Every engine requires a certain amount of oil flow to keep the bearings, camshaft, valvetrain and other moving parts lubricated. Oil forms a barrier that prevents metal-to-metal contact and reduces friction and wear within the engine.
In the bearings, it forms a wedge that actually lifts and supports the crankshaft and camshaft as they rotate. Oil also serves as the primary means of cooling the bearings as well as the pistons. Whatís more, oil also serves as a hydraulic fluid inside hydraulic lifters and lash adjusters to maintain proper valve lash. On some late model engines with variable valve timing, oil also moves the adjuster mechanism that advances and retards cam timing.
With so many important jobs to do, itís obvious that the engine needs a steady supply of oil. But how do we know if the bearings and other moving parts are being adequately lubricated? Since thereís no simple way of measuring oil flow inside an engine, we use oil pressure as a means of gauging oil flow and lubrication.
Oddly enough, oil pressure by itself doesnít give us the complete picture of whatís really going on inside the engine. It only tells us the oil is encountering resistance as it circulates through the engine.
Understanding oil pressure
Oil pressure is created when the positive displacement oil pump sucks up oil from the pan and pushes it through the filter into the engineís main oil gallery. The oil is then routed to where it is needed: the main bearings, rod bearings, cam bearings, upper valvetrain and timing chain or tensioner.
The oilís viscosity makes it thick and slows the flow as it passes through the passages and bearings. The amount of pressure created thus depends on the bearing clearances as well as the oilís viscosity. Tighter clearances and/or heavier viscosity equal more pressure.
As the pressure builds up in the oil system, it has to go somewhere. A spring-loaded pressure relief valve built in the oil pump (or next to the pump) opens when pressure exceeds a certain limit and either reroutes oil back into the pumpís inlet or the oil pan. This prevents a dangerous build-up of pressure that could rupture the oil filter or blow out pressed in oil gallery plugs. Most relief valves are set to open when oil pressure exceeds about 50 psi.
At idle, most oil pumps do not produce enough flow to force open the relief valve. Oil pumps that are camshaft driven only turn at half engine speed so output isnít great at idle and low rpm. Even pumps that are crankshaft driven and turn at engine speed (or double engine speed in a few instances) donít pump enough oil to overcome the relief valve spring. The relief valve generally only comes into play at higher rpms when the pumpís output pushes more oil into the system than it can handle. Then the relief valve opens to vent oil and limit maximum oil pressure until engine returns to idle or a lower rpm.
Low oil pressure
The first indication of low oil pressure is usually a flickering oil warning lamp at idle, or a lower-than-normal gauge reading. Unfortunately, most warning lamps wonít come on until the oil pressure is really low, like 4 to 5 psi. If the warning isnít noticed or heeded, the next indication may be valve clatter as the hydraulic lifters or lash adjusters run out of oil. If the loss of oil pressure continues much longer, the next sound will be the rods knocking followed by silence as the bearings seize.
Low oil pressure is bad news because it usually means the engine isnít getting enough oil, and that can lead to serious trouble very quickly for the rod and main bearings, as well as upper valvetrain components (especially overhead camshafts). For this reason, any indication of low oil pressure should not be ignored and should be diagnosed and dealt with immediately!
In a high mileage engine, low oil pressure is often due to a combination of worn bearings and a worn oil pump. The oil pump is a high wear component in all engines because it draws in unfiltered oil. The pickup screen on most engines is relatively coarse and will only stop relatively large chunks of debris from entering the pump. Yet the internal clearances in many pumps is only a few thousandths of an inch between the gears and between the gears and cover. Whatís more, many pickup screens have a relatively large bypass valve or slots that allow unfiltered oil to enter the pump when the oil is cold and thick, or if the oil filter pickup screen itself becomes plugged with debris.
When an oil pump ingests debris, it can wear or damage the gears. As wear increases over time, clearances increase causing the pump to work less efficiently. As the pump gets weaker, oil pressure drops. The cure here is to replace the worn pump with a new one.
Sometimes an oil pump will suck in something that causes it to lock up. This usually breaks the pump drive and totally starves the engine for oil. Unless the engine is shut off almost immediately, itís the end of the road for the bearings, too.
Installing a high volume oil pump can increase oil pressure somewhat at idle because the larger gears will displace more oil. But high volume oil pumps are designed primarily for high rpm, performance applications where increased bearing clearances and/or the use of a remote filter or dry oil sump requires more oil flow. A high volume oil pump is not a substitute for overhauling or replacing a worn engine.
