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.

[300EE320]

Hi Benz-LGB,

Carbon deposits can build up to the point where they retain heat from combustion and "glow". They continue to glow during the compression stroke and when the cylinder pressure reaches the right point, the glowing carbon can initiate combution (before the spark plug). This is "pre-ignition", causing the pinging sound your friend hears.

The symptom can also be caused by having the timing advanced too far where the spark plug is initiating the combustion, but too early.

The pinging is actually the explosion of combustion hitting the piston while the piston is still traveling up in the cylinder. In a severe case, this can bend a connecting rod or "hole" a piston.

I assume that he has tried the highest octane fuel he can get? How about trying a can of octane boost? High octane fuel burns slower and gives the piston more time to reach TDC before the explosion meets it, curing the problem. The next step would be to have the ignition timing checked.

I hope this helps.

Please let us know how it turns out.

[300EE320]

By the way, I love Chevys. Make sure that your friend understands that these problems can occur in any make or model and usually has a lot to do with the poor quality of fuel at the pump these days...

[BENZ-LGB]

...thank you for the reply. So the carbon deposits act as mini-glow plugs or spark plugs, when they get heated enough, they ignite. That makes sense.

I think he should look into the timing issue. His car is very well maintained and the pinging developed very suddenly.

I like Chevys too. Nice cars. Back in high school my brother and I restored a 57 Chevy to car show conditions. It was a real fun project.

Thanks for the info.

EJ