Low, no or fluctuating oil pressure may be a sign of catastrophic internal engine damage. Engines exhibiting these symptoms should not be flown until fully inspected by a licensed mechanic. Do not operate any engine that has operated without oil pressure. Connecting rod bearing damage occurs quickly and cannot be detected without internally inspecting the engine.
All things being equal a low but within limits oil pressure is more desirable than a high but within limits oil pressure. First, go back to the oil pressure limits in the engine operating manual. Use an oil pressure gauge of known accuracy. Look up the oil pressure limits in the Engine Operating Manual. If oil pressure falls within these limits then there isnt a problem.
Generally on Lycoming aircraft engines oil pressure at 650 rpm with hot oil can be as low as 25 psi and as high as 100 psi starting in cold weather. Low oil pressure can result in high oil temperature in marginally cooled engines. Increasing oil pressure increases oil circulation through the oil cooler. Low oil pressure results from either lost pressure or a weak pump. One oil pressure problem is low pressure at idle but too high oil pressure when the oil is cold. This may be caused by the piston oil squirts not closing on Lycoming engines (Lycoming oil squirts should be open at 50 psi and closed at 40 psi) or (more likely) the oil pressure seat leaking. At idle speeds the oil pressure responds to any leak in the seat. At higher engine speeds and oil pressure the relief valve is off the seat and oil pressure is less affected by seat to valve leakage.
1. Insufficient oil.
Bearing material flaking off due to fatigue failure of bearing surface from high loads and high temperature. When missing material reaches edge oil pressure can no longer form between the bearing and the journal. Journal runs dry causing remaining bearing material to melt. Increased clearance caused by the melted material results in the rod bolts pounding and eventually failing.
2. Pressure relief out of adjustment. If the pressure relief valve is not operating properly, the oil pressure follows the throttle because the oil pump is engine driven and speed sensitive. Lycoming limits are 60 to 90 pounds, with a range of normal 25 when idle go to 100 psi during starting and run-up. See oil pressure specifications in the applicable operators manual before making adjustment.
77808 Lycoming adjustable oil pressure valve.
Lycoming engines: External adjustable type (part number 77808) increase oil pressure by turning adjusting screw clockwise, and to decrease turn counter-clockwise. On types with solid tower, standard 1/4" flat washers ( part number STD-425 or AN960-10) are used to control oil pressure. Add washers to increase oil pressure and remove washers to decrease oil pressure. On short tower, a maximum of three washers may be used. On a tall tower, a maximum of nine washers may be used. Adjust oil pressure when engine is warm. Normal oil pressure can be expected to vary from 100 psi during starting and warm up to 25 psi at idle when the engine is warm. See the oil pressure specifications in the applicable Operators Manual for specific numbers. Refer to Lycoming Service Instruction 1172C. Lycoming produces no less than four oil pressure relief valve springs. You may choose anyone to suite your conditions. This may be preferable than stacking washers. The reason there are different springs has to do with the airframe installation. Lycoming engines take oil pressure from between the pump and the pressure relief valve. Oil Pressure Relief Valve Spring Free Length Wire Dia. 68668 2.04 .130 LW-11713 2.12 .144 LW-11138 2.64 .144
W-11713 Lycoming oil pressure spring
Oil to the propeller governor and oil cooler come directly off the oil pump. The size of the oil cooler, oil cooler lines, fittings, and length all has an effect on oil pressure before the relief valve. The amount of oil bypass through the governor also effects oil pressure. Different size oil pressure relief valve springs are used to compensate for these installation differences. Even though an engine overhaul facility adjusted oil pressure after overhaul and on the test stand, you might have to re-adjust after installation on the aircraft. Download Oil Pressure Relief Spring Program
Lycoming oil pump
3. Oil viscosity too low.
4. Foam in oil due to presence of alkaline solids in system.
5. Malfunctioning oil pump. If using the old two-piece style oil pump that uses a cotter pin to hold the stationary idler shaft in place then some loss of oil pressure will occur if the accessory housing has been drilled per Lycoming Service Instruction 1341 to provide oil passage to the idler shaft bore. This modification is not done one the two-piece oil pump housing.
6. Malfunctioning pressure gauge. Bordon oil pressure gauges will slightly straighten with excessively high pressures. This causes the gauge to read low.
7. Weak or broken oil pressure relief valve spring (Continental engines). The pressure relief valve spring in Continental engines is long and slender. It has a tendency to rub against the housing when it flexes. In high time engines it weakens the spring and eventually the spring breaks.
