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