0005 - Oil circuit, oil pressure, overpressure and bypass valve, oil filter Engine 102 (except 102.983/99) single-belt drive

18-005 Oil circuit, oil pressure, overpressure and bypass valve, oil filter

Oil circuit

Specified viscosity classes according to SAE at sustained outside temperatures.

Precisely adhering to the SAE classes on the basis of outside air temperatures would necessitate frequently changing the engine oil. The temperature limits for the SAE classes should therefore be regarded as a guideline which may be exceeded for short periods.

Further information regarding specified viscosity classes and approved engine oils is given in the "Specifications for Service Products".

Opening pressures of oil overpressure and oil filter element bypass valve bar gauge pressure

Function of oil circuit

The oil is drawn off at the lowest point of the oil sump through the strainer basket and the oil suction pipe (7) to the suction chamber (B) and pumped into the tooth gaps, which produce pressure chamber (A) as a result of the eccentric arrangement of the oil pump gears
(1 and 2).


To ensure that the oil remains in the tooth gaps on its way from the suction to the pressure chamber, a sickle (a) is attached between both gears in this area.

1 Inner oil pump gear

2 Outer oil pump gear

3 Oil pump cover

7 Suction pipe

11 M630 bolt (9 ea.)

11a Torx bolt

14 Overpressure valve

15 Compression spring

16 Screw plug

17 Seal

22 Drive sleeve

44 Timing case cover

A Pressure chamber

B Suction chamber

C Damping chamber

a Sickle

b To oil filter

1 Oil filter bottom section

2 Replaceable cartridge

3 Connection fitting

4 Valve cone

5 Compression spring

6 Spring plate

7 Locking ring

8 Filter element

9 Valve plate

10 Valve plate carrier

11 Compression spring

12 Valve holder

13 Compression spring

14 Spring plate

15 Rubber diaphragm

16 Contact-pressure plate

17 Seal

A Feed from oil pump

B Unfiltered oil to main oil gallery

C Filtered oil to main oil gallery

D Connection for electrical oil pressure sensor

The two check valves (9-12 and 13-16) prevent the filter cartridge from running empty when the engine is switched off and when changing the filter.

The bypass valve (4-7) opens when the difference pressure between dirty and clean oil side exceeds 3 bar. This is the case if the filter element is severely fouled. The oil then flows through the gallery (B) unfiltered to the main oil gallery.

When changing the filter, a slight quantity of oil flows out of the replaceable cartridge despite the check valves, which flows together with oil from the bottom of the oil filter along the galleries (A and C) to the engine.

Oil supply of chain tensioner
The oil flows through a passage which branches off vertically up the way at the front of the main oil gallery, to the cylinder head, flows in a cross groove (arrow) to a vertical feed passage in crankcase and on to the chain tensioner.

1 Crankcase

36 Chain tensioner housing

37 Thrust pin

38 Detent spring

39 Compression spring

40 Cap nut

41 Seal

a Feed passage 1.1 mm dia.

b Restriction passage 1.2 mm dia.

Lubrication of valve timing
The oil coming from the main oil gallery in the crankcase, flows through a 4.5 mm dia. jet (A) fitted into the cylinder head, and through the annular gap (arrows) direct to the longitudinal gallery (a) in the cylinder head (pressure-oriented oil feed). This oil supply system ensures that the oil pressure prevailing downstream of the oil filter depending on the operating state, always exists at the hydraulic valve clearance compensating elements. In addition, the nozzle (A) serves as an oil return flow stop which prevents the longitudinal passage (a) and the valve clearance compensating elements running dry. When performing repairs, install oil nozzle return lock 2nd version (18-415).

Part of the oil flows through the restriction passage (g) with dia. of 2.5 mm and the two grooves (arrows) in the rear rocker arm bearing bracket to the rear camshaft bearing and there into the hollow-drilled camshaft. The oil flows through the camshaft to the other camshaft bearings (flow-oriented oil feed).

The rocker arm bearing points are lubricated from the longitudinal passage (a) in the cylinder head and the hydraulic valve clearance compensating elements supplied with oil (b) from there through a passage in the rocker arm.

The cam lubrication is provided through the oil pipe (d) bolted to the rear camshaft bearing.

Lubrication of idler gear shaft and ignition distributor drive

The oil flows from the front camshaft bearing through a cross passage to the left front cylinder head bolt bore and from there along a cross groove (e) to the feed passage in the crankcase.

The oil flows on to the rear bearing bush (17) of the idler gear shaft and through the hollow-drilled idler gear shaft (18b) to the front bearing bush (6) in the timing case cover (44).

The ignition distributor drive (18) is lubricated by splash oil.

The cam (18d) which is press-fitted to the rear end of the idler shaft gear on engines 102.922/924 and drives the fuel pump, is lubricated by the oil splash jet (13).

Oil return

Four return passages (arrows) are provided on the righthand side of the cylinder head (looking in direction of travel) for the oil return to the sump; part of the oil flows back through the chain box.

In the crankcase, the oil flowing back is passed along an oil collecting gallery (arrows) to the cast return passage.

From here, it flows into the oil sump.

Checking oil level
The oil level depends, among other factors, on the oil temperature and on the time for the oil to flow back after the engine is switched off. For this reason, do not measure oil level until approx. 5 minutes after switching off the engine from normal operating temperature (approx. 80 °C oil temperature).
The dipstick must always be withdrawn and wiped off before checking the oil level.

To properly assess the oil level, the dipstick must be viewed from both sides. The determining factor is always the clear horizontal indication of the dipstick (arrow).