Nissan paladin 2.4 engine has abnormal noise.

1. Abnormal sound of piston knocking on the cylinder The so-called abnormal sound of piston knocking on the cylinder refers to the abnormal sound produced by the piston side slapping the cylinder wall. Generally speaking, when the direction of the axial force generated at the top of the piston changes from the compression stroke to the power stroke with the piston, the piston knocks abnormally. In other words, every time the piston moves downward through the top dead center, it will be forced to hit the cylinder wall, resulting in the piston knocking failure. Whether the piston knocks on the cylinder or not depends on the gap between the piston and the cylinder wall, which is what we usually call the piston gap (Figure 5). As for the specific influence of piston clearance on cylinder knocking, it is mainly manifested in the following characteristics: excessive piston clearance is prone to cylinder knocking, which is determined by the material and working characteristics of the piston. It gives an intuitive performance that when the engine is cold, the piston clearance is large and the knocking sound is loud, but with the increase of engine temperature, the piston clearance will decrease accordingly and the knocking sound will also decrease. With the continuous development of automobile technology, engine knocking has undoubtedly become a problem that people must solve in order to reduce engine noise and prolong engine maintenance cycle. Therefore, in order to reduce the chance of cylinder knocking, some engines adopt the design that the center of the piston pin slightly deviates from the center line of the piston, so that when the piston is subjected to combustion pressure, the thrust direction of the piston changes, so as to reduce the pressure of the piston knocking on the cylinder wall, thus achieving the purpose of reducing the chance of cylinder knocking. After understanding the causes of piston cylinder knocking, we combine several common piston cylinder knocking failures to analyze their causes and detection methods. 1. The abnormal noise of the piston knocking on the cylinder is serious after the cold start of the engine, and it decreases after the hot start. This phenomenon is caused by excessive piston clearance or piston deformation. Because the piston clearance is large in cold state, the abnormal noise is naturally more serious, and when the engine temperature rises, the piston will expand, thus reducing the piston clearance and reducing the abnormal noise. (The dark horse will take off from here ...) 2. The engine is basically normal when it is cold, and this kind of fault is mentioned when it accelerates when it is hot. Some people may feel that there is some contradiction with the abnormal sound of the first piston knocking on the cylinder. In fact, a careful analysis will be clear. As we know, with the increase of engine temperature, the combustion speed of mixture will be accelerated accordingly. If the ignition time is too early, the highest point of cylinder combustion pressure will advance, and the knocking force on the cylinder wall will increase, resulting in abnormal knocking. The inspection contents and methods of such faults are as follows. (1) Check the fuel grade used. Low-grade gasoline is prone to deflagration. (2) Check the engine temperature. If the engine temperature is too high, the combustion speed and pressure in the combustion chamber will rise, thus causing deflagration. (3) Check the ignition timing. Premature ignition time will advance the combustion pressure point, leading to cylinder knocking. For the electronically controlled ignition system, it is important to check the relevant signals that control the ignition timing, such as throttle position sensor, intake pressure sensor, knock sensor, air flow sensor and water temperature sensor. At the same time, check the ignition timing angle with timing lamp and computer detector. Here we emphasize to check the signal of knock sensor (Figure 6). As we all know, when an engine equipped with a knock sensor is accelerating, the knock sensor will send a knock signal to the engine computer in time according to the knock condition, and the computer will adjust the ignition timing accordingly. If the knock sensor fails or the engine knock signal is not fed back to the computer in time, the ignition time of the engine will be advanced, resulting in abnormal sound. (4) Check the air-fuel ratio of the engine mixture, that is, the CO content in the exhaust. Because, if the engine mixture is too rich, under the same working conditions, the ignition time will be relatively advanced, the combustion speed of the mixture in the cylinder will be accelerated, and the pressure in the combustion chamber of the cylinder will rise too fast, which will also produce knocking noise. In this case, check the oxygen sensor, air flow meter, intake pressure sensor and oil pressure regulator. (5) Check whether the negative grounding wire of the engine is well connected. Some models of engine knock sensor and oxygen sensor are grounded through the housing. When the engine grounding wire is not well grounded, these two signals can not be transmitted normally, which will also cause abnormal noise of the engine. (6) Check the carbon deposit in the engine combustion chamber. An engine that works for a long time will gradually form a large amount of carbon deposits in the combustion chamber. Because carbon deposits occupy the space of the combustion chamber, the compression ratio of the engine will increase, which will eventually lead to the increase of combustion temperature and pressure. At the same time, due to the heat insulation effect, when the engine runs at high speed, the carbon deposit keeps the heat of the last work. When the mixture is compressed to a certain pressure and temperature, the heat stored by carbon deposits will ignite the mixture, forming irregular combustion and causing abnormal detonation. Also known as spontaneous combustion and multipoint combustion. Therefore, the carbon deposit in the engine should be removed in time to prevent abnormal knocking of the cylinder. 