[99v6stang]

so when an engine knocks, whens does it do it and what exactly does it mean for the engine. Mine seems to be making a knocking noise whenever i shut of the car. Is this engine knocking? And if it is could this have somethig to do with why my car feels so slow, and what can i do to fix it? Or does it mean time for a new engine

[ESG-642T]

Knocking (also called pinking or pinging)— colloquially detonation—in internal combustion engines occurs when air/fuel mixture in the cylinder has been ignited by the spark plug and the smooth burning is interrupted by the unburned mixture in the combustion chamber exploding before the flame front can reach it. The engineered combusting process ceases, because of the explosion, before the optimum moment for the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber, creating a characteristic metallic "pinging" sound, and pressures increase catastrophically.

Wikipedia -

[stephen]

Originally Posted by SSStang Knocking (also called pinking or pinging)— colloquially detonation—in internal combustion engines occurs when air/fuel mixture in the cylinder has been ignited by the spark plug and the smooth burning is interrupted by the unburned mixture in the combustion chamber exploding before the flame front can reach it. The engineered combusting process ceases, because of the explosion, before the optimum moment for the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber, creating a characteristic metallic "pinging" sound, and pressures increase catastrophically.

This is partially correct. What is happening is that the mixture is igniting BEFORE the spark plug fires. So in effect the timing is way too advanced.
This usually occurs because of deposits of carbon in the combustion chambers, which become hot enough to ignite the mixture. The 3.8L Ford V6 is very prone to building up carbon deposits due to defective design of the PCV system. The fix is to decarbonize the engine, either with an additive or by removing the heads and scraping the carbon off the pistons etc.

[Cricket]

But since you say it happens when you shut your car off, it is not detonation. Car can't have detonation if it isn't on. It's your engine cooling down is all. Alls cars make pinging noises when they shut off after being driven for a while. Usually at night when the air is colder.

[Breakout]

if you're going to decarbonize the system...it's a good idea to look at the spark plugs, too. They will build up carbon and eventually fail to give sufficient spark.

[stephen]

Originally Posted by Cricket But since you say it happens when you shut your car off, it is not detonation. Car can't have detonation if it isn't on. It's your engine cooling down is all. Alls cars make pinging noises when they shut off after being driven for a while. Usually at night when the air is colder.

I'm not sure now what the problem is. If it is run on, that is also related to the carbon deposits, they continue to ignite the mixture, thus the engine continues to run for a while after the ignition is shut off.

As you say cars make snapping type noises as they cool down, which is normal.

[tomzac]

I don't know how it could be run on. When you turn your car off, the computer stops signaling the injectors to fire. No fuel, no run on. Unless you're running so ungodly rich, preignition gives it one last pop. Carberated cars with bad preignition can run after the key is swiched off.

[707Stang]

Well it seems that since it does it when he shuts it off in maybe dieseling? i guess that's what the word is. but look at your timing if you have made some mods it may have thrown it off. ( But i'm not too sure with the new cars tho, anyone sure about that one?)

[stephen]

Originally Posted by tomzac I don't know how it could be run on. When you turn your car off, the computer stops signaling the injectors to fire. No fuel, no run on. Unless you're running so ungodly rich, preignition gives it one last pop. Carberated cars with bad preignition can run after the key is swiched off.

Yes that sounds right. So I'm not sure what we are talking about.