I've had the thermostat on my M272 engine replaced by an indy mechanic. OEM was Wahler, looking at the original the the Wahler I got, the only difference was the spot where the MB logo was on the original, it was grounded out by some grinding disk on the replacement one.
I had two Wahlers and they leaked as soon as I fitted them. In exactly the way you described.
I got the wahler one in 2019, that would explain the really slow leak I have.
My Wahler thermostat starts to leak after 4 years. It's not a big leak (only a cup a month) but I'd swap it asap.
Replaced the new Mahle/Behr one with a new Wahler one, and now it runs at 97° under low load and 80° under high load, as it should, and seems to be fine.
I replaced the thermostat myself. It’s in a pretty bad place, hidden under the intake manifold. It took me about 3 hours to get the job done, had to use a mirror to see the screws. Mine was stuck open so the car in winter time wouldn’t even reach 190 F. I bought an OEM Wahler 07K121113B thermostat from Amazon for like 30$, so far has been working great.
My 2007 MKV 2.5 Jetta is still having overheat problems even after changing the tstat to a OEM (Wahler) and the waterpump to one with metal impeller, which works way better than the original.
I installed a Wahler thermostat a little over a year ago and it's still holding strong.
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