8. Clogged filter or strainer. If a direct pressure reading on the pump is low, it will be necessary to drain and remove the sump to check for restriction in the pickup tube. Check oil filter or screen for aluminum. A clogged screen may allow oil normal pressure after engine start but drop after a period of time. This is caused by particles sucking up onto the screen during engine operation and then falling off the screen when the engine is shutdown.
9. Carbon or metal chips under oil pressure relief valve. Any debris caught between the plunger and seat of the oil pressure relief valve will cause low oil pressure with hot oil; and possibly not so low with cold oil. The debris hold the
valve off its seat and bleeds off oil. Clean the plunger and seat. If debris are metal, further investigation is required.
10. (Lycoming aircraft engines) Damaged oil pressure relief seat. Seat not concentric. A leaking seat will dump idle oil pressure but result in normal oil pressure at higher engine speeds. If you try to adjust the oil pressure higher you will find that it is then too high in the morning. On later Lycoming engines the ball is centered somewhat within the housing. If the seat is not centered with the housing the ball wont contact completely. A few thousandths of an inch is enough to kill idle pressure. This might occur if the crankcase has slightly warped during welding. Inspect the inside of the housing near the end where the ball may contact for compete polishing. If the ball has been contacting the housing on one side but not the other then the housing is pushing the ball off the seat. Lycoming makes a tool that cuts the seat centered to the housing. If you feel the seat is centered with the housing but still leaking you might try taking a spare ball and brazing a rod to the ball. Lap the ball into the seat. Lapping however will not center the seat to the ball so. If the ball is contacting the housing then you need to countersink the oil relief ball seat. Instructions and tooling are explained in Lycoming Service Instruction 1172C or latest version.
11. High oil temperature.
12. Restriction at inlet side of oil pump.
13. Excessive internal spill-off of oil. May be caused by: excessive bearing clearance, cracked crankcase in oil gallery area, piston cooling squirts blocked open, or spun bearing. Lycoming geared engines and IO-720 used hyperbolic (barrel) shaped cam followers. Hyperbolic followers bleed more oil than straight followers. Excessive wear to the tappet bores will result in low oil pressure at normal oil temperature. When the oil is cold and thick oil pressure will be normal. On the IO-720 hyperbolic followers (p/n 73061 )can be replaced for straight followers (p/n 72877) per Supersedure notice 21779-250.
14. Pump inlet gasket deteriorated thus allowing pressure oil to return to the inlet side of the pump.
15. Lycoming O235 engines make sure plate p/n LW-12795 is installed between accessory housing and the oil filter adapter. The plate is sandwiched between two gaskets. The plate is designed with a hole that meters the amount of oil flow and prevents an excessive flow of oil to the idler gear. When this plate is not installed as required, it results in low oil pressure especially at idle.
16. Oil cooler lines kinked or otherwise blocked (Lycoming engines).
17. Oil cooler plugged.
18. Piper PA34-200T Seneca II - low oil pressure is normal. Cruise oil pressures (2300-2575 rpm) of 30 to 40 psi at oil temperatures of 180 to 240 degrees F. is normal. Note. minimum idle oil pressure is 10 psig. Oil pressure below 30 psi under the conditions outlined above should be checked. Verify pressure reading by mounting a remote pressure gauge connected at the turbocharger oil supply line tee fitting at the same position the cockpit oil pressure gauge line is attached. Theres a larger diameter turbocharger oil supply line and fittings, a single high load oil pressure relief valve spring, and an improved (low pressure drop) oil cooler available for these engines. See Continental Service Bulletin M78-2.
19. Collapsed oil filter. Usually associated with cold weather starts. See Champion Spark Plug Aviation Action Alert 91-1.
20. Continental A series engines (A65,A75) no oil pressure. Oil pump has lost its prime. If the oil pump gears are worn enough to let the oil flow back down into the sump between flights, then the dry gears might not be able to create enough suction to get the oil circulating again. For a quick fix, hold the tail as high as possible for several minutes and enough oil will run from the oil screen cavity to the top of the oil pump to prime it. If this doesnt work then squirt oil into the oil screen area.
21. Lycoming IO720 wide deck engines, install high capacity oil pump (20-25% greater flow) per Lycoming Change Number 21514- 260.
