Is it normal for the engine to knock at idle but stop when the throttle is applied?
3 Answers
It is not normal for the engine to knock at idle but stop when the throttle is applied. Causes of this phenomenon: During cold starts, there may be insufficient lubrication between the piston and cylinder wall, excessive clearance between the piston and cylinder wall, or excessive cylindricity of the piston. This causes the piston to swing and strike the cylinder wall under high-pressure gas during the power stroke, resulting in a metallic knocking sound. Hazards of engine idling: When the engine is idling, the lower temperature and pressure inside the cylinder prolong the ignition preparation time. Combined with reduced cylinder sealing, this makes knocking more likely, shortening the engine's service life.
Engine knocking at idle but disappearing when throttle is applied is a common abnormal phenomenon I've encountered. At low idle speed, cylinder pressure is insufficient, leading to incomplete combustion (causing detonation) or piston friction noise. When revving up, stabilized combustion and improved lubrication eliminate the sound. Typical causes include overly thin/insufficient engine oil, severely carbon-fouled spark plugs, worn piston rings, or incorrect ignition timing. Ignoring this not only increases fuel consumption but accelerates engine wear, ultimately raising repair costs. I recommend owners regularly check oil level/cleanliness, replace spark plugs, verify ignition system functionality, and conduct cylinder compression tests by professionals when needed. Maintaining stable idle operation during daily driving is crucial for prevention—don't wait until major repairs become necessary.
I've encountered this issue before - the engine suddenly starts knocking loudly at idle, but the noise disappears when lightly pressing the accelerator, which is clearly abnormal. A normal engine should idle as quietly as a kitten, and such irregular noises often indicate lubrication issues or incorrect valve clearance, especially common in older vehicles. At low idle speeds, insufficient oil pumping leads to friction noise; when the throttle is pressed and RPM increases, proper oil circulation eliminates the noise. A simple solution is to immediately check the oil color and level - replace it if substandard, and clean the throttle body while changing spark plugs. Delaying repairs may lead to bearing damage or high-speed failures, compromising driving safety. Regular maintenance like oil changes and air filter replacements can prevent such issues from developing.
