Low Oil Pressure
Low oil pressure is indicated when the oil indicator
light glows, oil gauge reads low, or when the engine
lifters or bearings rattle. The most common causes of
low oil pressure are as follows:
1. Low oil level (oil not high enough in pan to
cover oil pickup)
2. Worn connecting rod or main bearings (pump
cannot provide enough oil volume)
3. Thin or diluted oil (low viscosity or fuel in the
4. Weak or broken pressure relief valve spring
(valve opening too easily)
5. Cracked or loose pump pickup tube (air being
pulled into the oil pump)
6. Worn oil pump (excess clearance between rotor
or gears and housing)
7. Clogged oil pickup screen (reduce amount of oil
A low oil level is a common cause of low oil
pressure. Always check the oil level first when
troubleshooting a low oil pressure problem.
High Oil Pressure
High oil pressure is seldom a problem. When it
occurs, the oil pressure gauge will read high. The most
frequent causes of high oil pressure are as follows:
1. Pressure relief valve struckopen (not opening at
2. High relief valve spring tension (strong spring
or spring has been improperly shimmed)
3. High oil viscosity (excessively thick oil or use
of oil additive that increases viscosity)
4. Restricted oil gallery (defective block casting or
debris in oil passage)
Indicator or Gauge Problems
A bad oil pressure indicator or gauge may scare the
operator into believing there are major problems. The
indicator light may stay on or flicker, pointing to a low
oil pressure problem. The gauge may read low or high,
also indicating a lubrication system problem.
Inspect the indicator or gauge circuit for problems.
The wire going to the sending unit may have fallen off.
The sending unit wire may also be shorted to ground
(light stays on or gauge always reads high).
To check the action of the indicator or gauge,
remove the wire from the sending unit. Touch it on a
metal part of the engine. This should make the indicator
light glow or the oil pressure gauge read maximum. If it
does, the sending unit may be defective. If it does not,
then the circuit, indicator, or gauge may be faulty.
Always check the service manual before
testing an indicator or gauge circuit. Some
manufacturers recommend a special gauge
tester. This is especially important with some
LUBRICATING SYSTEM MAINTENANCE
There are certain lubricating system service jobs
that are more or less done automatically when an engine
is repaired. For example, the oil pan is removed and
cleaned during such engine overhaul jobs as replacing
bearing or rings. When the crankshaft is removed, it is
usual procedure to clean out the oil passages in the
crankshaft. Also, the oil passages in the cylinder block
should be cleaned out as part of the overhaul.
As a Construction Mechanic, you will be required
to maintain the lubrication system. This maintenance
normally consists of changing the oil and filter(s).
Occasionally you will be required to perform such
maintenance tasks as replacing lines and fittings,
servicing or replacing the oil pump and relief valve, and
flushing the system. The following discussion provides
information that will aid you in carrying out these
Oil and Filter Change
It is extremely important that the oil and filter(s) of
the engine are serviced regularly. Lack of oil and filter
maintenance will greatly shorten engine service life.
Manufacturers give a maximum number of miles or
hours a vehicle can be operated between oil changes.
Newer automotive vehicles can be operated 5,000 miles
between changes. Older automotive vehicles should
have their oil changed about every 3,000 miles. Most
construction equipment average between 200 and 250
hours of operation between oil changes. However,
depending on the climate and working conditions the