Timing Chain Guide

Replace worn timing chain guides on the M275 V12 BiTurbo engine to prevent catastrophic engine damage from timing chain failure.

Warnings

Tools required

Parts

• Timing chain guide kit (both banks) × 1 — Use OEM specification
• Timing chain tensioner set × 2 — Use OEM specification
• Valve cover gasket set × 2 — Use OEM specification
• Timing cover gasket × 1 — Use OEM specification
• Crankshaft seal × 1 — Use OEM specification
• Oil filter × 1 — Use OEM specification
• Coolant expansion tank cap × 1 — Inspect and replace if needed

Fluids

• Mercedes-Benz 229.5 5W-40 — 8.5 qt
• Mercedes-Benz Coolant (Blue) — 18 qt

Preparation

1. Park vehicle on level surface and allow engine to cool completely (minimum 3 hours).
2. Disconnect negative battery terminal and wait 10 minutes for system discharge.
3. Remove engine cover and intake plenum assembly.
4. Drain engine oil completely and remove oil filter.
5. Drain coolant from radiator and engine block drain plugs.
6. Remove serpentine belt and all accessory drive components.
7. Support engine with overhead hoist or engine support fixture.
8. Remove front engine mount to allow engine tilt for access.
9. Label and photograph all electrical connectors before disconnection.
10. Clean exterior of timing cover area thoroughly to prevent contamination.

Procedure

1. 1

   Remove valve covers (both banks)
   Disconnect ignition coils and fuel injector harnesses from both banks. Remove valve cover bolts in reverse of tightening sequence (outside to inside pattern). Carefully lift valve covers away, noting location of any alignment dowels. Inspect valve train components for wear while exposed.
   Torque spec

   Valve Cover Bolts10 Nm (7 lb-ft)
2. 2

   Rotate engine to TDC cylinder 1
   Rotate crankshaft clockwise using crankshaft pulley bolt until cylinder 1 is at Top Dead Center on compression stroke. Verify timing marks on both camshaft sprockets align with marks on cylinder heads. Install camshaft locking tools on both banks to prevent rotation.
   ⚠Both intake and exhaust camshaft marks must align simultaneously. If marks do not align, timing chain has already jumped and further inspection is critical.
   Torque spec

   Crankshaft Pulley Bolt180 Nm (133 lb-ft)
3. 3

   Remove timing cover
   Remove all timing cover bolts, noting different lengths and locations (8mm bolts vs standard bolts). Carefully separate timing cover from engine block using plastic pry tools only at designated pry points. Do not damage sealing surfaces. Remove crankshaft seal from timing cover.
   ⚠Some timing cover bolts thread into coolant passages. Keep track of bolt locations to prevent coolant leaks on reassembly.
   Torque spec

   Timing Cover Bolts18 Nm (14 lb-ft)
4. 4

   Inspect timing chain system
   With timing cover removed, thoroughly inspect timing chains, sprockets, tensioners, and guides on both banks. Check for excessive chain slack (more than 1/2 inch deflection), worn guide surfaces (grooves deeper than 2mm), or metal fragments in oil passages. Measure timing chain wear using manufacturer specifications. Document findings with photos.
   ⚠️If metal fragments or excessive wear debris is present, complete oil system flush and internal engine inspection is required before proceeding.
5. 5

   Release timing chain tension
   While maintaining camshaft locks, carefully release timing chain tensioners on both banks by removing tensioner bolts. Remove tensioner assemblies and set aside. Verify chains are now slack but still engaged on all sprockets. Do not allow chains to fall off sprockets.
   ⚠️Keep camshaft locking tools installed at all times. Camshafts will rotate under valve spring pressure if unlocked, causing valve damage.
   Torque spec

   Tensioner Bolt27 Nm (20 lb-ft)
6. 6

   Remove timing chain guides
   Remove timing chain guide bolts from both fixed guides and pivoting guides on each bank. Note orientation and position of each guide (upper, lower, left bank, right bank). Carefully extract worn guides, watching for broken pieces that may have fallen into oil pan. Clean all guide mounting surfaces thoroughly.
   ℹ️Guides are specific to each position and bank. Label each guide as removed to ensure correct replacement guide selection.
   Torque spec

