🚗 Classic Fault — Mercedes-Benz W115 (1968–1976)
Spark plug incorrect heat range causing carbon fouling
💰 Estimated Repair Cost
$15 – $30 USD
Parts and labour estimate
🔧 Tools Required
Plug socket, torque wrench
📋 Overview
The M115 requires a specific spark plug heat range. Plugs too cold foul quickly at idle; plugs too hot cause pre-ignition. Wrong plugs after a service cause misfiring or carbon fouling.
🩺 Symptoms
- Carbon fouling shortly after service
- pinking or detonation
- misfiring at idle or under load
🔍 Possible Causes
- Wrong heat range spark plugs fitted
- non-OEM equivalent with different heat range
🛠️ Diagnosis Steps
Remove plugs and inspect. Bosch W8D is correct. Check heat range designation on existing plugs.
✅ Most Confirmed Fix
Replace with correct heat range plugs. Torque to 25-30 Nm.
86% of users confirm this fix worked (30 reports)
🏷️ Parts Needed
Bosch W8D spark plugs (set of 4)
🔗 Other Mercedes-Benz W115 Faults
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