vague but a long story cut short,

    The turbo actuator on the BV43 was holed and losing vacuum... I guess this might have been the same on the previous one.

    With the new BV43, all appears fine.

    One last question, where on the pic below would the vacuum ball fit in? And is it really necessary for the system to run properly, to have it installed?


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