🚗 Classic Fault — Mercedes-Benz W115 (1968–1976)
Vacuum advance unit diaphragm failure
💰 Estimated Repair Cost
$20 – $55 USD
Parts and labour estimate
🔧 Tools Required
Screwdrivers, vacuum pump
📋 Overview
The vacuum advance unit diaphragm fails meaning timing does not advance at cruise, causing flat economy and performance in the mid-range.
🩺 Symptoms
- Poor fuel economy
- flat mid-range performance
- engine feels lazy at cruise
🔍 Possible Causes
- Failed vacuum advance diaphragm
- blocked or kinked vacuum pipe
🛠️ Diagnosis Steps
Disconnect vacuum hose from advance unit. Apply suction by mouth — should hold. Bleeds back immediately if diaphragm failed.
✅ Most Confirmed Fix
Replace vacuum advance unit.
79% of users confirm this fix worked (21 reports)
🏷️ Parts Needed
Bosch vacuum advance unit for W115 distributor
🔗 Other Mercedes-Benz W115 Faults
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