1995-1997 BMW 750i Crankshaft: Why the Sensor Fails, Not the Steel

Known Issues: The Crankshaft vs. The Crankshaft Sensor

The forged steel crankshaft in the 1995-1997 BMW 750i's M73 V12 engine is exceptionally durable. Catastrophic failure of the crankshaft itself is virtually unheard of. Most issues that owners suspect are crankshaft-related are actually caused by a much simpler and more common failure: the crankshaft position sensor (CKP).

The M73 engine uses two crankshaft position sensors, one for each bank of six cylinders, which are managed by separate engine computers (DMEs). A failure of one of these sensors will prevent the engine from starting or cause it to stall. Internal BMW field reports from the late 90s noted a higher rate of failure for these sensors on pre-1998 models, often due to heat exposure in the tight engine bay.

Other problems more common than crankshaft failure on the E38 750iL include:

• Oil Leaks: The upper timing covers and lower oil pan gasket are common leak points.
• Cooling System Failures: Water pumps can fail, and the plastic components of the radiator and expansion tank become brittle with age.
• Electronic Faults: Intermittent no-start conditions can be caused by a failing ignition switch or issues with the EWS anti-theft system. Problems with the electronic throttle bodies (EML) can also cause driveability issues.

Symptoms of Failure

Because a true crankshaft failure is so rare, this section focuses primarily on the symptoms of a bad crankshaft position sensor, which are often misdiagnosed as a major engine problem.

Common Symptoms (Likely Crankshaft Position Sensor)

• Engine Cranks but Won't Start: This is the most common symptom. The starter will turn the engine over, but it will never fire up. The engine computer (DME) requires a signal from the CKP sensor to trigger the fuel pump and injectors; without it, it cuts fuel as a safety measure.
• Engine Stalls While Driving: The engine may suddenly shut off and refuse to restart. This can be intermittent.
• Rough Idle or Misfiring: While less common, a failing sensor can send an erratic signal, causing poor engine performance.
• Check Engine Light: A failing CKP sensor will almost always trigger a Check Engine Light and store fault codes like P0335 or P0336.

Rare Symptoms (Catastrophic Crankshaft or Bearing Failure)

• Loud, Rhythmic Knocking from Lower Engine: A deep knocking sound that increases with engine RPM can indicate a spun connecting rod bearing.
• Engine Seizure: In a complete failure, the engine will lock up and will not turn over at all.
• Metal Shavings in Engine Oil: Finding significant metallic debris during an oil change is a sign of severe internal engine damage.

Buying a Used Crankshaft: What to Check

If you are rebuilding an M73 engine and need a replacement crankshaft, a used OEM unit is an excellent and cost-effective choice due to their durability. Cheap aftermarket crankshafts for this engine do not exist, and for good reason—the precision and material quality required are extremely high. A used original part is far superior to a non-existent cheap alternative.

When inspecting a used M73 crankshaft, here is what to look for:

1. Verify the Part Number: Ensure the crankshaft is from a 5.4L M73 engine (part numbers 11211704831 or 11211704832). It is not interchangeable with the crankshaft from the earlier 5.0L M70 V12.
2. Inspect the Journals: Run your fingernail across all main and rod bearing journals. They should be perfectly smooth. Any ridges, deep scratches, or discoloration (blueing from heat) are major red flags indicating oil starvation or a spun bearing.
3. Check for Cracks: Carefully inspect the entire surface, especially around the journal fillets (the curved transition from the journal to the counterweight) and keyways, for any signs of cracking.
4. Examine Threads and Keyways: Ensure the large thread on the front snout (for the main pulley bolt) and the threads for the flywheel are clean and undamaged. The keyway for the crank sprocket should be sharp and not worn.
5. Confirm Straightness: While a visual check is difficult, ask the seller if the engine it came from was known to be in good running condition. A crankshaft from an engine that suffered a major impact or catastrophic failure could be bent. Professional machine shop testing is the only way to be 100% certain.

Replacement Difficulty & Labor

• Difficulty Rating: 5 out of 5 (Professional Only)
• Estimated Time: 30-40+ hours

Replacing the crankshaft is one of the most intensive automotive repairs possible. It is not a DIY job. The procedure requires the complete removal of the engine and transmission from the vehicle. The engine must then be mounted to an engine stand and fully disassembled—including the cylinder heads, timing components, oil pans, and all 12 pistons and connecting rods—just to access the crankshaft. Reassembly requires specialized tools for measuring bearing clearances and torquing bolts to specific angles. The crankshaft itself weighs approximately 55 lbs (25 kg) and requires careful handling.

Cost Breakdown for Crankshaft Replacement

Because this repair involves a full engine rebuild, the costs are significant. In many cases, replacing the entire engine with a tested, used unit is more economical.

Frequently Asked Questions (FAQ)