Again, I'm not an MB expert, but in general to check for a blocked exhaust (most likely cat converter), you can do a vacuum check at idle and then at 1500 RPM. If the vacuum is say 21 inches at idle and like 15 inches at 1500 then the cat is likely clogged.

Others remove the oxygen sensor and start it up. The hole where the oxygen sensor was will relieve the back pressure and it should run a tad better. If so, then disconnect the exhaust pipe at the exhaust manifold and try it again. If it is a clogged cat it will scream like a banshee and rev up to its normal 4000 rpm.

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1999 SL500