Timing Chain

Complete timing chain replacement on the M275 V12 BiTurbo engine, including chain tensioners, guides, and cam phasers. This is a complex procedure requiring precise timing alignment and special tools.

Warnings

Tools required

Parts

• Timing chain kit (complete with chains, guides, tensioners) × 1 — Use OEM Mercedes-Benz specification
• Timing cover gasket set × 1 — Use OEM specification
• Valve cover gaskets (left and right) × 2 — Use OEM specification
• Valve cover spark plug tube seals × 12 — Use OEM specification
• Camshaft position sensors O-rings × 4 — Use OEM specification
• Crankshaft position sensor O-ring × 1 — Use OEM specification
• Timing cover oil seals × 1 — Use OEM specification
• Crankshaft front seal × 1 — Use OEM specification
• Oil filter × 1 — Use OEM specification
• Oil drain plug crush washer × 1 — Use OEM specification
• Coolant × 1 — Mercedes-Benz Coolant (Blue)
• Engine oil × 1 — Mercedes-Benz 229.5 5W-40
• VVT solenoid O-rings × 4 — Use OEM specification

Fluids

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

Preparation

1. Ensure vehicle is on level ground with parking brake applied and rear wheels chocked
2. Disconnect negative battery terminal and wait 10 minutes for system capacitors to discharge
3. Remove engine cover and decorative trim pieces
4. Drain engine coolant completely into appropriate container by opening radiator drain valve and lower hose
5. Drain engine oil completely and remove oil filter
6. Remove front bumper cover and radiator support for access to front of engine
7. Remove cooling fan assembly and fan shroud
8. Remove drive belts (serpentine and accessory)
9. Support engine from above using engine support fixture - transmission must remain supported
10. Remove engine mount bolts and raise engine slightly for timing cover access
11. Label and photograph all electrical connections, vacuum lines, and hose routing before disconnection
12. Clean work area thoroughly - any debris entering engine will cause damage

Procedure

1. 1

   Remove intake system and valve covers
   Remove air filter housing, intake manifold, and throttle bodies for access. Disconnect ignition coils from all 12 spark plug wells. Remove valve cover bolts in reverse sequence (consult service manual for specific pattern). Carefully lift valve covers from cylinder heads, inspecting gasket condition. Clean all gasket surfaces thoroughly with solvent and plastic scraper - do not gouge aluminum surfaces.
2. 2

   Remove crankshaft pulley and harmonic balancer
   Remove crankshaft pulley center bolt using impact wrench or breaker bar while holding crankshaft from rotating (can use starter motor engagement or flywheel locking tool). This bolt requires significant torque to remove. Use appropriate puller to remove harmonic balancer from crankshaft snout - do not pry or hammer. Inspect balancer for rubber deterioration or separation.
   ⚠Crankshaft pulley bolt may be torque-to-yield (TTY). Inspect service manual to determine if replacement is required.
3. 3

   Remove timing covers
   Remove all timing cover bolts noting different lengths and locations - there are 6mm, 8mm, and 10mm bolts in specific positions. Carefully separate timing covers from engine block using plastic wedge - do not pry on sealing surfaces. Remove covers completely exposing timing chain assembly. Inspect cover oil seals and replace if worn. Clean all gasket surfaces on covers and engine block.
   Torque spec

   Timing Cover Bolts12 Nm (9 lb-ft)
4. 4

   Rotate engine to TDC and install locking tools
   Slowly rotate crankshaft clockwise (viewed from front) using socket on crankshaft bolt until cylinder 1 is at TDC on compression stroke. Verify timing marks on camshaft sprockets align with cylinder head marks. Install crankshaft TDC locking pin through block into crankshaft. Install camshaft alignment bars into slots on rear of camshafts for both cylinder banks - bars should slide in freely when properly timed. If alignment bars do not fit, timing is incorrect - recheck position.
   ⚠️Never force camshaft alignment bars. If they do not fit easily, camshaft timing is incorrect. Forcing tools will damage camshafts.
5. 5