22. Continental IO346 A engines and up through GTSIO520 experiencing low or erratic oil pressure, make sure Continental Service Bulletin M79-2 Rev. 1 or latest edition is complied with. This bulletin provides for a new oil pressure relief valve. New pressure relief valve is suppose to have a yellow dot on the relief valve housing.
23. Continental "E" series engines, check to make sure high capacity oil pump was installed per Continental Service Bulletin M53-2 dated November 4, 1953.
24. Oil squirts leaking on Lycoming engine. Leaking piston oil squirts will dump oil pressure at idle. Oil squirts should shut off below 45 psi.
25. (Continental) Loose oil pressure relief valve fitting.
26. Clogged oil pick-up tube.
27. Full flow lifters on Continental 470 and 520 series engines have the capacity of reducing oil pressure 5 to 7 pounds. Full flow lifters can be removed from the intake side (if used). Full flow lifters are sometimes referred to as constant flow lifters because the wide oil groove allows oil to flow during the entire travel of the lifter in the boss. Intermediate feed lifters have a narrower oil groove that only registers with the oil passage in the crankcase during part of the travel.
28. Continental 360 series. Poor fit on oil pump cover.
29. Crankcase or accessory case crack into oil galley.
30. Loss of oil pressure, and oil, on Cessna turbocharged aircraft from leaking actuator piston rings passing oil out the drain line. If the aircraft isnt flown high, the bore of the actuator may become pitted on the dry side of the piston, past the normal (for that airplane) range of travel. In this case oil loss might only happen on higher altitude trips.
31. Continental O470 U, IO470 K, IO520, IO550, TSIO520 engines using oil pump assemblies with integral oil filter adapters an incorrectly cut oil pump gasket (p/n 643749) may not adequately cover the machined orifice in the accessory case that provides access to the gallery plug for the 1-3-5 crankcase half. Inspect this gasket per Continental Service Bulletin SB94-11
32. Leaking oil filter adapter gasket. On Lycoming 76 series engines (the ones with the Bendix D2000/D3000 dual drive magneto). Gasket attaches to oil filter adapter plate install per Lycoming Service Bulletin 543a and Service Instruction 1453 - gasket is suppose to be bonded to plate. There is an airworthiness directive covering this subject on 76 engines.
33. Lycoming engine - missing Allen plug in oil pump body.
The P/N STD-1339 Allen type pipe plug is used to close the end of a hole drilled in the oil pump housing during manufacture. When this plug is installed, the hole becomes an internal passage in the body of the oil pump that allows oil to flow at oil pump discharge pressure during engine operation.
This pressure is higher than the oil system pressure of 55 psi to 90 psi. Lycoming Service Bulletin No. 555 was issued as a result of one engine (a TIO-540-AK1A) that was returned to Lycoming due to low oil pressure. The engine oil pressure dropped from 65 psi to 30 psi during flight. At engine disassembly, the following was found:
The P/N STD-1339 Allen type pipe plug was missing from the oil pump body. This explained the reason for the drop in oil pressure. There was light damage to the accessory gears. The Allen type pipe plug was found in the engine oil sump. This explained the reason for the damage to the accessory gears.
34. Continental - Oil pump drive shaft broken. Reference Airworthiness Directive 81-13-10-R1 or latest edition.
Lack of oil in Lycoming engine damages the connecting rod bearing
The rod journal rides on an oil film (called hydrodynamic lubrication) and does not touch the bearing during operation. Without adequate oil flow to the bearing, the oil film is ruptured and contact is made between the journal and the bearing.
Notice the evidence of high heat on the connecting rod and the piece of bearing back sticking out the right hand side of the rod.
Frictional forces melt the bearing Babbitt until the bearing shell starts to extrude out the side of the connecting rod (shown on the right). Excessive clearance causes the rod bolts to pound and they eventually fail in fatigue. Notice also the heat damage to the rod bolts.
Pilot Description of Symptoms
The pilot assumed that it was a loss of power maybe due to an ignition problem, so he made an emergency landing. With the engine still running, he tried to take off but the engine could not develop enough power to do that, so he turn the engine off.
When maintenance check the engine, it was found that there were not oil at all in the sump and the engine was covered of oil. After opening the engine, we could see the damage caused when there is an oil starvation and the engine is not turn off immediately. The connecting rods were burnt, the bearing destroyed including the crankshafts journals. One bearing was extruded between the connecting rod and the crankshaft cheek. Pistons stroke the crankcase with their skirts.
Lycoming engine giving birth to a new connecting rod. This may occur after operating without oil pressure.