Air compressors rely on oil carryover in order to provide lubrication to the compressor's pistons, valves, and piston rings. The discovery of lubricating oil at the inlet port, or the discharge port, of the air compressor could be an indication of normal operation of the compressed air system.
The presence of lubricating oil at these other components within the air system may also be an indication of proper system operation:
Air dryers - Most desiccant air dryers incorporate a separator, or a filter, that is used to remove liquid oil from the compressed air. The proper operation of the air dryer minimizes the amount of oil that passes through the system to the supply reservoir. The oil, and moisture, that is allowed to collect in the air dryer is periodically purged from the dryer. Ensure that the drain for the dryer is in a location that will not allow the system to be purged onto other components. If the air dryer is purged onto the fuel tank or onto the fender, the concern over excessive oil carryover is sure to be amplified.
Aftercooler - Aftercoolers are occasionally used in order to cool the air that is used by the compressed air system. Aftercoolers that utilize condenser assemblies to cool the air may replace the air dryer completely. Many aftercoolers utilize designs that promote the collection of oil and moisture. Collection of a small amount of oil carryover in this type of aftercooler will indicate the proper operation of the aftercooler. The location of drain cocks, or valves, that are used for expelling the contaminants from the aftercooler should be considered during the investigation of a complaint that involves excessive oil carryover. Ensure that the drain for the aftercooler is not in a location that may amplify the concern of excessive oil carryover.
Supply reservoir - The supply reservoir is also a system component that has been designed to collect oil carryover and moisture. Again, ensure that the drain for the supply reservoir is not in a location that may amplify the concern of excessive oil carryover.
Information that is specific to your application can be found in the manufacturer's service manual. Use the drain intervals in order to verify that the air compressor is producing excessive oil carryover.
Identifying Excessive Oil Carryover
As a preliminary check, you are instructed to perform a walk-around inspection. A visual inspection of the air system will allow you to identify system problems such as fluid leakage or air restrictions.
Excessive oil carryover will be identified by evaluating the quantity of oil that is produced by the system during a specific interval of time.
Use the following procedures in order to accurately identify the possibility that the air system is producing an excessive oil carryover.
Visual Inspection of the Air System Components
Check each component of the air system for external oil leakage. Use the following procedure to check for oil leakage:
Check the compressor, and other components, for signs of oil leakage. If the compressor shows signs of leakage, clean the compressor and investigate the leak. Check for oil leakage from other components that may be leaking onto the compressor.
Note: Most manufacturers agree that the use of air valves that will automatically drain the reservoirs to 0 kPa (0 psi) on a daily basis are best. Compressed air systems must be equipped with air drain valves. The compressed air system should also be equipped with a desiccant air dryer.
Drain Intervals for the Air System
If an excessive amount of oil is passing from the air compressor, the filter element that is inside the air dryer and the desiccant cartridge may have become saturated. One of the first signs of a filter element or desiccant cartridge that has become saturated is the appearance of an excessive amount of moisture in the supply reservoir.
An air compressor that is operating under normal conditions will pass an average of 3 cc (0.2 in3) of oil per hour. Use the following procedure to determine that your air compressor may be passing an excessive amount of oil carryover.
- Drain the air system reservoirs. All reservoirs should be drained to 0 kPa (0 psi). Check all reservoir drain valves for proper operation. Valves that are malfunctioning or valves that are leaking should be replaced.
If you know that the system has been drained within the last 6450 kilometers (4000 miles) or 75 hours of operation and an excessive accumulation of oil is drained from the system, proceed to "Check for Leakage in the Air System" in order to continue troubleshooting the system.
If you can not verify the last drain interval, or you can verify that the last drain interval has not been within the last 6450 kilometers (4000 miles) or 75 hours of operation, continue with this procedure in order to verify that a problem exists.
- After draining the system, return the engine to service for an additional two service intervals. If an excessive amount of oil is drained from the system after the second interval, continue to troubleshoot the system.
Replacement of the air compressor should not be performed at this time. Prior to replacing the air compressor, determine the cause of the oil carryover problem.
Check the Air System for Leakage and Check the Compressor's Duty Cycle
Excessive leakage in the air system can cause a dramatic increase in the duty cycle of the air compressor. An increase in the duty cycle of the air compressor will cause the performance of the air compressor to degrade. As the duty cycle of the air compressor increases, oil carryover will also increase.
