Just took off the plastic cover over the air filter box and MAF and found a couple of 2mm plastic/rubber cables which had been worn through to make holes in them, by rubbing on the cover, which I was aware had been vibrating and was noisy. I don't know what these small cables do, but they are hollow inside and I suppose could be part of the air intake. Does anyone know if these holes could be the cause of my lack of power at higher speeds ?
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These will be the small bore vacuum pipes that control various items on the engine including the turbo.
The ones that terminate in the air box are just bleed off pipes from the controlling valves so won't affect anything.
However, if the rest of the vacuum pipes are also in poor condition they may be leaking & affecting the control of the turbo which could be causing your lack of power.
The rubber tubing used by VW is poor quality & prone to perishing.
For the sake of a few pounds worth of tube I would just replace them all, one at a time to avoid any mix-ups with the conexions. There are 2 different bore sizes used, the other is about 4mm.
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The vac feed to the infamous N75 valve which controls the boost takes this route. If the smaller pipes are damaged there will be no vac to the VNT actuator on the turbocharger, giving the symptoms you describe.
The vac "plumbing" on VAG engines is at best, casual and uses poor materials as stated above. Repair carefully using good materials which will withstand engine heat without softening and do not mis-route any pipes.
659.
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As 659 said, there are a few small-diameter hard plastic tubes around there.
One takes vacuum to the N75 controller for the VNT mechanism, one is a return from the N75 back to the vac system, and there's another which links to the anti-shudder "flap" valve which only actuates when you switch off.
A pinhole in ANY of these hoses will cause a vac leak, which will mean low boost and low power ... simply because the VNT mechanism is not getting fully opened.
You might be best getting Samco hoses from a specialist like Demon Tweeks or similar, they will sell it in metre lengths and the hose is designed for high temp applications.
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Thanks to those who replied to my post. I have taped the holes on a temp basis and will replace the plastic tubing. My power seems to be back to normal. I wonder how much a main dealer would have charged me and how many spare parts they might of tried ?
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Snipped off small section of each offending vac line to show dealer which lines I needed. Problem getting the right size because they have changed the size and type of vac line they use. Why ? because it kept rubbing on the plastic cover over the air box and wearing holes in it. Exaxctly my problem. Managed to get some slightly bigger hard plastic vac line and used existing rubber hose to join it to the T joints over the air box and the N75 valve port. Road tested it. Problem solved. Power back to normal. Lesson learnt - Look for holes or splits in vacuum lines for this problem, before replacing things like MAF or N75 valve. Vaccum line problems are the most common problem when TDI's go in to limp mode at high speeds. See great article by CANADIAN GRIZZLY on TDI Website forum. Explains every check needed in detail. Be wary of fault codes, vaccum leaks will often come up as N75 Valve problem or MAF problem or Coolent Temp Sensor problem.
Edited by 1964johnr on 11/04/2009 at 11:03
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