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.

1. Disconnect battery
2. Drain oil
3. Drain coolant
4. Lift and support front half of car to access exhaust system from underneath
5. Set engine position to TDC

1. Remove false firewall
2. Remove wire harness holder
3. Remove valve cover
4. Remove and disconnect smog pump / EGR switchover vacuum valve
5. Remove fan clutch
6. Remove accessory belt
7. Remove smog pump
8. Remove alternator
9. Remove front lifting hook and coolant pipe
10. Unplug camshaft advance solenoid and camshaft position sensor
11. Remove heater pipe
    1. Disconnect hose clamp from auxiliary water pump hard line and pull hose off
    2. Remove 10mm screw from water pump
    3. Remove 5mm allen screw from idler pulley / fan bracket
12. Remove belt idler pulley
13. Remove 6x 13mm screws for front top timing cover
14. Remove timing chain guide rail pin cover
    1. 5mm allen screw cover located at bottom right of the upper timing cover
15. Use a slide hammer or a puller to remove the timing chain guide rail pin
16. Pry upper front timing cover off. Avoid damaging the mating surface with prying tools.
17. Remove chain tensioner
18. Remove camshaft chain sprocket

1. Remove smog pump / alternator bracket from engine block
2. Remove EGR valve
   1. Disconnect pipe at rear
   2. Disconnect pipe at manifold
   3. Remove fasteners at flange and remove
3. Disconnect both downpipes from exhaust manifolds underneath car
4. Remove downpipes support clamp at transmission
5. Remove copper self locking nuts securing exhaust manifolds to cylinder head
6. Remove exhaust manifold

1. Disconnect fuel injectors wire plugs
2. Disconnect resonance flap wire plug
3. Bleed pressure from fuel rail using test port schrader valve
4. Disconnect fuel supply and fuel return hoses
5. Remove 5 screws holding fuel rail to intake manifold
6. Pull fuel rail off
7. Remove remaining screws on intake manifold
8. Loosen hose clamps connecting upper and lower manifolds
9. Pull air intake away from cylinder head slightly
10. Reach down between gap and disconnect knock sensor below cylinder 5
11. Remove air intake manifold

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

1. Clean mating surfaces and check for flatness.
   1. 0.004“ tolerance between front and rear of engine block / cylinder head mating surface
2. Clean cylinder head bolt holes
3. Set engine timing to 30 degrees BTDC.
   1. Pistons are in position such that valves will not interfere when adjusted later
4. Install new head gasket
5. Install cylinder head
6. Torque cylinder head bolts
   1. Lightly coat threads with engine oil for wet torque
   2. Follow tightening stages, order, and torque specs
   3. 1st stage: 55 Nm
   4. 2nd stage: 90 degrees
   5. 3rd stage: 90 degrees

1. Set camshafts to TDC using a tool 4mm in diameter such as a punch or a 5/32” drill bit
2. Turn intake camshaft advance magnet to the fully retarded position by turning it clockwise until stop
3. Install timing chain with exhaust cam timing gear
   1. Tighten screws to 22 Nm

1. Clean mating surfaces for upper timing cover and install U-shaped seal.
2. Apply oil resistant sealant to metal seal surfaces of upper timing cover
3. Replace o-ring for water passage in upper timing cover
4. Install new timing chain guide rail
5. Install upper timing cover, secure bottom screws first.
   1. Tighten screws to 21 Nm
6. Lubricate chain guide rail pin with oil and reinsert through the timing cover
7. Reinstall the cover for the pin by hand until fully seated. Tap the guide rail pin further if necessary. Avoid using the threads to push the pin in because they are made of aluminum and will strip.

1. Place intake manifold against cylinder head mating surface and install with only 4 screws
   1. Use the 4 screws that are not shared by the fuel rail.
   2. Reconnect knock sensor wiring while intake is loose
2. Tighten hose clamps connecting upper and lower intake manifolds
3. Reconnect resonance flap electrical connector
4. Reinstall fuel rail and the remaining screws to secure both fuel and intake manifold to cylinder head
5. Reconnect fuel supply and return line