# Glow Plugs & Glow Plug Control Module Replacement ## Overview Failing glow plugs are the leading cause of hard cold starts, white smoke on startup, and rough cold-engine running on the OM642 V6 diesel. Plugs and the glow plug control module (GPCM) wear together: when one plug starts to draw too much current, the GPCM is stressed; when the GPCM begins commanding incorrect voltage, it accelerates plug wear. **Always replace all 6 plugs AND the GPCM as a single set.** Never replace individual glow plugs. Mismatched plug ages will cause uneven heating, repeat misfires, and premature GPCM failure. ## Tools Required - 10mm deep socket - Torque wrench (calibrated for 15 Nm range) - Penetrating oil (PB Blaster or equivalent) - **apply 24 hours before** on high-mileage engines - Glow plug extraction kit (in case of breakage) - Multimeter (to test plugs before removal) - OBD2 scanner with Mercedes coverage (to clear glow plug DTCs) - Compressed air - 6 new glow plugs - Bosch 0 250 403 008 (OEM equivalent) - 1 new glow plug control module (GPCM) ## Procedure ### Step 1: Allow engine to fully cool Never work on glow plugs on a warm engine. Allow at least 2 hours of cool-down time. Hot aluminum heads make plug removal much more difficult and dramatically increase the risk of breakage. ### Step 2: Apply penetrating oil Spray penetrating oil around the base of all 6 glow plugs. Allow to soak **a minimum of 24 hours** on engines over 80,000 miles. This step is critical. Snapped glow plugs require extraction kits or professional repair (often a head removal). Skipping the soak step on a high-mileage engine is the #1 cause of broken plugs. ### Step 3: Disconnect the GPCM harness Locate the glow plug control module on the engine. Disconnect its wiring harness connector. Remove the GPCM mounting bolts and set the module aside. ### Step 4: Remove glow plugs one at a time Using a 10mm deep socket, carefully remove each glow plug with slow, steady torque. **Do not use impact tools.** If you feel excessive resistance, stop, apply more penetrating oil, and wait. Forcing a stuck plug risks snapping it off in the head - a very expensive repair. Apply slow, steady pressure only. ### Step 5: Inspect threads Inspect each glow plug bore for debris, carbon buildup, or thread damage. Clean threads with compressed air before installing new plugs. ### Step 6: Install new glow plugs Thread each new glow plug in **by hand first** to avoid cross-threading. Torque to exactly **11 ft-lb (15 Nm)** with a torque wrench. Do not overtighten. Glow plugs in aluminum heads are easy to strip. Hand-thread first, then use the torque wrench only - never an impact or extension cheat bar. ### Step 7: Install new GPCM and reconnect harness Mount the new glow plug control module, torque mounting bolts to spec, and reconnect the harness connector until it clicks. ### Step 8: Clear DTCs and test Use an OBD2 scanner to clear any stored glow plug fault codes. Start the engine cold and verify smooth startup. Monitor for white smoke on startup - it should be gone after the new plugs are seated. If white smoke persists after glow plug replacement, check the GPCM output voltage with a multimeter to confirm the new module is commanding the plugs correctly. Each plug should see ~11V briefly during preheat. ## Common Mistakes - Skipping the 24-hour penetrating oil soak - Working on a warm engine - Replacing only one plug instead of the full set - Reusing the old GPCM with new plugs - Using an impact gun on stuck plugs - Overtightening (stripping aluminum threads)