3. The engine always has abnormal noise. When the engine is cold, the abnormal noise is relatively light, and when it is hot, it is generally more common in the engine that has just been overhauled. Therefore, in order to avoid this kind of fault, we should pay attention to the installation direction of the piston during assembly, especially for the engine with offset piston pin. This design is originally designed to reduce the chance of the engine piston knocking. Once it is installed backwards, the abnormal sound of the cylinder is inevitable. 4. Single-cylinder knocking cylinder guide sound. Most of these faults are caused by serious deformation of engine piston, piston strain caused by overheating or cylinder strain. Generally, the piston of a cylinder can be found by fire breaking test or by endoscopy. Generally, when the cylinder is pulled, the cold and hot engines will produce abnormal noise. As for the abnormal sound caused by the impact of foreign matter entering the cylinder or valve bending on the piston, there will always be abnormal sound whether the flameout test is carried out or not. Generally speaking, abnormal piston sound and cylinder knocking can be judged by temperature, ignition test and endoscope. In a word, only by combining several methods can we judge accurately, otherwise it is easy to make mistakes. Three, crankshaft bearing (big tile) and connecting rod bearing (small tile) can be distinguished from the sound position and sound frequency. Generally, the sound of large tiles (Figure 7) is low, while that of small tiles is high. Secondly, it can be distinguished by the broken cylinder, because when some small tiles make abnormal noise, if the broken cylinder is ignited or injected with oil, the abnormal noise will weaken or disappear; If the tile rings, the sound will only weaken or remain the same when the cylinder breaks, but it will never disappear. Let's analyze the causes of abnormal sound of crankshaft bearings, mainly including: ① loose bolts for fixing bearings. ② The gap between big tile and small tile is too big. ③ Poor lubrication or ablation of large and small tiles. ④ When the oil pressure is too low or the oil is too little, generally, the small tile is easy to be ablated at high engine speed, and the large tile is less ablated. ⑤ When the gap between the important official and the tile is too large, it is easier to ablate than when the gap is small. Therefore, when overhauling the engine, there should be a proper clearance between the shaft and the pad, but it should not be too large. 4. Abnormal noise of the intake and exhaust system When the intake manifold leaks air, the engine will generate noise when idling, and the noise will be aggravated with the increase of engine speed; When the exhaust manifold leaks air, a sudden explosion will be heard when the engine accelerates; When the air filter is blocked, the engine will feel bored and will produce abnormal noise when accelerating; When the exhaust system is blocked and the engine accelerates, sometimes there will be tempering explosion; When there is air leakage from the air flow meter to the rubber hose of the throttle valve (Figure 8), sudden sonic boom of tempering will also occur during rapid acceleration. V. Abnormal sound caused by engine bracket damage. When the engine is idling, the engine vibration will be too large. When you are in gear (especially automatic transmission), the engine will produce abnormal noise because of excessive swing. When the gear hears the click sound, check whether the engine bracket is intact and whether the engine is installed in place. Abnormal sound of crankshaft with disc (with torsional vibration damper) or flywheel (with torsional vibration damper) Because the crankshaft produces periodic torque, in order to eliminate vibration, many engine crankshaft limbs are connected with pulley through rubber to absorb the vibration generated when the engine accelerates. However, when the pulley is damaged by aging, it is easy to produce abnormal sound. Its noise feature is that when the engine is idling, the abnormal noise is serious due to uneven pulsating torque, and when the engine is slowly accelerating, the abnormal noise becomes smaller or disappears. When the engine is idling and the air conditioning compressor is turned on, the abnormal sound changes. In addition, some vehicles have also designed torsional dampers on the flywheel disc of the engine (Figure 9). When the damping rubber in the flywheel is aged and damaged, the clutch will make a "click" noise when it is engaged. Vehicle road test sometimes produces abnormal sound, but if you have no experience, it is easy to diagnose whether the fault is abnormal sound of transmission or differential.