Help needed! not holding boost

[s2_bo]

Also check the cabling to the sensors, they can perish and that will give false readings.

[S2central.net]

Did i read coolant temp -49 degrees??

Yep - with engine off, you cannot get valid readings from sensors such as IAT and others - very annoying actually, but its true.

However - would be good to get some VAGCOM screenshots with engine on warm idle for starters. Group 0,1,2 enough to kick things off

[220bhp]

I have been through all this with the same symptoms, and it turned out to be the front knock sensor, the cable slowly degrades and its dificult to see without moving stuff.

[iib3327]

i did clean the rear knock sensor but if im gonna get to the front i might replace it out of ease.

will get some screen shots with it warm shortly..

cheers for all your help guys really appriciate it

[LHD_Kid]

I have just today cured the EXACT same problem with my 3B.... and almost precisely as Lou (220BHP) says, it was a knock sensor.... in my case it was the rear one...

My advice, buy a front (brown) knock sensor and change it carefully, if this makes no difference use the front one you took off to check the rear one... This is what I did and the car flies again...

Front and rear knock sensors are the same aprt from cable length and plug colour...

was about 50 quid from Pete at Quattro corner (plus delivery)



Chris

[iib3327]

i have today fitted a new front knock sensor (no change)

fitted the front nock sensor to the rear (no change)

tried a vr6 MAF as one was to hand and just to rule out faulty new MAF (still no change)

but i have noticed that now i have less boost

here's a quick video

first pull 3rd gear 2500rpm onwards
second pull 4th gear think 3000rpm onwards
third pull 4th gear 2500rpm to almost redline..



sorry its not very clear.

boost gauge reads from 0.2-1.0 bar

[AndyP]

only thing i cant seem to get an answer on is why when checking for leaks the air bleeds off into the wastegate through the vacume and i dont know if this is normal?
Matt

Are the graduations on the gauge 0.2, 0.4, 0.6, 0.8, 1.0? So it looks like it peaking at 0.8 in 4th and dropping off?

Also, can you elaborate on the above quote. If your leaking testing with the breather system plugged, you should be able to get it to hold pressure. How much pressure are you putting into it and how quickly is it leaking away?

[iib3327]

Are the graduations on the gauge 0.2, 0.4, 0.6, 0.8, 1.0? So it looks like it peaking at 0.8 in 4th and dropping off?

Also, can you elaborate on the above quote. If your leaking testing with the breather system plugged, you should be able to get it to hold pressure. How much pressure are you putting into it and how quickly is it leaking away?

yup graduations are 0.2, 0.4, 0.6, 0.8, 1.0.

yes with breather system and vacume to wastegate plugged will hold 15psi, creeps away very slowly u can hear it bubbling back into the engine very slowly (am guessing turbo water/oil lines to engine maybe)

i still wouldnt be suprised to find a boost leak but i've removed all the pipes and checked for leaks swopped all jubilee clips etc.


this cars days are numbered will either be stripped for bits or sold on as a headache for someone else very soon as i have a growing hatred for the dam thing

[Jamo]

Take it to MRC

[gavs20v]

Matt, give Ben a ring at AVS on that number i gave you
Gav

[iib3327]

Matt, give Ben a ring at AVS on that number i gave you
Gav

MRC seemed puzzled at what i'd changed and not solved it so not gonna be going there

Gav- spoke to your mate ben (really nice chap) i was just rulling out the last few things as i havent the chance to get it over this week..

[Jamo]

you can't expect a phone diagnosis, it needs to go there so doug can health check it

[220bhp]

This is a problem I struggled with for a long time and its no fun,you just have to keep at it.Has the car been chipped in the past, could even be a problem with the ECU.
Have you checked all the vacuum hoses including the one to the ECU, there is also connection under the back seat which has been known to leak, and quite a few at the back of the IM.
As with the knock sensor I am not saying this is your problem but if you don't eliminate it then it could be. you have to be symptomatic if you are going to get to the bottom of it,but when you do its a great feeling,I went through a tank of petrol the day I fixed mine. Whatever the problem the answer is somewhere on this Forum. The search engine is your friend.thumbs:

[max_s2]

I have the same problem, the boost is falling off to 0.8bar after receiving 1.1-1.2bar. Seems like overboost is holding only for max 0.5-1 sec.
BTW, no faults in ECU.

[iib3327]

problem is they said i've checked more than they would (also a lot more times than they would) and some things they seemed puzzeled that i've checked. so seems a waste to pay them to go over a few of the things i have..

all vacumes have been replaced (or platic hard lines checked and double checked)

paid to have the ecu full health checked

i think i've been more than patiant with it 3 months of owning a car i've always wanted to do 17.4mpg at its best and be slower than a mk4 golf gti thats doing 30+mpg.. seems a waste of time money and effort atm..