The air compressor's duty cycle compares the run time of the compressor with the rest time of the compressor. The charging cycle is the duration of continuous operation of the compressor during normal system operation. The list that follows will detail the effects that are produced by an excessive duty cycle or an excessive cycle time:
- An increase in oil carryover
- An increase of operating temperature for the air compressor
- An increase in the temperature of the compressor's air discharge
- Excessive carbon buildup within the air compressor
All of the conditions that are mentioned above can cause damage to air system components.
Check for Leakage in the Air System
Use the following procedure to check for excessive leakage in the air system:
- Allow the system to build pressure. The system should be allowed to cycle at the system's peak pressure.
- Wait for one minute in order to allow the air system to stabilize.
- Use the air system's pressure gauge in order to check that system leakage does not exceed a drop in pressure that exceeds 7 kPa (1 psi) per minute. Ensure that no demand is placed on the air system during the test.
Note: For on-highway truck applications, apply the air brakes prior to allowing the system to stabilize. The system can not experience a drop in pressure that is greater than 21 kPa (3 psi) over a test period of three minutes. The air brakes must remain applied during the entire test period.
Duty Cycle of the Air Compressor
The following guidelines indicate the normal duty cycle for an air compressor.
- The duty cycle of an air compressor should not exceed 25 percent of the total running time of the compressor. This calculation should be performed during a period of normal operation of the air system.
- An average in the duration of the compressor's charge cycle should not exceed a measurement of 90 seconds. This measurement should be obtained during a period of normal operation of the air system.
Note: In order to obtain accurate data that pertains to the air compressor's cycle times, the system should be observed for an extended period of time. Information that relates to normal operation of the system over the duration of one day's operation would be an acceptable term.
If the duty cycle and cycle time of the air compressor is within specifications, refer to "Troubleshooting the Compressor's Air Inlet System" in order to continue troubleshooting the system.
If either of these conditions are exceeded, the useful life of the air compressor will decrease due to the increase in work output that is required from the compressor. The replacement of the compressor with a compressor that has a rating with a higher output may be necessary. If replacing the compressor is not an option, the following modifications to the air system should be considered in order to extend the life of the compressor:
- Increase the system's maintenance by increasing the frequency of the following maintenance items: drain intervals, replacement of the air dryer cartridge and replacement of the discharge line.
- Install an air dryer system that is rated for extended duty air systems.
- Install a filter system that has been rated for extended duty air systems.
Although all of the modifications that have been outlined will counteract the effects of an extended duty cycle on an air compressor, the normal life of the air compressor will still be diminished.
Troubleshooting the Compressor's Air Inlet System
Operating an air compressor that is supplied with unfiltered air will result in excessive wear to the piston rings and the cylinder bores of the compressor. If the system is not repaired, critical damage to the air compressor will occur in a very short period of time and the compressor may require replacement.
Air inlet restrictions cause an excessive vacuum to buildup in the air compressor. An excessive vacuum will produce symptoms that are similar to the symptoms that are produced by a compressor that is worn or in need of repair. This high vacuum will cause an increase in the amount of oil that is drawn from the compressor crankcase into the air that has been compressed.
The following conditions will result in the introduction of unfiltered air into the air system. Check for these conditions:
- Damaged inlet air lines
- An inlet system that is leaking
- An air filter that is dirty or clogged
Repair the inlet air system or service the system filters immediately if any of these conditions are discovered.
Use a test gauge to check the inlet restriction during normal operation. The maximum value for restriction of the air inlet system is 6.2 kPa (25.00 inches of H2O). If the air inlet restriction is observed to be excessive, investigate the cause of the restriction. Check for the following problems:
- Bends in the air supply line that are at sharp angles
- An excessive amount of bends in the air supply line
- An air supply line that is kinked
- Installation of an incorrectly sized air supply line
- A poorly routed air supply line
- An inlet line that may collapse from excessive vacuum
Repair any problems that have been discovered in the air inlet system.
Troubleshooting the Air Compressor's Discharge System
The discharge line delivers compressed air to the air dryer, or to the supply tank from the air compressor. The discharge line also serves to cool the compressed air. If the compressed air is not adequately cooled prior to entering the air dryer, oil that is in a gaseous state will be allowed to pass through the air dryer. This oil will travel with the compressed air throughout the system. As the oil cools, the oil will condense in the other system components.
A restriction in the discharge line will cause abnormally high charging pressures. These high charging pressures can contribute to the high operating temperatures of all of the air system components.
