G Klasse Diesel Engine Issues

That sounds eerily familiar! I had a similar issue with my old BMW 5 series a few years back. Reduced power, rough idle, the whole shebang. Turned out to be a cracked intake manifold. The manifold pressure readings were all over the place, which messed with the fuel-air mixture and caused engine misfires. Did you check for any obvious signs of damage on the manifold itself? Any hissing sounds that might indicate a vacuum leak?

I visually inspected the manifold, but I didn't detect any obvious cracks or hissing sounds. The manifold sensor readings are erratic. Where did you find the crack on your BMW's manifold, and how did you pinpoint the vacuum leak?

On my BMW, the crack was subtle, almost invisible to the naked eye. It was located on the underside of the manifold, near where it connects to the engine block. The hissing sound was only audible when the engine was cold. Given your manifold sensor readings and the other symptoms, I'd strongly advise taking it to a trusted mechanic for a proper diagnosis. They can perform a smoke test to pinpoint any vacuum leaks and confirm the manifold's condition. It's probably not worth the hassle of DIY-ing this one, especially considering the potential damage to engine performance if misdiagnosed.

Thanks for the detailed insights. I appreciate you sharing your experience. I took it to my mechanic, and it turned out the intake manifold was indeed leaking due to scorching. The manifold was replaced for 1180€, and the engine is now running smoothly again.