The sudden appearance of a strange sound in the operation of the power unit always causes alarm in the owner of the car. engine-knock It occurs when the gas is discharged, which is when the fuel supply is reduced dramatically and the engine is braking, and unlike the knocking under load, which often indicates problems with the rod liners, the sound on the reset is of a completely different nature and requires a specific approach to diagnosis.
The mechanics of the process is such that when releasing the accelerator pedal in drain-manifold This pressure change affects mixing, the function of the valves, and even the behavior of the piston group, and ignoring these symptoms can lead to serious damage, so it is important to immediately identify the source of vibration and sound to avoid costly overhauls in the future.
In this article, we will discuss in detail the main causes of metal clang, slurring or ringing when gassing and braking the engine, you will learn how to distinguish critical malfunctions from acceptable noise, and what steps you need to take to localize the problem in a garage or service environment.
Mechanics of the process: why there is a knock on the reset
To understand the nature of sound, we need to look at the physical processes that occur in the cylinders as they transition from acceleration to braking. When the driver abruptly releases the gas pedal, the throttle closes, but the inertia of the car continues to rotate the crankshaft. pistons They start working like pumps, pumping gases out of the cylinders.
A sharp change in pressure puts a specific strain on the details of the crank-shaking mechanism. If oil normally creates a reliable cushion between rubbing pairs, then if you jump the operating modes, you can briefly disrupt the oil wedge. It is in these fractions of a second that gaps between parts that do not normally manifest themselves can become a source of audible noise.
⚠️ Attention: Knocking on a gas discharge is often confused with detonation, but these are different processes: detonation is accompanied by ringing under load, and mechanical knocking on the discharge indicates backlashes in the nodes of friction or gas distribution mechanism.
It is important to consider that the nature of the sound can vary depending on the temperature of the engine. On cold oil is more viscous, and some knocks can only be heard for the first minutes of operation. On a hot engine, when the lubricant becomes thinner, there are problems with worn-out slip bearings or thermal gaps of the valves.
Problems of the gas distribution mechanism (GRM)
One of the most common causes of knocking during gas discharge are malfunctions in the GRM system. At the time of a sharp decrease in revolutions, the inertia of the valves and pushers can lead to their impacts on the saddle or against each other if thermal gaps are broken.
If your vehicle is installed hydrocompensatorAnd when you knock on a reset, it's often a sign of a coking or a low oil pressure in the system, and the oil doesn't fill the compensator cavity when you change mode abruptly, and you get a characteristic clinking sound. In motors that are mechanically adjusted, the problem is that there are too large gaps that start to ring when you brake the engines.
The stretching of the HRM chain can also be felt in transition modes, the weakened chain begins to rattle against the walls of the casing or sedatives when the inertia of the camshafts changes the tension vector, this is especially noticeable on engines with high mileage, where the life of the tensioner is already exhausted.
- 🔧 Wear cams cams camshaft leads to the appearance of knocking, which changes with the frequency of rotation of the crankshaft.
- 🔧 Damage or wear of the gears of the GRM drive causes periodic metal clang.
- 🔧 Weakening the bolts of the camshaft bed fastening can cause vibration and knocking when the load changes.
How to distinguish the thud of the GRM from the thumping of the piston?
The GRM is usually more vocal and frequent, with a rhythm that matches half the rotation of the crankshaft (on a four-stroke engine), the piston beat is more often deaf and (low-frequency). Simple test: apply a long screwdriver to the cylinder block and cylinder head, where the sound is louder, there is the source of the problem.
Diagnosis of HRM requires removal of the valve cover, which allows you to visually assess the condition of the chain, tensioners and valves themselves. Do not ignore these symptoms, as a break in the chain or jamming the camshaft will lead to a meeting of the valves with the pistons and expensive repair of the head of the block.
Failures of the piston group and the CSM
The pistons knocking in the cylinders is a more serious problem, and it often occurs when the piston's force changes direction, and when the piston is discharged, the force vector that presses the piston against the cylinder wall changes to the opposite, and if the gap between the piston and the cylinder is large due to the output, the piston skirt hits the wall.
It is also worth paying attention to the state of piston-fingerFingers are often described as a ringing metal click, and on some engines, this sound only manifests itself on a warmed-up motor when the heat expansion of parts reaches its peak and the lubrication becomes less efficient.
Diesel or direct-injection engines may have a knock associated with piston shifting, but if the sound gets louder over time and is accompanied by a drop in compression, it is a sure sign of critical cylinder wear or rings.
| Type of knocking | Nature of sound | Temperature dependence | Probable cause |
|---|---|---|---|
| Piston | Deaf, metallic | Harder on cold, subsides on hot | Wear of the piston skirt, big gap |
| Finger | Calling, high tone | Often heard only in the hot | Wear of the sleeve of the upper head of the rod |
| Fast-wired | Deaf, rhythmic. | Increases with warming | Wear of the rod liner (more often under load) |
| Valve | Twisting, frequent. | It doesn't depend on temperature. | Large heat gap |
Use a medical stethoscope or a long screwdriver to listen to the engine, which will localize the sound source to the cylinder, without disassembling the engine completely.
