Ping, knock and related subjects

[BENZ-LGB]

I recently searched the terms "ping" and "knock." I learned a lot about ping and knock, but I still have a few questions:

Ping and knock are terms that describe a similar event: pre-ignition. But...

1. Why does carbon deposits on valves and piston heads cause ping? What is the relationship between carbon deposits and ping?

2. Ping usually occurs under load, when ambient or engine temperature is high. What's the relationship between heat and ping?

3. In modern engines, where engine timing is usually not an issue (due to knock sensors that advance or retard timing), are tehre any other "timing" factors that can be a cause (like a timing chain that has slipped out of place?

I was researching the ping issue because a friend of mine, with a high mileage 420SEL, has been complaining about knocking or pinging. His car does not consume much oil (less than 1 qt. every 3K miles), the engine idles smoothly and the car runs well otherwise. The only problem is pinging, under load, when the engine is hot (or when it is hot outside). He has tried all the usual fixes, changing gas brands, using something called Moly valve cleaner, but the car still pings. His mechanic says he needs an engine overhaul. He lives out of the state, otherwise I'd refer him to my own mechanic. I told him that an engine overhaul seems a bit of overkill.

What do you guys think? My friend is ready to drop the Benz and get a Chevy

Thanks.