====== M104 Head Gasket Replacement ======

This article is a reference for replacing the head gasket on a Mercedes Benz M104 engine equipped with the HFM injection system. Certain details may be specific to M104 implementations in a W124 sedan chassis.

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^ Time | 10 hours |
^ Cost | {{icon>dollar?color=#008000}} {{icon>dollar?color=#008000}} {{icon>dollar?color=#000000}} {{icon>dollar?color=#000000}} {{icon>dollar?color=#000000}} |
^ Difficulty | {{icon>cog?color=#0000ff}} {{icon>cog?color=#0000ff}} {{icon>cog?color=#0000ff}} {{icon>cog?color=#0000ff}} {{icon>cog?color=#000000}} |
^ Parts Required |  |
^ Applicable models | All HFM injected M104 engines |
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</grid>

===== Preparation =====
  - Disconnect battery
  - Drain oil
  - Drain coolant
  - Lift and support front half of car to access exhaust system from underneath
  - Set engine position to TDC

===== Remove Front Cover, Belt, and Accessories =====
  - Remove false firewall
  - Remove wire harness holder
  - Remove valve cover
  - Remove and disconnect smog pump / EGR switchover vacuum valve
  - Remove fan clutch
  - Remove accessory belt
  - Remove smog pump
  - Remove alternator
  - Remove front lifting hook and coolant pipe
  - Unplug camshaft advance solenoid and camshaft position sensor
  - Remove heater pipe
    - Disconnect hose clamp from auxiliary water pump hard line and pull hose off
    - Remove 10mm screw from water pump
    - Remove 5mm allen screw from idler pulley / fan bracket
  - Remove belt idler pulley
  - Remove 6x 13mm screws for front top timing cover
  - Remove timing chain guide rail pin cover
    - 5mm allen screw cover located at bottom right of the upper timing cover
  - Use a slide hammer or a puller to remove the timing chain guide rail pin
  - Pry upper front timing cover off. Avoid damaging the mating surface with prying tools.
  - Remove chain tensioner
  - Remove camshaft chain sprocket

===== Remove Exhaust Manifold =====
  - Remove smog pump / alternator bracket from engine block
  - Remove EGR valve
    - Disconnect pipe at rear
    - Disconnect pipe at manifold
    - Remove fasteners at flange and remove
  - Disconnect both downpipes from exhaust manifolds underneath car
  - Remove downpipes support clamp at transmission
  - Remove copper self locking nuts securing exhaust manifolds to cylinder head
  - Remove exhaust manifold

===== Remove Intake Manifold =====
  - Disconnect fuel injectors wire plugs
  - Disconnect resonance flap wire plug
  - Bleed pressure from fuel rail using test port schrader valve
  - Disconnect fuel supply and fuel return hoses
  - Remove 5 screws holding fuel rail to intake manifold
  - Pull fuel rail off
  - Remove remaining screws on intake manifold
  - Loosen hose clamps connecting upper and lower manifolds
  - Pull air intake away from cylinder head slightly
  - Reach down between gap and disconnect knock sensor below cylinder 5
  - Remove air intake manifold

===== Remove Cylinder Head =====
  - Remove EGR support bracket
  - Loosen hose clamp on coolant hose at back of cylinder head and pull off hose
  - Remove cylinder head bolts, starting with bolt #14 on the outside working towards the center
  - Lift cylinder head off engine block. Cylinder head weight approximately 80 lb.


===== Installing Cylinder Head =====
  - Clean mating surfaces and check for flatness. 
    - 0.004" tolerance between front and rear of engine block / cylinder head mating surface
  - Clean cylinder head bolt holes
  - Set engine timing to 30 degrees BTDC.
    - Pistons are in position such that valves will not interfere when adjusted later
  - Install new head gasket
  - Install cylinder head
  - Torque cylinder head bolts
    - Lightly coat threads with engine oil for wet torque
    - Follow tightening stages, order, and torque specs
    - 1st stage: 55 Nm
    - 2nd stage: 90 degrees
    - 3rd stage: 90 degrees