What causes pinging or knocking under the hood?

Answer 1

Pinging or knocking could be any number of things, unfortunately. Experienced car folks can usually tell serious problems from minor ones just by listening.

Number one problem is low grade (low octane) or cheap gas. It doesn't ignite as well as the others and may lag behind the spark just a bit to allow you to hear the combustion from the cylinder from the open valve.

Secondly is ignition timing. Retarded or advanced timing could accomplish the same problem. Burning the compressed air/fuel vapor at the wrong time resulting in a ping. My experience in California is that the manufacturer's "smog test" suggested timing makes my car run really bad. Every time they test it, I have to set my timing at the mfgrs setting for the test (or else they wont do it). It passes but runs like heck.... pinging and no power. I just set the timing back to what runs well and doesn't ping after the test passes. Kind of a joke really.

The two above are usually diagnosed as a rattle or "ping" from the valve train (top of the engine) and occurs typically when the engine is under load. Going uphill, starting out from a stoplight or accelerating quickly. Both of these will build up carbon in the valves and pistons when associated with stop and go city driving. If the carbon stays hot, it acts like a spark pulg and lites the compressed fuel/air vapor and will cause the car to run-on (dieseling) after you shut it off.

Now (not to scare anyone) but "knocking" is another thing altogether. Again, a trained or experienced ear can tell the difference. Knocking could be a worn part in the engine that may require a rebuild. Not likely though. Chase all the other (cheaper) ideas first.

HEY. CHECK YOUR MOTOR OIL LEVEL. When oil gets low, rattling from the valve train and oil pump may be heard. Oil leaves cars in various ways, though burning, dripping and big gashes in the oil pan...... don't ask :) Make sure you keep the right oil in the car (check your owner's manual) The wrong viscosity of oil can ruin hydraulic lifters.... another ratttle. Get the right stuff at the right dipstick level.