As the saying goes, do what you love, do what you know. Mastering some simple truck repair knowledge will bring us some convenience, whether in work or money. The card friend “Jiayan” of the existing forum posted “don’t think it’s so difficult to repair the car, in fact, you will”, introducing the characteristics and causes of the abnormal mechanical noise of the diesel engine. You can also see a doctor for your car. Let’s share it with you.
The abnormal noise of diesel engine mainly includes piston knocking, piston head knocking, piston topping off, piston rubbing “shoulder arm” sound, piston pin sound, connecting rod bearing < small tile > sound, crankshaft bearing (large tile) sound, air leakage sound, valve sound, tappet sound and timing gear sound, etc.
If the fit clearance between the piston skirt and the cylinder liner is too large, a clear and rhythmic knocking sound will be sent out on the upper part of the cylinder block when the diesel engine is idling. The gasoline engine makes a ‘click’ sound. The diesel engine has a “just” sound. The piston diameter is small and the knocking sound is crisp. The piston diameter is large and the knocking sound is heavy.
The noise is obvious at low speed, and disappears at medium and high speed. The accelerator shakes at low speed with a ‘click, whine’ sound. When the water temperature of the diesel engine is lower than 40 ℃, the sound will weaken or disappear with the increase of temperature. The sound of serious piston knocking will not weaken at any temperature. Internal listening and real listening are more obvious than virtual listening. When the oil supply of a single cylinder is cut off, the sound weakens or disappears. The cylinder with changed sound is the faulty cylinder with piston knocking sound.
Virtual listening: it refers to standing in front of the engine and listening to abnormal sounds through the air.
Internal listening: insert a hollow tube into the crankcase and directly listen to the abnormal sound of the piston, crankshaft and connecting rod.
Real listening: use a metal rod or long handle against the diesel engine body to listen to the abnormal sound by the vibration of the body.
Oil cut-off method: loosen the high-pressure oil pipe joint nut of a high-pressure cylinder to stop the oil supply of the cylinder.
Piston topping refers to the fracture of the head and skirt from the oil ring groove of the piston. Generally, the piston head remains at the top of the cylinder, and the skirt still moves up and down with the connecting rod and collides with the piston head.
A loud ‘click’ sound can be heard above the diesel engine at any speed, and the sound increases at high speed; In the cylinder head of the engine, the false hearing is obvious and the real hearing is shocking; The sound does not change after the oil is cut off; The faulty cylinder does not work. After removing the fuel injector, start the diesel engine. There is no compressed air in the faulty cylinder, and the abnormal noise of the cylinder is very loud.
When the piston decapitation fault occurs and the maintenance conditions are not available, you can remove the cylinder head, first press the piston head to the bottom to point position, then quickly rotate the crankshaft to eject it, then install the cylinder head, and take out the push rod and lifter of the two valves of the cylinder, so that the inlet valve and exhaust valve of the cylinder are always closed, so that the cylinder does not work at all. After such treatment, the diesel engine works without one cylinder.
In repairing the diesel engine, if the height of the cylinder shoulder (where the uppermost part of the cylinder liner is not rubbed by the piston ring, a step will be formed, which is called the cylinder shoulder) is not checked, and the new piston ring is not chamfered, a crisp ‘rattle’ sound will be heard in the test run, which will sound at any speed; In the upper part of the diesel engine cylinder block, false hearing and real hearing are obvious; When the oil is cut off, the sound does not change; The diesel engine feels jittery during operation.
The piston pin and connecting rod bushing have been improperly repaired due to wear, resulting in excessive matching clearance. When the diesel engine runs at a slightly higher idle speed, a more clear and rhythmic knocking sound is sent out on the upper part of the cylinder block than that of the piston.
There is a noise at idle speed, which is obvious when the idle speed is slightly high; When the oil is cut off, the sound of the faulty cylinder becomes louder and the knocking frequency increases; It weakens at medium speed and disappears completely at high speed.
Serious piston pin noise. The noise is obvious at low speed and does not disappear at medium speed: the noise is more obvious when refueling at medium speed, and the abnormal noise disappears when the oil is cut off.
The fitting clearance between the connecting rod bearing and the connecting rod journal is too large due to wear or improper maintenance. When the diesel engine is running at medium speed, a short metal knocking sound can be heard at the lower part of the cylinder block.
