Crankshaft failure during engine startup is a common mechanical failure, especially in low-temperature environments or lack of maintenance of the vehicle is more frequent. This kind of failure is often accompanied by starter idling, gear noise or even completely stuck phenomenon, if not dealt with in time may lead to serious damage to the engine. In this article, we will analyze the system from the cause of the failure, diagnosis method to repair skills, to provide you with engine troubleshooting methods.
1. Failure causes in-depth analysis
(1) Gear mesh abnormality: starter and flywheel “bite failure”.
When the starter gear and flywheel gear ring mesh poorly, often accompanied by metal impact sound. Failure to mesh can be caused by wear of more than 3 teeth on one side of the ring or by deformation of the teeth (e.g. expansion of the ring due to high temperatures). For example, long-term violent startup may cause the tip of the tooth chipping, the formation of “gear misalignment stall”.
(2) Cylinder sticking: “metal welding” at high temperatures
Engine overheating shutdown, the piston ring and the cylinder wall due to thermal expansion adhesion. When the temperature plummets, aluminum alloy piston and cast iron cylinder shrinkage differences will intensify the adhesion, the formation of “cold lock-up”. At this time, the crankshaft rotation resistance can be up to 5 times the normal value.
(3) Crankshaft lock-up: the fatal consequences of lubrication failure
More than 90% of crankshaft lock-ups originate from lubrication system failures:
Worn oil pump resulting in oil supply pressure below 1.5 bar (normal 2.5-4.5 bar)
Oil film breakdown due to excessive clearance (more than 0.15mm) between the axle shafts.
The oil sludge formed by poor quality oil clogs the oil channel. Typical cases can be seen in Mercedes-Benz GL350's repeated shaft-holding failures caused by oil quality problems.
(4) Injector pump stall: diesel engine “thrombosis crisis”.
Plunger coupling due to water intrusion or fuel impurities jammed, high-pressure oil circuit can not establish pressure. At this time, the injector can not be normal atomization of fuel, resulting in abnormal mixture concentration in the cylinder, crankshaft resistance increased dramatically.
2.Diagnostic process and repair program
Step 1: Initial Gear System Examination
Use a crowbar to manually rotate the flywheel, if it can rotate and the engine is successfully started, it indicates that the gear ring is partially damaged.
Check the fixed state of the ring: loose ring will produce periodic noise, need to be replaced with new parts using the hot mounting method (heated to 200 ℃ and then pressed).
The ring worn on one side can be turned 180° and reused, saving 60% of maintenance costs.
Step 2: Internal mechanical fault diagnosis
Crankshaft hold-up verification: After removing the spark plug/injector, the crankshaft still cannot be rotated, and the oil pressure gauge shows <1bar, the lubrication system can be determined to be faulty.
Sticky cylinder detection:
Inject 20ml of oil into the cylinder, let it sit for 2 hours and then try to rotate it.
Use the endoscope to observe the cylinder wall scratches, the depth of scratches > 0.1mm need to be repaired by boring the cylinder.
Injector pump detection:
Dismantle the pump body to check the plunger movement resistance, normal should be <5N
Use ultrasonic cleaning machine to remove carbon deposits, replace the plunger coupling with excessive wear.
3.Key restoration technology points
(1) Gear system repair
Welding and repairing the gear ring need to use nickel-based welding rod, preheating temperature control at 300 ℃ to prevent thermal cracking.
After installing the starter, it is necessary to adjust the gear lash (standard value 1.5-2.5mm), too large will lead to tooth beating, too small will increase the starting resistance.
(2) Crankshaft holding treatment
After disassembling and measuring the clearance of the shaft tile, the excess difference of 0.05mm needs to replace the shaft tile.
Use honing machine to repair crankshaft neck, roundness error should be <0.005mm.
Clean the oil passage with high pressure oil gun (pressure>8bar) with special cleaning agent.
Mechanical faults such as crankshafts that fail to rotate may appear simple, but often hide deeper problems such as lubrication failures and assembly errors. By using a step-by-step diagnostic method from the outside to the inside, combined with quantitative means such as pressure testing and dimensional measurements, the point of failure can be pinpointed accurately. If you want to know more about engine troubleshooting, please visit our website.
