Okay, this is by far the strangest problem I've ever had with my wagon.

I've had my max boost pressure set to 13psi for years, using a boost controller (works great).

Recently I noticed if I was accelerating hard it would get to 15psi. I tried to readjust the boost controller down to 13 to no avail - the gauge kept reading 15 max no matter how low I set the controller. So I disconnected the boost controller altogether, along with the wastegate, so the intake manifold was now open to the outside world - theoretically I should get little or no pressure on my gauge.

I still get 15psi.

I can feel air blowing out the outlet on the turbo that the wastegate hose normally connects to; if I rev the engine it blows harder. Just to be sure, I blew through it with an air compressor and it seems to be open and unobstructed. I borrowed another boost gauge from my mechanic (dad ) to be sure it isn't my gauge that's messed up, and all it did was confirm the impossible...

So how am I getting 15psi boost?!

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)