[Fourt1thirty]

Putting this in here since I had to type it all out:

But you said you made 260hp... so by all standards you know what you're doing. So when you ask beginner questions don't be surprised to get condescending answers.

This question has been asked enough I think we actually had to ban someone to get them to stop asking it over and over. You've been told a bov is bad but you keep pushing it because that's not the answer you want.

So even though I urge you to search, I'll lay it down for you because you refuse to. This is going to be long...

The purpose of a diverter valve or bov is to release the pressure spike between the turbo and throttle body. This pressure spike builds when the turbo is still spinning but the throttle body closes. If there were no valve what we would get is compressor surge. Compressor surge is, when broken down to its main component, you turbocharger going from 110,000rpm to 0 in less than a second. This is bad. It's a really good way to ruin a turbo. That's the purpose of the valve.

OK, so now that you know what the valve is for... on to why a bov is bad for a 1.8t.

The engine sucks in air. The MAF measures that air. The ECU sends a certain amount of fuel to the engine based on what the MAF tells it. Because of this anything between the MAF and the cylinder MUST be part of a sealed system. Any leak is excaping metered air. The engine will provide fuel for the air that isn't there anymore and the fuel mixture will be rich.

The dv/bov is after the MAF so any air passing through is has already been measured. We can't have any leaks here. A dv reroutes the air back to the inlet of the turbo, after the MAF and still in the metered air system. Totally acceptable. The BOV lets the air go to atmosphere. That air just escaped the system... but the ECU is going to fuel for it anyway. Boom... hard rich. That's the puff of black smoke from your exhaust when the BOV hits.

Now listen up, because this is the important part that kills motors. Motronic 7 is a complex system. The front O2 sensor from a 1.8t is what all the aftermarket companies use for their widebands. Your car is DAMN good at knowing its own mixture and compensating (it has a WIDE fuel trim range. It will actually compensate over problems and you'll never know it happened). So what happens is every time your BOV goes off and the engine makes a rich condition the ECU logs it and leans out the system just a bit to compensate. You keep doing it over and over and it'll keep leaning itself out to compensate. The problem is it doesn't need to be leaned out, it's only too rich when the bov is open. It's still going to trim the whole fuel map. Now you're too lean under boost, too lean at idle, everywhere.

Lean conditions are dangerous. Lean is when you get detonation, piston damage and outright engine failure.

Tightening it down all the way just makes it hard to open which will cause compressor surge and damage the turbo