From Knocking to Smooth: Why Your Engine Quiets Down With Acceleration

Engine knocking is when there are knocking or tapping noises coming from the engine when it is running. They often keep time with the speed of the engine and speed up as you accelerate. Sometimes, however, the engine knocking will disappear completely when you accelerate. What causes this to happen and how do you fix it?

In general, engine knocking that goes away during acceleration can be caused by inefficient combustion, uneven fuel burning, low engine oil levels, low oil pressure, a bad oil pump, damaged hydraulic valve lifters, a bad idle air control valve, or a bad mass airflow sensor.

Normal engine knocking that is present all of the time from the time you start the engine is caused by a problem with the combustion of the fuel in the cylinders.

What Causes Engine Knocking At Idle That Goes Away When Accelerating?

Engine knocking that disappears when accelerating can also be caused by a lack of sufficient lubrication within the engine, especially when the knocking appears to be coming from the top of the engine.

When an engine is turned off, the engine oil will pool in the oil pan underneath the engine. After the engine is first started, the oil pump will start to pump oil around the engine, building up pressure as quickly as possible so as to prevent damage to unlubricated components.

Engine knocking noises can also be generated by problems with the combustion part of the engine combustion cycle. These knocking noises can also occur when the engine is idling, but will usually get worse as the engine accelerates.

This is because a problem with the timing of the engine, the spark plugs, or how efficiently the fuel is being used will generally worsen as the engine speeds up and more power is required.

Here are a few of the most common reasons why engine knocking may appear when idling but go away when the car accelerates.

1. Low engine oil level

One of the most common reasons for an engine knocking at idle is a lack of sufficient engine oil within the engine. Engine oil is critical for the engine to run smoothly and to reduce wear and tear. One of the first places to experience adverse wear and tear is the camshafts and valve train at the top of the engine.

This is because engine oil is usually pumped from the bottom of the engine to the top and pressure will increase more, the faster the engine is running. When the engine is idling, the top of the engine is more likely to become starved of oil if there is not enough circulating.

This can create a telltale tapping or knocking noise when the camshaft is turning and the valves are moving up and down without adequate lubrication

2. Low oil pressure

Low oil pressure is just as bad for an engine as low oil levels. If the oil pressure is too low, this will affect the ability of the oil to continuously travel to all parts of the engine and keep them sufficiently lubricated.

Oil pressure is maintained within an engine by the oil pump. Oil pumps can be located inside or outside of the oil pan and are traditionally driven by the rotation of the crankshaft.

The oil pressure is affected by the pressure generated by the oil pump, the viscosity of the engine oil, and the ability of the oil to flow within the engine lubrication lines and through the oil filter.

If oil flow is hindered at any point within the engine lubrication system, or if the oil pump is not working properly then this can lead to parts of the engine not being properly lubricated. This can create knocking noises as moving parts are not moving smoothly when the engine is idling.

As the engine speeds up, oil pressure usually increases with the increased RPM, and this can sometimes eliminate engine knocking that occurs at idle speeds.

3. Damaged hydraulic valve lifters

Hydraulic valve lifters operate within the top of the engine to control the valve pushrods and rockers to open and close the valves.

This is a common place for engine knocking and tapping to occur if there is a lack of proper lubrication within the engine. Hydraulic valve lifters operate under pressure that is generated by oil pressure in the cylinder head.

When an engine first starts, there will generally be a lack of oil in and around the lifters as this usually drains off when the engine stops. It’s not uncommon for valve lifters to make noise when an engine first starts and is cold.

This noise should go away after a few minutes, but if it doesn’t then this may indicate wear or damage to the lifters and possibly a problem with oil pressure or oil flow in the engine.

4. Bad idle air control valve

Another cause of engine knocking is a bad idle air control valve. This is not related to the lack of lubrication but instead is caused by the lack of sufficient air getting into the engine when the throttle is completely closed.

The idle air control valve opens when the engine is idling to allow air to bypass the closed throttle body so that the engine can continue to run when the vehicle is stationary. If engine knocking disappears completely when accelerating this can be because air is now entering the engine via the throttle body and not the faulty idle air control valve.

A broken or sticky idle air valve will mess with the idle speed of the engine and will affect the amount of air that is getting into the engine cylinders for combustion to occur. Too much or too little air will affect the fuel mixture and this will cause the engine to run rough or cut out completely.

If there is too little air getting in the engine may run rich, resulting in unburnt fuel residue finding its way into the exhaust. This can cause knocking or mini detonations as the unburnt fuel burns up in the hot gases present in the exhaust.

Lack of sufficient air can also cause engine misfires, localized fuel detonation within the cylinder, and lack of engine power that can also lead to engine knocking.

5. Bad Mass Airflow sensor

Another common cause of engine knocking and overall rough engine idling is a bad mass airflow sensor. This sensor is located before the intake manifold and is used by the ECU to determine the volume of air entering the engine in real time.

A bad or dirty mass air flow sensor will not be able to accurately monitor the airflow especially when the engine is idling and doesn’t need much air to stay running. This can lead to an engine running rough at idle while idling due to too much air entering the cylinders and causing the engine to run lean.

The ECU is generally better at compensating for a bad mass air flow sensor when the engine is running faster when accelerating. This is because there is more scope to adjust the amount of fuel entering the engine in response to readings from the oxygen sensors.

As a result, an engine with a faulty or dirty mass air flow sensor will usually run smoother with less chance of knocking or misfiring when accelerating.

What Next – Engine knocking at Idle goes away when accelerating

If your engine is knocking or misfiring when idling but this goes away when accelerating then there are a few things you can easily check to help pinpoint the cause of the problem.

1. Start by running a diagnostic using a compatible OBD-II reader. This may uncover relevant engine error codes that are stored in the ECU. Common oil pressure codes include P0522 and P0520 which indicate problems with the oil pump and oil pressure sensor. P0101, P2096, and P2099 can indicate a problem with the mass air flow sensor, whilst P0505 can point to a problem with the idle air control valve.
2. Check the oil level. A lack of engine oil can cause knocking near the top of the engine if the oil pump cannot generate enough oil pressure throughout the engine. Also, ensure that the engine is using the correct specification oil and that it has been changed regularly along with the oil filter.
3. Remove and check the idle air control valve. Blockages and damage to the idle air control valve can hinder its operation resulting in a rough idle and a lack of adequate air entering the engine. Sometimes removing and cleaning the valve can remedy the situation. If possible, you should also check the throttle body to ensure the throttle actuator is opening and closing easily and fully. On some engines, if the throttle is not closing completely when idling this can cause problems.

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