   Remove VVT solenoids and cam phasers
   Disconnect electrical connectors from all four VVT (variable valve timing) solenoids. Remove VVT solenoid mounting bolts and withdraw solenoids from cylinder heads. Remove cam phaser bolts while holding camshafts stationary using hex section - do not allow camshafts to rotate. Mark phaser orientation before removal. Remove cam phasers from camshafts, noting dowel pin positions.
   ⚠VVT solenoids contain small screens that can become clogged. Inspect and clean or replace as needed.
6. 6

   Remove camshaft sprockets
   Using camshaft holding tool on hex section to prevent rotation, remove camshaft sprocket bolts from all four camshafts. Carefully slide sprockets forward off camshafts while supporting timing chain - do not allow chain to fall into engine. Suspend chains using wire or zip ties to prevent dropping into crankcase.
   Torque spec

   Camshaft Sprocket Bolts80 Nm (59 lb-ft)
7. 7

   Release tensioners and remove timing chains
   Using tensioner compression tool, carefully compress each timing chain tensioner and lock in compressed position using retaining pin or clip. Remove tensioner mounting bolts and withdraw tensioners from engine. Remove timing chain guide bolts and guides. Carefully remove timing chains from crankshaft sprockets and idler sprockets, noting routing path. Remove idler sprockets if worn or damaged.
   ⚠Timing chain tensioners are under significant spring pressure. Compress gradually and maintain control to prevent sudden release.
   Torque spec

   Timing Chain Tensioner25 Nm (18 lb-ft)

   Timing Chain Guide Bolts12 Nm (9 lb-ft)
8. 8

   Inspect components and prepare for installation
   Thoroughly inspect all timing chain guides for wear grooves, cracks, or material loss. Inspect idler sprockets for wear or rough rotation. Check crankshaft and camshaft sprockets for wear patterns. Inspect tensioners for proper operation - they should extend smoothly when pin is removed. Clean all timing chain contact surfaces. Verify new timing chain kit includes all necessary components. Compare old and new chains for correct length and link count.
   ℹ️M275 engines are known for timing chain guide wear. Replace all guides even if some appear acceptable to prevent premature failure.
9. 9

   Install new timing chain guides and idler sprockets
   Install new timing chain guides in their respective positions, ensuring proper seating in locating dowels. Install guide bolts hand-tight initially. Install idler sprockets on their mounting posts. Torque all timing chain guide bolts and idler sprocket bolts to specification. Verify guides are secure and properly positioned before chain installation.
   Torque spec

   Timing Chain Guide Bolts12 Nm (9 lb-ft)
10. 10

    Install new timing chains
    Verify crankshaft locking pin is still installed and camshaft alignment bars are in place. Route new timing chains around crankshaft sprockets and idler sprockets following exact path of original chains. Ensure chains engage all sprocket teeth properly. Bring chain ends up to camshaft sprocket positions. Verify colored links on new chains (if present) align with timing marks per service manual specifications.
    ⚠️Incorrect chain routing will cause immediate engine failure. Verify routing matches service manual diagrams exactly.
11. 11

    Install camshaft sprockets
    Position camshaft sprockets onto camshafts ensuring dowel pins engage properly. Align timing marks on sprockets with marks on cylinder heads while engaging timing chain. Install camshaft sprocket bolts hand-tight. Using camshaft holding tool on hex section to prevent rotation, torque sprocket bolts to specification. Verify all timing marks remain aligned after torquing.
    ⚠Do not allow camshafts to rotate while tightening sprocket bolts. Use holding tool on camshaft hex section.
    Torque spec

    Camshaft Sprocket Bolts80 Nm (59 lb-ft)
12. 12

    Install timing chain tensioners
    Compress new timing chain tensioners fully and lock with retaining pins or clips. Install compressed tensioners into engine and torque mounting bolts to specification. Carefully remove retaining pins/clips to allow tensioners to apply pressure to chains. Verify tensioners extend properly and chains are under correct tension - chains should have minimal slack but not be over-tight.
    ⚠Remove tensioner pins slowly and verify tensioner extends smoothly. If tensioner does not extend, do not proceed - replace tensioner.
    Torque spec

