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 oil)

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 entering pump)

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 specified pressure)

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.

NOTE

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 computer-controlled systems.

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 duties.

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