🚗 Classic Fault — Mercedes-Benz W115 (1968–1976)
Engine knock at low RPM — pinking or detonation
💰 Estimated Repair Cost
$5 – $50 USD
Parts and labour estimate
🔧 Tools Required
Timing light, feeler gauge
📋 Overview
Metallic pinking or knocking sound when accelerating from low RPM, particularly under load. On the M115 engine this is most commonly ignition timing too advanced or fuel octane too low.
🩺 Symptoms
- Metallic pinking sound under light acceleration
- worse when engine hot
- most noticeable between 1500-2500 rpm under load
🔍 Possible Causes
- Ignition timing too advanced
- fuel octane too low (W115 requires 91 RON minimum, 95 recommended)
- carbon deposits in combustion chamber
- incorrect heat range spark plugs
🛠️ Diagnosis Steps
- Check ignition timing — retard 2-3 degrees if pinking.
- Ensure 95 RON fuel being used.
- Check spark plug heat range — Bosch W8D or equivalent.
✅ Most Confirmed Fix
Retard ignition timing 2 degrees. Switch to 95 RON fuel. Replace spark plugs with correct heat range.
70% of users confirm this fix worked (10 reports)
🏷️ Parts Needed
Bosch W8D spark plugs
🩺 Related Symptoms
🔗 Other Mercedes-Benz W115 Faults
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