This weekend, my air filter housing supports broke loose. All the rattling knocked the breather tube loose from the air intake manifold. (It was still attached at the valve cover). I immediately noticed a decrease in power, a loud exhaust type sound, and a HUGE increase in blowby. I fixed the mounts, and reseated the breather tube, and the problems seemed to have gone away. So maybe just check to see that your crankcase ventilation is working properly.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT