Strange Boost Issues

[h100vw]

If the N75 is failed electrically, it won't spike at all.

VCDS will get to the bottom of this quickly. Please don't buy any parts, just because. It can be a few things.

An output test of the N75 hot and cold, using VCDS will help. Check it cold. Then when it fails, check again without cycling the ignition.

Gavin

[kdubGTI]

It doesn't fail unless you switch off the ignition and leave it for 10 mins.

Is VCDS the only way of monitoring the fault codes/engine etc. Would much prefer to have a non laptop hand held device. Refuse to have a windows machine in the house and at 150-180 per shot at all the so called specialists here in Perth is not happening either. Been to a couple of them before xmas to be told fault on the N75, its reset and all good. For it to return as soon as you switch the car off.

Let it cool down and it disappears.

Which to me pushes me in the direction of a faulty coil being subjected to heat (windings breaking down) or something sticking in a position.

Is the N75 failed open / energised closed. Because the minute it goes to 4psi if I disconnect the vacuum hose I get exactly 17psi which is identical to normal operation when everything seems to work fine.

[sambb]

if the N75 or its wiring/plug connections fail electrically it would cause the valve to be in its open position. Its designed this way as a fail safe so that all the available boost pressure in the boosted side of the inlet pushes through the valve straight to the actuator = a max boost of 6-7psi which is actuator spring pressure. Now if the N75 was mechanically sticking you could get boost anywhere from massive spikes if it stuck closed, to 6-7psi if it stuck open That you saw 4psi tells me maybe your wastegate flapper is loose as that's the only way I can think of that would give you boost below spring pressure - but then you said it can come good and boost again which sort of flies in the face of that. Whatever fault you have could be openeing the N75 but also throttle limiting you too which could explain the very low boost. So you see it could really be anything so like Gav said you really need to scan it. if your service guys are turd and charging big time for it, then try to find a VAG enthusiast near you that can do a log and fault scan and post the results here. Don't stress, you will get it sorted but it kind of starts with some data.

[sambb]

just to rule it out look up VAG 1.8T diverter valve testing on youtube and do the same on yours to check its ok. Also check/spray the plug connection on the other solenoid on the cam cover that has pipes running from it to the vacuum canister (black).

[kdubGTI]

Ordered diagnostic gear today. Hopefully be here in the coming weeks and the saga can continue lol.