BEW low boost issue.

So I'm fighting a low-boost on my 2005 Jetta Wagon (BEW) with ~225kmi on the clock. I threw a vag-com on it and the requested boost looks normal, but the actual boost lags wayyyy behind. It eventually throws an underboost code and goes into limp. I've tried throwing an n75 from an ALH onto it, but that didn't seem to resolve the issue. I've was also able to get to the actuator and move it by hand, and it moves smoothly and freely. This all happened shortly after the short rubber hose between the fuel pump and the fuel rail sprung a huge leak into the driver's side of the engine bay. I haven't been receiving any MAF or MAP codes aside from the low-boost. I was thinking toasted turbo, but I am not seeing any excessive oil or smoke.

Any thoughts?

Great step-by-step scientific HOW-TO listed in my sig.

Check the vacuum line from the N75 to the turbo...replacing them if they haven't been is a good idea.

Check to see if the turbo actuator holds vacuum. Mine didn't due to internal rust.

EvilLawnGnome said:

This all happened shortly after the short rubber hose between the fuel pump and the fuel rail sprung a huge leak into the driver's side of the engine bay.

Click to expand...

Here's your root cause. You have a rubber vacuum hose that's leaking or collapsing now that it's soft and spongy after taking a bath in diesel. Start at the tandem pump and work your way from there. I would simply start replacing each hose with silicone.

Might be a crack in the larger plastic hose that goes from the tandem pump to the brake booster. That's supposed to be a common problem. (Hasn't happened to me yet).

Thanks for the info, guys. I don't think there's a crack in the pre-n75 vacuum system. I smoke tested it and didn't see any leaks. I'll go ahead and replace the n75->wastegate line and the n75 and see if that helps.

While your testing it check if the vacuum pump is producing at least 20 inches of vacuum.
Wiggle the hoses to see if the vacuum varies.

I've read the plastic hose going to the brake booster is known to crack where it's hard to see. Shake it also.

Tee into the line going from the N75 to the actuator. You should have about 10 inches of vacuum (or so) at idle.
That will drop lower when you start to drive.