High operating temperatures will accelerate wear on the compressor's internal components. These high operating temperatures will shorten the life of the air compressor.
Check the Compressor's Discharge Line
Utilize the following guidelines in order to troubleshoot problems with the compressor's discharge line:
- Check the temperature of the compressed air at the inlet of the air dryer. The system should not experience a temperature that is greater than 71 °C (160 °F) at this point. If this temperature is exceeded, the installation of a discharge line that is longer, or the installation of a discharge line that has a larger diameter than the line that is currently installed could reduce the temperature to a value that is within specifications.
- Carefully inspect the compressor's discharge line for restrictions that may be caused by the accumulation of carbon. Also check the discharge port of the air compressor for a buildup of carbon. Clear any carbon buildup from the discharge line and the discharge port of the compressor.
Repair any deficiencies that have been identified in the air compressor's discharge system.
Troubleshooting the Air Compressor's Cooling System
High coolant temperatures can damage an air compressor beyond repair. During an overheating condition, the following conditions may contribute toward damaging the air compressor:
- A distortion of the cylinder bore which will cause a drop in the efficiency of the compressor and an increase in oil carryover
- A breakdown of lubrication between the cylinder bore and the cylinder wall that can be the cause of excessive ring wear and excessive oil carryover
- A breakdown of the compressor's lubricating oil which will promote the formation of carbon deposits
- An increase in the temperature of the compressed air at the dryer inlet that will cause the degradation of dryer efficiency
Use the following procedure in order to troubleshoot and repair cooling system problems.
Check the Cooling System for Deficiencies
Check these indicators in order to identify a condition of insufficient cooling:
- Carbon deposits in the inlet port of the air compressor
- Heavy carbon deposits will be present in these components: air compressor cylinder head, compressor discharge line and line fittings
- Discoloration of the compressor cylinders or the cylinder head
If any of these indicators are present, measure the coolant temperature at the coolant outlet port of the air compressor while the air compressor is operating under normal conditions. Refer to the air compressor's Service Manual for specifications that relate to coolant temperature for your application. If the cooling system is overheating, use the following procedure in order to locate the problem and repair the cooling system:
- Remove any grease or dirt that may have accumulated on the air compressor's cooling fins. Clear any obstructions from the fins.
- Inspect the coolant lines and line fittings for an accumulation of rust. Check the coolant lines for bends that are at sharp angles. Check for lines that are kinked. Reroute any coolant lines that have been routed poorly. Repair restricted lines and replace lines that are damaged.
Ensure that the coolant lines are attached to the correct coolant port on the air compressor cylinder head. For information on identifying the locations of the coolant ports for your application, refer to the air compressor's Service Manual.
- Install an in-line flow meter in order to check that the coolant flow through the air compressor is within specifications. Check the coolant flow at low idle and at high idle.
Note: The state of the engine thermostat could affect the coolant flow through the air compressor. Be sure to check the coolant flow through the compressor with the thermostat in the open and in the closed position.
Refer to the air compressor's Service Manual, "Specifications" for specifications that relate to coolant flow.
- If the air compressor is located within 76 mm (3 inch) of an exhaust pipe or a turbocharger, some of the manufacturers require the installation of a heat shield in order to protect the compressor from radiant heat. If this is required, ensure that the heat shield is installed properly. Refer to the air compressor's Service Manual for more information.
If the air compressor's cooling system conforms to all of the specifications that have been mentioned in the tests that have been outlined, and the coolant temperature at the coolant outlet port remains out of specifications, then the problem may be originating from the engine cooling system. Check the engine cooling system for proper operation. Repair any deficiencies that may be discovered.
If the air compressor has been allowed to operate during an overheating condition, repair or the replacement of the air compressor will be necessary. Test the cooling system and make necessary repairs prior to allowing the system to return to normal operation.
Troubleshooting the Air Compressor's Lubrication System
The engine provides a continuous supply of oil to the air compressor. Oil is routed from the engine to the oil inlet of the air compressor. This oil is used to lubricate all of the components of the air compressor. Return oil is allowed to drain back into the engine through the mounting flange of the air compressor or through a drain tube that is external to the engine.
Utilize the following procedure in order to verify the correct operation of the air compressor's lubrication system.