    Timing Chain Tensioner25 Nm (18 lb-ft)
13. 13

    Install cam phasers
    Align cam phasers with dowel pins on camshaft ends and slide into position on camshafts. Ensure phasers are fully seated. Apply threadlocker to cam phaser bolts as specified. Hold camshafts stationary using hex section and install phaser bolts. Torque cam phaser bolts to specification while preventing camshaft rotation.
    ℹ️Cam phaser orientation is critical. Verify phaser alignment marks match service manual before tightening bolts.
14. 14

    Verify timing alignment and remove locking tools
    With all components installed and torqued, verify all timing marks are correctly aligned. Remove camshaft alignment bars - they should slide out freely. Remove crankshaft TDC locking pin. Manually rotate crankshaft two complete revolutions clockwise using socket on crankshaft bolt. Reinstall locking pin and alignment bars - if they do not fit, timing is incorrect and must be corrected before proceeding.
    ⚠️This is final verification before closing engine. If timing tools do not reinstall easily after rotation, DO NOT proceed. Timing is incorrect and engine will be damaged.
15. 15

    Install VVT solenoids and timing covers
    Install new O-rings on VVT solenoids lubricated with clean engine oil. Install solenoids into cylinder heads and torque bolts to specification. Install new crankshaft front seal into timing cover if removed. Apply thin bead of RTV silicone to timing cover mating surfaces as specified in service manual. Install timing covers ensuring proper gasket alignment. Install timing cover bolts in correct locations (6mm, 8mm, and 10mm) and torque in sequence from center outward to specifications.
    ℹ️Allow RTV silicone to cure per manufacturer specifications before adding fluids if specified in service manual.
    Torque spec

    Timing Cover Bolts12 Nm (9 lb-ft)
16. 16

    Install crankshaft pulley and complete reassembly
    Install harmonic balancer onto crankshaft ensuring proper seating on keyway. Install crankshaft pulley bolt and torque to specification - verify if bolt is torque-to-yield and requires replacement per service manual. Install new valve cover gaskets and spark plug tube seals. Install valve covers and torque bolts in sequence from center outward. Reinstall all remaining components in reverse order of removal including intake system, engine mounts, cooling system, and drive belts.
17. 17

    Refill fluids and perform initial startup
    Install new oil filter and install oil drain plug with new crush washer, torquing to specification. Fill engine with correct quantity and specification of engine oil (8.5 quarts Mercedes-Benz 229.5 5W-40). Refill cooling system with Mercedes-Benz Blue coolant (18 quarts total capacity). Reconnect battery negative terminal. Without starting engine, crank engine for 10 seconds to build oil pressure. Start engine and verify oil pressure immediately. Allow engine to idle while monitoring for leaks, unusual noises, or warning lights. Check for timing cover, valve cover, and oil system leaks.
    ⚠️If any unusual noises occur during startup, immediately shut down engine. Knocking or rattling may indicate timing error or component failure.
    ⚠Monitor oil pressure closely during initial startup. Low pressure may indicate oil pump issue or insufficient priming.

Reassembly

1. Torque all fasteners to exact specifications using calibrated torque wrench
2. Follow specified tightening sequences for all multi-bolt components (timing covers, valve covers, bearing caps)
3. Replace all one-time-use fasteners including TTY bolts if specified
4. Use new gaskets and seals throughout - never reuse timing cover or valve cover gaskets
5. Apply threadlocker to cam phaser bolts as specified
6. Ensure all electrical connectors are fully seated with locking tabs engaged
7. Verify all timing marks align correctly before final assembly
8. Double-check that no tools or parts remain in engine before closing covers

Verification

• Perform crankshaft rotation test with timing locking tools - tools must reinstall easily after two full rotations
• Start engine and verify immediate oil pressure indication on instrument cluster
• Listen for any unusual timing chain noise, rattling, or knocking during idle and light acceleration
• Verify no coolant, oil, or vacuum leaks from timing covers, valve covers, or related components
• Check engine codes using Mercedes-Benz Star Diagnostic system - no timing-related codes should be present
• Verify VVT system operation using diagnostic scanner - cam timing should adjust properly at various RPMs
• Perform test drive monitoring engine performance, smooth acceleration, and proper power delivery
• Recheck all fluid levels after test drive and top off as necessary
• Inspect for leaks again after engine reaches full operating temperature
• Consider performing compression test and leak-down test to verify valve timing is correct if any concerns exist

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