Diagnosing Filters with Collapsed Centertubes or
Full flow oil filtration is a system in which all of the oil from the oil
pump (that oil directed to the mating parts of the engine) must pass through
the oil filter.
This means that a filter must remove engine damaging dirt and
grit from the oil on the first time around. To ensure a supply of oil to
lubricate the engine under all conditions, a relief valve is built into the
filtering system. This filter relief valve could be located in the engine or
oil filter base (attached to engine) or in the filter itself. Under normal
operating conditions this valve is closed. However, this valve will open and
supply oil directly to the engine whenever the filter becomes plugged with
contaminants and too restrictive to oil flow. Manufacturers believe that it
is better to supply unfiltered oil to the bearings than to burn out bearings
by oil starvation.
Bypass or part flow systems
Bypass or part flow oil filtration systems take only a small
portion (about 10%) of the oil flow from the pump. Actually,
this amount is borrowed from the excess oil that would
ordinarily be returned to the engine oil pan or inlet side of
the pump through a passage-way controlled by the
engine pressure regulation valve.
Oil flow through a bypass filter is regulated by the use of a
metering orifice within the filter. Oil which has been filtered
is returned to the engine oil pan directly and not to the mating
parts of the engine as with the full flow system.
Over-Pressurized lube oil filters
From time to time every filter manufacturer has a filter that
has been severely over-pressurized returned from a customer. Often the
deformed filter is the only sign the car owner has that a problem existed in
his lube oil system. It's possible that the damaged filter was not noticed
until it was removed during the next scheduled oil change. However, if the
pressure was sufficient to blow out the gasket or unroll the lockseam, the
car owner may have experienced immediate and costly problems.
With the "evidence" in his hands, he tends to put the blame on
the damaged filter. It's not surprising that his more than a little
aggravated when the filter manufacturer denies any responsibility for the
What, then, has caused the over-pressurization?
A look at how a lube oil system functions will show that oil
pressure is created by the oil pump.
The upper limit of this pressure is
controlled by a pressure regulating valve which is usually an integral part
of the pump. Figure 1 is a simplified diagram of the lube oil system showing
the pump, regulating valve, filter and bearings
The pump supplies sufficient flow to lubricate the bearings and
other moving parts of the engine. This oil must be under pressure if it is
to properly separate the highly loaded parts of an engine and prevent
excessive wear. The purpose of the regulating valve is to provide this
pressure which on most passenger cars is between 40 and 60 PSI.
The regulating valve is made up of a ball or plunger which
regulates pressure with aid of a spring. The spring is calibrated so that
the plunger will lift off its seat when the oil pressure reaches the desired
amount. Once the valve is open, the pressure remains fairly constant with
only small changes occurring as the engine speed varies.
The filter and all other components in the system are subjected
to the pressure established by the regulating valve. If this
pressure is excessive, filter damage may occur. This is the
point that many people who are not familiar with lube systems
fail to realize. What can cause the pressure in the system to
exceed the regulating valve setting? The answer is that either
the valve must be stuck in the closed position or it is sluggish
and slow to move to the open position after the engine has
Figure 2 shows the system operating with the regulating valve
stuck in the shut position. Under these conditions the pressure
builds up equally on all components in the system until
something happens to relieve the pressure. If the regulating
valve becomes unstuck, the pressure will return to normal. If it
remains stuck, something has to break. Normal operating pressure
caused no permanent deformation of the filter body. When the
system pressure reaches 150 PSI due to a faulty regulating
valve, most filters become permanently deformed. At this
pressure the gasket usually will not blow out and the lockseam
will remain sound.
If the regulating valve still remains stuck, the pressure will
increase further and the gasket between the filter and the base
can be blown out. This will probably cause the loss of all the
oil in the system.
If the filter has been installed on the tight side, the gasket
may not blow out and the lockseam will unwind as the pressure
continues to rise.
If the customer is alert and shuts the engine off at the first
sign of trouble (red light on or reduced oil pressure) he can
limit his loss to a tow job, oil change and new filter. If he
drives to the nearest garage, he will probably burn up the
engine due to lack of oil. The main point is that the deformed
filter is not the cause of this excessive pressure, but is the
victim of a faulty regulating valve.
The customer may ask if a filter that is completely plugged
could have caused the over-pressure conditions in the system.
The answer is no. If the regulating valve is functioning
properly it will maintain the pressure on the filter at 40 or 60
PSI even if filter is plugged.
In summary, if a filter distorts due to overpressure in the
system, the fault lies with the regulating valve and not with