1996-1997 BMW M3 Crankshaft: A Guide to the S52's Strong Core and Its Tricky Sensor

Is It the Crankshaft or the Crankshaft Sensor?

Before diagnosing a major engine problem, it's critical to understand the difference between two parts that are often confused: the crankshaft itself and the crankshaft position sensor (CPS). For the 1996-1997 M3, it is far more likely that the sensor has failed, not the crankshaft.

• The Crankshaft: A massive, durable, rotating metal component inside the engine. Failures are extremely rare and catastrophic, usually caused by other major problems like oil starvation or extreme abuse.
• The Crankshaft Position Sensor (CPS): A small, electronic sensor that reads the crankshaft's speed and position. It sends this vital information to the engine computer (DME) to control fuel injection and ignition timing. These sensors are a common failure point and were subject to a recall.

Symptoms of Failure

Because sensor failures are much more common, we have separated the symptoms for each part.

Symptoms of a Bad Crankshaft (The Part Itself)

A failing crankshaft is a sign of a catastrophic engine problem. Symptoms are severe and should not be ignored.

• Loud, Rhythmic Knocking or Rumbling Noise: A deep knock from the bottom of the engine that increases with RPM is a classic sign of failed rod or main bearings, which will quickly destroy the crankshaft journals.
• Severe Engine Vibration: If the crankshaft is bent or has lost a counterweight, the entire engine will vibrate violently.
• Metal Shavings in Engine Oil: When you drain the oil, it may look like metallic paint or contain visible flakes of metal. This indicates the bearings and crankshaft journals are grinding themselves apart.
• Engine Seizure: In the worst-case scenario, a failed bearing will cause the crankshaft to lock up, stopping the engine instantly and causing extensive internal damage.

Symptoms of a Bad Crankshaft Position Sensor (The Common Problem)

If your M3 is experiencing these issues, the crankshaft position sensor is the most likely culprit.

• Engine Cranks but Won't Start: The engine turns over but never fires up. The DME doesn't see the crank spinning, so it won't trigger spark or fuel.
• Car Stalls Suddenly: The engine may cut out while driving for no apparent reason, as if the key were turned off.
• Check Engine Light: A faulty sensor will trigger the Check Engine Light with codes like P0335 (Crankshaft Position Sensor 'A' Circuit Malfunction).
• Rough Idle and Misfires: The engine may run poorly, shake, or misfire because the DME has incorrect timing information.
• Tachometer Doesn't Move During Cranking: The tachometer gets its signal from the CPS, so if it reads zero while the engine is turning over, it's a strong clue the sensor is bad.

Known Issues, TSBs, and Recalls

The S52 crankshaft itself is known to be very robust and reliable. There are no widespread known issues, TSBs, or recalls for the actual crankshaft. It is a forged steel unit designed to handle the demands of a performance engine. Most failures are not due to a defect in the part, but are a consequence of other issues like lubrication failure or improper engine assembly.

Buying a Used S52 Crankshaft: A Step-by-Step Inspection Guide

Since new OEM crankshafts are expensive or may no longer be available, a used OEM unit is a common choice for engine rebuilds. A thorough inspection is not just recommended; it is mandatory to avoid building a new engine around a bad part. Here’s what to check:

1. Visual Inspection (On the Bench)

• Check the Journals: These are the polished surfaces where the bearings ride. Run your fingernail across every main and rod journal. You should not be able to feel any grooves or scoring.
• Look for Discoloration: Blue or dark brown discoloration on or around the journals is a major red flag. It indicates the crank has been severely overheated, which can alter the hardness of the steel and lead to future failure.
• Inspect Keyways and Threads: Check the keyway at the front of the crank (for the main pulley/harmonic balancer) for any signs of damage or wear. Verify all threaded holes, especially the large center bolt hole and the flywheel mounting holes, are clean and undamaged.
• Look for Welds: Never buy a crankshaft that has been welded. This is a sign of a previous catastrophic failure and repair that is not safe for use.

2. Professional Machine Shop Inspection (Crucial Step)

A visual inspection is not enough. A used crankshaft MUST be taken to a reputable automotive machine shop for professional analysis before installation.

• Crack Testing (Magnafluxing): The shop will use a magnetic particle process to check for microscopic cracks that are invisible to the naked eye. A cracked crank is junk.
• Check for Straightness (Runout): The crank will be mounted on V-blocks and measured with a dial indicator to ensure it is perfectly straight. A bent crank will destroy bearings.
• Measure Journals: The shop will use a micrometer to measure every journal to ensure they are within factory specifications and are not out-of-round. If they are lightly worn, they can often be polished. If wear is more significant, the crank may need to be ground to an oversize bearing specification.
• Balancing: The rotating assembly (crank, rods, pistons) should be balanced as a unit. This is especially important if you are using any different components from the original engine.

Cost Comparison for a 1996-1997 M3 Crankshaft

Installation and Replacement

Replacing a crankshaft is one of the most complex jobs on any vehicle. It requires a complete engine disassembly, often referred to as a 'nut and bolt' rebuild.

• Difficulty: 5 out of 5 (Expert Level)
• Process: The engine must be removed from the car. It is then mounted on an engine stand and completely disassembled. The crankshaft is the last major component to be removed from the block.
• DIY Feasibility: This is not a feasible DIY job for anyone without extensive engine building experience and a full range of specialized tools, including an engine hoist, engine stand, torque wrenches, and precision measuring instruments.

Frequently Asked Questions (FAQ)