Resolving a Catastrophic Snapped Crankshaft in a Land Rover Discovery 4 SDV6

Land Rover Discovery 4 SDV6 Crankshaft Failure & Engine Rebuild

The owner of this Discovery 4 arrived after searching Google for: “Discovery 4 3.0 diesel loud knocking then engine stalled – won’t start now, what happened?”

The vehicle’s specific problem was a sudden and total mechanical failure: while driving, the owner reported a sudden, extremely loud metallic banging from the engine bay, followed immediately by a complete loss of all power and engine stall. The vehicle would crank but not start. The owner’s primary concern was understanding the scale of the damage and whether a cost-effective, reliable repair was feasible for a vehicle they depended on.

Our Diagnostic Process: Finding the Root Cause in the Land Rover Discovery 4

The symptoms pointed to a sudden, major internal seizure. Our diagnosis focused on determining the exact point of failure without internal inspection tools first.

1. Initial Non-Start Investigation: A basic check confirmed the engine would crank but not fire. Checking for fuel rail pressure via the scanner showed pressure building correctly, ruling out a primary fuel system failure as the cause of the no-start.
2. Compression Test Revelation: We performed a manual compression test on all six cylinders. The results were definitive: cylinders 1, 2, and 3 showed near-zero compression, while cylinders 4, 5, and 6 showed normal, healthy compression. This pattern—a complete loss of compression on one bank of cylinders only—is a classic hallmark of a snapped crankshaft on a V-configuration engine like the SDV6.
3. Visual Clue & Oil Analysis: Removing the oil filter and cutting it open revealed it was packed with glitter-like metallic debris. A magnet confirmed it was ferrous (steel) material, not aluminium from pistons, pointing directly to crankshaft or bearing failure.
4. Borescope Inspection: Inserting a borescope through the glow plug holes of the low-compression bank showed the pistons were intact and at the top of their stroke. Attempting to manually rotate the crankshaft via the crank pulley revealed that only one bank of pistons would move.

The Critical Factor Often Overlooked

Following such a failure, a common path is to source a second-hand SDV6 engine. However, without a forensic examination of the failed engine, the underlying trigger for the crankshaft failure remains a mystery. The SDV6 crankshaft issue is often a symptom of a preceding problem, such as a seized ancillary component (like an A/C compressor or alternator) applying sudden, extreme torsional shock to the crank, or advanced wear in the crankshaft main bearings creating harmonic imbalances. Simply swapping the engine without inspecting and replacing these potential trigger components risks a repeat failure.

The Root Cause Explained

In this SDV6, the root cause was a fatigue fracture of the crankshaft, almost certainly originating at the journal fillet radius for cylinder #3 main bearing. This is a known stress concentration point in this specific engine. The failure is progressive: microscopic cracks form due to cyclic stress over high mileage, potentially exacerbated by engine harmonics or pre-existing bearing wear. Once the crack propagates to a critical size, the crankshaft snaps under normal driving load, instantly disconnecting one bank of pistons. Imagine a pencil (the crankshaft) being bent back and forth repeatedly at the same spot. A small crack forms, weakens the structure, and eventually, it snaps clean through with one final bend.

Our Repair Strategy: The Most Comprehensive Solution for Long-Term Reliability

For a confirmed snapped crankshaft, the options are limited and significant:

1. Second-Hand Engine Replacement: Installing a used engine.
   • Potential Outcome: Returns the vehicle to the road. The labour cost is similar to a rebuild.
   • Long-Term Consideration: The biggest gamble. Used SDV6 engines carry the same inherent design risk. Their service history and the reason for removal are usually unknown. There is no durability improvement over the original failed unit.
2. Fitting a Factory-New Land Rover Short Engine: Installing a brand-new, Land Rover-supplied partial engine assembly (block with crankshaft, pistons, and rods).
   • Potential Outcome: Excellent reliability with a factory warranty on the core assembly.
   • Long-Term Consideration: Extremely high parts cost. It also does not typically include critical ancillaries like turbochargers, injectors, or the high-pressure fuel pump, which may have been damaged by debris or require testing.
3. Complete Engine Rebuild with Uprated Components (Our Chosen Strategy): Stripping the original engine, salvaging undamaged components, and rebuilding it with a new, uprated crankshaft and bearings.
   • Why This Was the Most Comprehensive Choice: This strategy allowed us to conduct a full forensic examination, identify and replace the failed component and any other damaged or worn parts (e.g., turbocharger, oil pump). Crucially, we could install a new, uprated crankshaft from a specialist manufacturer that addresses the metallurgical weaknesses of the original, alongside uprated bearings. This transforms the engine's greatest weakness into its strongest asset.

Our Detailed Repair Procedure:

• Engine Removal & Forensic Strip: The engine was removed and fully disassembled. The crankshaft was found snapped cleanly between the 3rd and 4th main bearing journals. The block and cylinder heads were intact.
• Salvage, Cleaning, and Machining: The block and cylinder heads were sent for ultrasonic cleaning, crack testing, and deck inspection. The oil cooler was replaced as mandatory practice. We sourced and fitted:
  • A new, uprated forged steel crankshaft from a reputable specialist (e.g., Turners Engineering or Darton Sleeves).
  • A matched set of performance-grade main and big-end bearings.
  • New OEM piston rings and a new Land Rover OEM oil pump.
  • Both turbochargers were inspected and rebuilt due to potential debris ingestion.
  • All seals, gaskets, and the timing chain system were replaced.
• Precision Reassembly & Systems Check: The engine was rebuilt with lab-level cleanliness. All bearing clearances were verified. After installation, the fuel system was purged, and the engine management system was calibrated. A careful run-in procedure was followed.

Results & "Before/After" Proof

• Function: The engine started immediately and ran smoothly. The catastrophic knock was, of course, completely absent.
• Performance: Full power and torque were restored. The vehicle drove without fault.
• Oil & System Health: Post-repair oil analysis after 1,000 miles showed no contaminants, confirming a clean, healthy internal environment.
• Owner Confidence: The vehicle was returned with a 24-month warranty on the repair, underpinned by the use of uprated core components.

Summary:

• Core Problem: Fatigue failure resulting in a snapped crankshaft.
• Solution:Complete engine rebuilds utilising the original block and heads, a new uprated crankshaft, performance bearings, and rebuilt turbos.
• Outcome: An engine rebuilt to a specification that exceeds original factory durability in its critical weakness, restoring long-term reliability.