A high oil supply pressure can result in an increased oil level in the crankcase of the air compressor. During the operation of an air compressor that has an increased oil level, there is a constant saturation of the cylinder walls with oil. The increased amount of oil on the compressor's cylinder walls also increases the amount of oil that migrates past the compressor's piston rings. This oil eventually collects in the air dryer and the supply reservoir of the air system.
In order to maintain the compressor's oil level, the engine oil pressure must remain within specifications. Use a test gauge that is known to be accurate in order to test the engine oil pressure. If the engine oil pressure is not within the factory specifications, repair the engine lubrication system in order to bring the pressure into specifications.
Note: Do not attempt to correct a high engine oil pressure by placing a restriction in the air compressor's oil supply line. Damage to the air compressor may result.
The compressor's oil return to the engine should not be restricted in any manner. Restrictions that are present in the oil drain passage from the compressor to the engine will cause the oil level in the air compressor's crankcase to increase. This will cause excessive oil carryover to occur. A field test procedure is not available for testing the compressor's crankcase oil level.
For compressors that drain the return oil directly to the engine through a flange mounting hole, check for the following problems:
- Ensure that the drain ports are properly aligned.
- Check for the proper alignment of the mounting gasket. Ensure that the drain passage is not obstructed by the gasket.
- Check that the drain passage is not obstructed by a buildup of gasket sealant.
Adjust problems that have been discovered with port alignment and remove any restrictions in the oil drain passage.
For compressors that utilize a drain tube for the return oil, check for the following problems:
- Check that the oil drain tube has a continuous descent from the compressor's drain port to the fitting that connects to the engine crankcase.
- Check that the routing of the drain tube does not obstruct the flow of oil. Check for bends in the tube that have excessive angles. The drain tube should not be kinked in a manner that would restrict the flow of oil.
- Remove the drain tube from the engine and inspect the passage for obstructions. Ensure that the tube is free of sludge deposits.
Make any repairs that ensure that the optimum amount of oil is allowed to drain from the compressor to the engine.
Engine crankcase pressure that is excessive can inhibit the compressor's ability to drain oil. When the compressor's ability to drain is reduced, the crankcase oil level will increase a proportional value. This condition can also cause the crankcase pressure of the air compressor to increase. An increase in the compressor's crankcase pressure can cause oil to be forced past the compressor's piston rings. When these two conditions are combined, a sharp increase in the amount of oil carryover that occurs will be realized.
Use a test gauge that is known to be accurate in order to check the engine crankcase pressure. If the crankcase pressure is above specifications, make the repairs that are necessary in order to correct the problem.
Outside Influences that Affect Air Compressor Operation
Engine Application and Operation
Verify that the engine is rated and configured for the current application. Ensure that the capacity of the air compressor conforms to the requirements of the application.
Check the other components in the air system for proper operation. The performance of the other components in the air system will affect the performance of the air compressor. Ensure that the capacity of the air system's components conform to the requirements of the compressor.
Evaluate the engine's operating environment. Verify that the engine and engine systems has been equipped for extreme conditions that may exist. Ensure that the maintenance intervals are applicable to the operating conditions of the engine.
Compressors that are loosely mounted or incorrectly mounted can be the cause of incorrect oil levels in the compressor's crankcase. Consider the engine's tilt angle. For compressors that are flange mounted, the tilt angle of the compressor is controlled by the tilt angle of the engine. The tilt angle of the engine may be allowed to change during the operation of the application. The tilt angle for some compressors will contribute severely to the compressor's oil carryover, performance, and service life.
Contact the compressor's manufacturer in order to obtain information on the correct mounting angle for your compressor. Also look for information that is available on the steps that can be taken in order to improve the characteristics of your air compressor's oil drain.
The condition of the engine must be considered when you evaluate the condition and the performance of the air system. In most applications, the engine and the air compressor share lubrication oil, coolant, and supply air. A poorly maintained engine will adversely affect the mechanical condition of the compressor.
Gather any information that is available from the operator or from the field mechanic. Utilize the maintenance records and the history of the engine in order to determine the root cause of any complaint.
The compressor's original equipment manufacturer has determined that the use of Oil Test Cards for Oil Passing Diagnosis is NOT a reliable method for diagnosing problems with the air system. The use of oil test cards is not recommended. Refer to , SEBD6774, "A New Diagnostic Kit for the Evaluation of Air Compressor Operation is Introduced" for more information.
Prior to replacing the air compressor, all checks that have been provided by this document must be performed. If the deficiencies that have been identified by these checks are not corrected, the performance and life of the replacement components may be affected.