Symptoms
- Deep knocking or pinging sound when accelerating or under load
- May improve or disappear at steady cruising speed
- Often worse going uphill or with a heavy load
- Could be paired with reduced power
Likely causes
- CommonEngine knock/detonation from low-octane fuel or bad timingFuel igniting at the wrong point in the combustion cycle causes an audible knock, often fixable with proper fuel or timing correction.
- PossibleCarbon buildup in the combustion chamberExcess carbon can cause hot spots that trigger pre-ignition knock.
- PossibleWorn rod or main bearings (more serious)A deeper, more mechanical knock that doesn't relate to fuel quality can indicate internal engine wear.
- PossibleFailing knock sensor causing the engine computer to mistime ignitionA faulty sensor can prevent proper timing correction, allowing knock to occur.
- Less commonOverheating engineExcess heat can contribute to detonation-related knocking.
What to do
- Check what octane fuel you've been using and compare it to what your owner's manual recommends; try a tank of the recommended or higher octane fuel.
- Note whether the knock happens mainly under hard acceleration or load versus at all times, which helps distinguish detonation from a mechanical issue.
- Check the engine temperature gauge to rule out overheating as a contributing factor.
- Have a mechanic scan for trouble codes, including any related to the knock sensor.
- Avoid continuing to drive hard or tow/haul heavy loads if the knock is pronounced, until it's diagnosed.
- Have a mechanic perform a compression test if the knock sounds deep and mechanical rather than a lighter pinging.
- If carbon buildup is suspected (common with certain direct-injection engines), ask about a carbon cleaning service.
- Treat a knock that persists even with correct fuel and no overheating as a sign of a bigger mechanical issue and get it inspected soon.
Tools and parts
Knock sensorSearch: “knock sensor for [year make model]”
When to call a pro
A knocking noise under acceleration, especially if it doesn't resolve with correct fuel, should be diagnosed by a mechanic with a compression test before continued regular driving.
Safety
Use caution
Avoid continuing to drive under hard acceleration or heavy load if you hear a pronounced knock, since this can worsen internal engine damage.