The most common problem with the W163 is transmission failure. The second generation ML320s (W164) have a common problem of engine oil leaks. These leaks typically occur around the oil cooler lines or at the front crankshaft seal. The W166 was mainly reported for electrical issues and the W167 had problems with air suspension.
The problem I am seeing is engine idling/revs behaviour, from cold it idles at just over 900 rpm and won't drop until a good 15 minutes of driving. When it does eventually drop to just over 800 rpm it will on occasion sit back at 900 even though the engine has warmed up.
Symptoms displayed-. NO smoke from muffler. (Small drips of water observed) NO water observed in crankcase oil or signs of whipped oil. Oil Level about normal. Tranny fluid level about normal. No signs of water in fluid. Motor runs with no signs of overheating for extended periods.. 289 276 324 184 126 111 8 277 188