Oil filters renewed at regular intervals. If it is left in service

7.22.2 Oil filters renewed at regular intervals. If it is left in service

too long it may become blocked. When this hap- Even new engines can contain very small par-

pens the buildup of pressure inside the filter forces ticles of metal left over from the manufacturing

open a spring-loaded relief valve in the housing process or grains of sand which have not been

and the oil bypasses the filter. This valve pre- removed from the crankcase after casting. Old

vents engine failure, but the engine will be lubri- engines continually deposit tiny bits of metal

cated with dirty oil until the filter is renewed. This worn from highly loaded components such as the

is better than no oil!

piston rings. To prevent any of these lodging in

A bypass filtration system was used on older bearings or blocking oil ways, the oil is filtered.

vehicles. This system only filters a proportion of The primary filter is a wire mesh strainer that

the oil pump output. The remainder is fed directly stops particles of dirt or swarf from entering the

to the oil gallery. At first view this seems a strange

Figure 7.52 Lubrication system

Engine systems 153

idea but all of the oil does eventually get filtered. Crankase breathing or ventilation of the engine The smaller amount through the filter allows a

was first done by what is known as an open sys- higher degree of filtration.

tem, but this has now been completely replaced by the closed system. The gases escaping from

7.22.3 Oil pumps

an engine with open crankcase ventilation as described above are very toxic. Legislation now

In its simplest form an oil pump consists of two demands a positive closed system of ventilation. gear wheels meshed together in a tight space so

This makes the pollution from cylinder blow-by that oil cannot escape past the sides. The engine

gases negligible. Positive crankcase ventilation drives one wheel. As the gears rotate in opposite

is the solution to this problem. directions, the gap between each tooth in each

In early types of closed system crankcase ven- wheel traps a small quantity of oil from an inlet

tilation, the lower pressure at the carburettor air port. The trapped oil is carried round by each

cleaner was used to cause an airflow through the wheel towards an outlet port on the opposite side

inside of the engine. The breather outlet was sim- where it is forced out by the meshing teeth.

ply connected by a pipe to the air cleaner. This The principle of squeezing oil from an ever-

caused the crankcase gases to be circulated and decreasing space is also used in the rotor type

then burned in the engine cylinders. A flame trap pump. An inner and outer rotor are mounted on

was included in the system, to prevent a crankcase different axes in the same cylinder. The inner

explosion if the engine backfired. rotor, which commonly has four lobes, is driven

In modern closed systems the much lower pres- by the engine. It meshes with an outer rotor, which

sure within the inlet manifold is used to extract has five lobes. As they rotate, the spaces between

crankcase gases. This has to be controlled in most them change size. The inlet port is at a point where

cases by a variable regulator valve or pressure con- the space between the rotor lobes is increasing.

scious valve (PCV). The valve is fitted between the This draws the oil in to the pump. The oil is then

breather outlet and the inlet manifold. It consists of carried round the pump. As rotation continues, the

a spring-loaded plunger, which opens as the inlet space between the lobes gets smaller. This com-

manifold pressure reduces. When the engine is sta- presses the oil out of the outlet port.

tionary the valve is closed. Under normal running Oil pumps can produce more pressure than is

conditions the valve opens to allow crankcase required. A valve is used to limit this pressure to

gases to enter the inlet manifold with minimum

a set value. The pressure relief valve is a simple restriction. At low manifold pressures during idling device, which in most cases works on the ball

and overrun (pressure is less than atmospheric), and spring principle. This means that when the

further travel of the valve plunger against its spring pressure on the ball is greater than the spring, the

closes it in the opposite direction. This reduces gas ball moves. The pressure relief valve is placed in

flow to the inlet manifold. This feature makes sure the main gallery so that excess pressure is pre-

that the fuel control process is not interfered with vented. When the ball moves oil is simply

under these conditions. The valve also acts as a returned to the sump.

safety device in case of a backfire. Any high pres- sure created in the inlet manifold will close the

7.22.4 Crankcase ventilation –

valve completely. This will isolate the crankcase

engine breather systems and prevent the risk of explosion.

Breathing is very important; without being able to breathe, we would die! It is almost as important

7.23 Diagnostics –

for an engine breathing system to work correctly. There are two main reasons for engine breathers.

lubrication

● Prevent pressure buildup inside the engine

7.23.1 Systematic testing

crankcase due to combustion gases blowing If the reported fault is that the oil pressure light past the pistons. The buildup of pressure will comes on at low speed proceed as follows. blow gaskets and seals but also there is a high

1. Run the engine and see when the light goes off ● Prevent toxic emissions from the engine.

risk of explosion.

or comes on.

Emission limits are now very strict, for good

2. Is the problem worse when the engine is hot? reason – our health!

Check the oil level! When was it last serviced?

154 Advanced automotive fault diagnosis

3. If oil level is correct then you must investigate

7.23.3 Test results

further.

4. Carry out an oil pressure test to measure the Some of the information you may have to get actual pressure.

from other sources such as data books, or a work-

5. If pressure is correct then renew the oil pres- shop manual is listed in the following table. sure switch. If not engine strip down is likely.

6. Run and test for leaks.

Test carried out

Information required

Oil pressure

Oil pressure is measured in bars. A

7.23.2 Test equipment

typical reading would be about 3 bar

Crankcase pressure

By tradition pressures less than

Note: You should always refer to the manufac-

atmosphere are given in strange

turer’s instructions appropriate to the equip-

ways, such as inches of mercury or

ment you are using.

inches of water! This is why I like to stick to absolute pressure and the bar! Zero bar is no pressure,

Oil pressure test gauge (Figure 7.53)

I bar is atmospheric pressure and so on. Two to 3 bar is more than

This is a simple pressure gauge that can be fitted

atmospheric pressure like in a

with suitable adapters into the oil pressure switch

tyre. The trouble is standards

hole. The engine is then run and the pressure

vary so make sure you compare

readings compared to data.

like with like! Back to crankcase pressure – it should be less than atmospheric, check data

Vacuum gauge

Oil condition

Recommended type of lubricant

A simple ‘U’ tube full of water is often used. This is connected to the oil dipstick tube and the