   Guide Bolts33 Nm (24 lb-ft)
7. 7

   Install new timing chain guides
   Install new timing chain guides in exact positions as removed, ensuring correct left/right bank orientation. Thread guide bolts finger-tight initially. Verify guides do not interfere with chain routing. Torque timing chain guide bolts first, then larger guide bolts to specification in sequence from top to bottom.
   ⚠Do not overtighten guide bolts as aluminum threads in engine block strip easily. Use calibrated torque wrench for all fasteners.
   Torque spec

   Guide Bolts33 Nm (24 lb-ft)
8. 8

   Install new timing chain tensioners
   Pre-load new timing chain tensioners according to Mercedes-Benz procedure using tensioning tool. Install tensioners on both banks while maintaining pre-load. Thread tensioner bolts and torque to specification. Verify chains have proper tension (slight slack but no excessive droop between sprockets).
   ⚠Tensioners must be pre-loaded before installation. Installing without pre-load will result in immediate chain slack and potential engine damage.
   Torque spec

   Tensioner Bolt27 Nm (20 lb-ft)
9. 9

   Verify timing alignment
   With camshaft locks still installed, verify all timing marks remain aligned. Check crankshaft position at TDC. Remove one camshaft locking tool at a time and verify camshaft does not rotate. Reinstall lock and repeat for all camshafts. Manually rotate engine two full revolutions clockwise and recheck all timing marks align at TDC.
   ⚠️Do not start engine if timing marks do not align perfectly after rotation. Double-check all installation steps before proceeding.
10. 10

    Install new timing cover seal and gasket
    Clean all timing cover mating surfaces on engine block with brake cleaner. Install new crankshaft seal into timing cover using seal driver tool. Apply thin bead of Mercedes-Benz approved RTV silicone to timing cover gasket areas as specified in service manual. Position new timing cover gasket.
    ℹ️Allow RTV silicone to set for 10 minutes before installing timing cover. Do not exceed 15-minute window or RTV will skin over.
11. 11

    Install timing cover
    Remove camshaft locking tools. Carefully position timing cover onto engine block, ensuring gasket remains properly aligned. Install all timing cover bolts finger-tight, being careful to place correct length bolts in proper locations. Torque 8mm bolts first in specified sequence, then torque standard bolts in sequence.
    ⚠Follow specified tightening sequence to prevent timing cover warping and ensure proper seal. Sequence typically starts at center and works outward.
    Torque spec

    Timing Cover Bolts18 Nm (14 lb-ft)
12. 12

    Reinstall valve covers and accessories
    Install new valve cover gaskets on both banks. Position valve covers and install bolts, torquing in sequence from center outward. Reinstall front engine mount and remove engine support. Reinstall all accessory drive components, serpentine belt, intake plenum, and engine covers. Reconnect all electrical connectors per documentation.
    Torque spec

    Valve Cover Bolts10 Nm (7 lb-ft)
13. 13

    Refill fluids and initial start
    Install new oil filter and refill engine with 8.5 quarts Mercedes-Benz 229.5 5W-40 oil. Refill cooling system with proper coolant mixture. Reconnect battery negative terminal. Perform initial engine start and monitor for oil pressure within 3 seconds. Listen for any abnormal timing chain noise. Allow engine to reach operating temperature while monitoring for leaks.
    ⚠️If oil pressure does not register within 3 seconds or any metallic rattling is heard, immediately shut down engine and reinspect timing chain installation.

Reassembly

1. Torque all fasteners to exact specifications using calibrated torque wrench in specified sequences.
2. Use only new gaskets and seals; do not reuse old components.
3. Apply RTV silicone only where specified; excess sealant can block oil passages.
4. Route all wiring harnesses and hoses in original positions to prevent chafing.
5. Verify serpentine belt alignment on all pulleys before tensioning.

Verification

• Check engine oil level on dipstick after initial run; top off if needed to full mark.
• Verify no coolant, oil, or vacuum leaks around timing cover or valve covers with engine at operating temperature.
• Monitor oil pressure gauge shows normal pressure (40+ psi at idle when warm).
• Listen for smooth, quiet engine operation with no timing chain rattle or ticking noises.
• Perform test drive and verify normal engine performance with no misfires or unusual vibrations.
• Scan for diagnostic trouble codes; clear codes if only related to battery disconnection.
• Recheck all fluid levels after test drive; inspect under vehicle for any leaks.
• Schedule follow-up oil change at 500 miles to remove any break-in debris.

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