It can’t be heard at low speed, and gradually becomes obvious with the increase of rotating speed. It is most obvious at medium speed, and it weakens or disappears at high speed: there is an obvious “woo, Da, Da, woo” sound when shaking the accelerator at medium speed, especially at the lower part of the cylinder block; At the moment when the diesel engine is stopped for ten minutes and restarted, the sound is more prominent: when the single cylinder cuts off the oil to the faulty cylinder, the sound will disappear, while the serious bearing sound will only weaken.
The fit clearance between crankshaft bearing and crankshaft journal is too large, which makes the journal collide with the bearing. The sound appears at medium speed and is obvious at high speed, and there is always a sound in the medium and high speed range: it is heavier than the connecting rod bearing; The sound increases with the increase of load, and the most obvious shaking of throttle at medium and high speed: after multiple bearings are seriously damaged, the oil pressure will decrease significantly.
Middle and high speed real listening or middle and high speed virtual listening are obvious; When the diesel engine is stopped for more than 10min, the instantaneous sound is particularly prominent; The oil cut-off sound of the single cylinder of the head and tail bearings will disappear, and the oil cut-off sound of the adjacent two cylinders of the faulty bearing will disappear at the same time.
When the compressed gas in the cylinder passes through the unsealed piston, valve, cylinder gasket, injector hole and spark plug hole, it sounds like air leakage. What we are talking about here is the sound of air leakage caused by unsealed piston and damaged cylinder gasket.
The piston and cylinder liner leak slightly. When running at low speed, you can hear a “Chi Chi” sound, a “bang bang” sound at medium speed, weakening at high speed, pulsating smoke at the crankcase vent, and a large amount of blue smoke is discharged from the exhaust; If the cylinder gasket between two adjacent cylinders is damaged, the diesel engine may have “tempering” and “shooting”; The cylinder gasket is damaged and communicated with the outside of the cylinder block, and the “bang bang” sound can be heard outside the diesel engine; If the damaged cylinder head gasket is at the water jacket position, white steam will be sprayed out; if the damaged cylinder head gasket is at the oil hole position, oil will be sprayed out.
When the single cylinder is cut off, the sound of the faulty cylinder will weaken or disappear, and the pulsation and smoke of the crankcase vent will weaken; If most cylinders leak, the vent will smoke continuously, and the abnormal sound of single cylinder oil cut-off will not disappear. At this time, it is difficult to start the diesel engine.
The matching clearance between the valve and its driving mechanism is too large. When the diesel engine is at low speed, a clear and rhythmic knocking can be falsely heard at the valve chamber, which is called valve sound; If a similar sound can be heard at the tappet chamber, it is called tappet sound. Their common feature is that the oil cut-off sound does not change cylinder by cylinder. Only when the oil is cut off, the knocking frequency slows down.
The valve mechanism of medium-sized and above diesel engines is generally driven by timing gear, and light vehicles are generally driven by timing gear or timing chain. Due to wear, damage, looseness, too large or too small axial clearance of camshaft, improper use and other reasons, the teeth and clearance of timing gear are abnormal, resulting in gear teeth and noise; Unstable speed and obvious noise; The sound did not change after the oil was cut off.
There is a noise at idle speed, which is obvious at slightly high idle speed; Listen falsely at the gear room, there is a “clatter” or “clatter, clatter”. The timing chain makes a “click” sound; If the teeth and clearance of the timing gear are too small, there will be a “whistling” sound at medium speed.
When the speed of the diesel engine is unstable or accelerates rapidly, there is a loud “bang, bang” sound at the flywheel; The clutch trembles when the car starts; There is a “clatter” sound at the moment of oil cut-off; After the diesel engine is shut down, rotate the flywheel by hand, it will feel loose.
For the combustion chamber with non flat top piston (such as spherical and shallow circular combustion chamber), it is easy to leave screws or nuts on the piston crown during maintenance, and a clear knocking sound can be heard during the test run of diesel engine.
The sound exists at any speed; Cut off the oil cylinder by cylinder, the sound does not change, and each cylinder works normally; The actual listening is most obvious in the cylinder head, and it is more obvious in the upper part of the diesel engine than in the lower part. Check that the valve clearance is normal.
The abnormal sound has nothing to do with the rotating speed, and the oil cut-off does not change. Check the valve clearance according to the position where the sound is located, and it is normal. It should be considered that there are foreign matters in the cylinder.
Jiayan, the card friend of the forum, said: “there are bosom friends in the sea, and the ends of the world are like neighbors. Card friends all over the world are one.” I hope you can bring help here. I also hope you can discuss more and help each other.
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