It is important to note that the knocking of the rod liners (the “cracker”) is most often manifested under load, but in the advanced stages of wear it can be heard at idling speeds and when the gas is discharged. If you hear thumps from the bottom of the engine, which are faster with a set of turns, operation should be stopped immediately.
Impact of the ignition system and detonation
It's not always mechanical, sometimes drivers mistake it for the engine. knock-off This is when the cylinders continue to burn the fuel mixture or when the hot lobes self-ignite.
When the throttle is closed abruptly, excess oil (through worn-out oil caps or rings) or fuel (with faulty nozzles) can enter the cylinder, disrupting the octane number of the mixture and causing explosive combustion, which is perceived by ear as a metal ringing.
- 🔥 A faulty knock sensor does not correct the angle of ignition advance, causing ringing.
- 🔥 Kalyl ignition occurs due to overheating of candles or the presence of foci of sodium in the combustion chamber.
- 🔥 Ignition too early can cause knocks when the load on the engine changes dramatically.
To diagnose this problem, you need to check the ignition system, the condition of the candles and the angle of ignition ahead. Using fuel with a higher octane number can temporarily remove the symptom, but will not solve the problem if the cause is overheating or soaking.
It is also worth checking the exhaust gas recirculation system (EGR). If the EGR valve is jammed in the open position, excess exhaust gas can be released into the cylinders, which disrupts the combustion process and causes uneven engine operation, perceived as knocking.
Diagnosis: step-by-step instructions
Determining the exact cause of knocking requires a systematic approach, always starting with a visual inspection and checking of the levels of technical fluids. Low oil levels are the most common, but common cause of extraneous noise in the engine.
Then you have to test it in different modes, warm it up to operating temperature, then let it cool down a little bit, listen to the engine at idle speed, when you press the gas sharply, and of course, when you drop it sharply, and use the phonendoscope to localize the source.
☑️ Checklist of primary diagnostics
Be sure to measure the compression in the cylinders. A scatter of values greater than 1 atmosphere or a low compression in one of the cylinders will indicate problems with the piston group or valves. It is also critical to measure the oil pressure in the system on the warmed engine.
⚠️ Attention: If the oil pressure on the heated engine at idle speeds falls below 0.8-1.0 bar (depending on the model), further operation of the engine is prohibited until the malfunction of the oil pump or liners is eliminated.
If mechanical causes are excluded, proceed to check the electronic part, the error scanner will help to identify ignition gaps or sensor malfunctions that can cause uneven engine operation, simulating knock.
Remediation and prevention
The problem with knocking depends on the diagnosis, and if the problem is with the hydraulic compensators, sometimes it helps to flush the engine with special oil or replace the compensators themselves.
When the piston group wears, the options can be different: from replacing the rings and liners to smelting the cylinder block for the repair size. The critical point is that when there is a knock of rod liners, further driving can lead to a twist of the liner and the destruction of the neck of the crankshaft in a matter of kilometers.
The prevention of such problems is the timely replacement of oil and filters. The use of high-quality lubricants with the correct viscosity tolerance prolongs the life of hydraulic compensators and reduces the wear of rubbing vapors during cold start-up and sudden load changes.
Timely oil change is the cheapest way to prevent major engine repairs. Don't skimp on filters and replacement intervals.
Regular diagnosis of the condition of the belt or the HRM chain will also avoid problems with gas distribution. Do not wait for the scheduled replacement period if the vehicle is mileage large or the operating conditions are heavy (city traffic jams, short trips).
FAQ: Frequently asked questions
Can I drive if the engine is knocking on the gas discharge?
If the knock is quiet and only appears in the cold, you can reach the service in gentle mode. If the knock is loud, metal and heard on hot, further movement can lead to a jamming of the engine.
Will replacing the oil with a more viscous one help?
In some cases, switching to oil with slightly higher viscosity (e.g., from 5W-30 to 5W-40) may temporarily reduce the noise of hydraulic compressors or worn liners, but this is not a repair, but only a temporary measure to mask the problem.
Why does it only knock on cold?
On a cold engine, the gaps between parts are maximum, and the oil has not yet been distributed to all nodes. Knocking on a cold engine often indicates wear of pistons or problems with hydraulic compensators, which "go away" when parts are thermally expanded.
How to distinguish the knock of the engine from the knock of attachments?
Try to loosen the belt of the drive of the attachments (generator, pumps) for a short time. If the knock is gone, the problem is in the rollers or bearings of the hinged one. If the sound is left, the source is inside the engine.