As for high pressure oil pumps, installing one usually wonít make any difference in idle pressure because the relief valve doesnít come into play until higher rpms. All this type of pump does is raise maximum oil pressure at higher rpms. If you replace an oil pump, always replace the pickup, too.
Swishing an old pickup in some solvent may only loosen the crud that has accumulated over the years inside the screen, allowing it to be sucked into the new pump as soon as the engine is started. Priming the pump prior to starting the engine is also a good idea so thereís no delay in oil reaching the bearings. Also, use care when installing a pickup tube and screen so that it is properly mounted and does not leak.
As for worn bearings, increased clearances allow the oil to leak out of the bearings at a faster rate. Only about .001˝ of wear in the main bearings can reduce oil pressure up to 20%! Wear also increases the amount of oil splash inside the crankcase, which can overwhelm the rings and cause the engine to burn more oil. Excessive bearing clearances will also increase engine noise and pounding, which will eventually lead to bearing fatigue and failure.
The only fix for worn bearings is to replace them or the entire engine. Installing a set of standard sized bearings (assuming the crankshaft is still within acceptable tolerances) can help restore normal oil pressure at idle, but the amount of improvement will depend on wear elsewhere in the engine, too. Bearings alone canít rejuvenate a tired engine that may also have low compression and worn valvetrain components.
Troubleshooting low pressure
Start by checking the oil level on the dipstick. If the oil level is low, the engine may be leaking and/or burning oil. Adding oil may temporarily restore normal oil pressure, but unless the oil level is maintained the problem will return.
If the engine is leaking oil, recommend new gaskets or seals to fix the leak. If the engine is burning oil, the valve guides and seals are probably worn, and the rings and cylinders might be worn, too. A wet compression test and/or leakdown test can help isolate the wear and determine the best course of action.
Also note the condition of the oil and make sure it is the correct viscosity for the vehicle and prevailing weather conditions. For most late model vehicles, the crankcase should contain 5W-30 or 10W-30. For older high mileage engines, 10W-40, 20W-50 or even straight 30W or 40W oil can help maintain good oil pressure in hot weather. But 20W-50 and straight 30 and 40 are too thick for cold weather driving and may cause start-up lubrication problems especially in overhead cam engines. Light viscosity oils such as 5W-20 and straight 10W can provide good lubrication in cold weather but may be too thin for warm weather or high mileage engines.
If the oil level is OK, the next thing to check would probably be the oil pressure sending unit. Disconnect the unit and check the warning lamp or gauge reading. If the warning light remains on with the sending unit disconnected, thereís probably a short to ground in the warning lamp circuit. Likewise, if thereís no change in a gauge reading, the problem is in the instrumentation, not the engine.
Bad oil pressure sending units are fairly common. But if the warning lamp is still on after replacing the unit, guess what? A sending unit has just been replaced unnecessarily. A better approach is to unscrew the sending unit, hook up a pressure gauge to the fitting on the block and take an actual pressure reading with the engine running.
If oil pressure reads low at idle, check for a plugged oil filter. Is the filter OK? Then youíll probably have to drop the oil pan to inspect the oil pump, pickup screen and bearings.
With a crankcase mounted pump, you can remove the pump cover and check the pumpís internal clearances for wear or damage. If the pump appears to be in good condition, that leaves the bearings. Check the clearances on the main bearing closest to the pump (since this has the greatest effect on pressure), and clearances on the furthest rod bearing (since this will show the greatest wear). If the bearings are worn, your customer either needs a new set of bearings, more extensive machine work and/or a rebuilt engine. But donít install any bearings until youíve carefully measured the crankshaft journals with a micrometer. If you find excessive wear, taper, out-of-round or any damage, the crank will also have to be reground or replaced.
Other checks might include camshaft end play, and/or pulling a valve cover or the intake manifold to check the cam bearings and lifters. Remember, excessive clearances or leaks anywhere in the engineís oil supply system can contribute to low oil pressure.
Erratic oil pressure
If the oil pressure seems fine one minute, then suddenly drops, it usually means air is being sucked into the oil pump. This can be caused by a low oil level, an improperly mounted oil pickup (too high in the oil pan), leaks between the pickup and pump, or too much oil in the crankcase. In the case of too much oil in the crankcase, windage from the spinning crankshaft whips the oil into foam. A missing or loose windage tray inside the oil pan may be a contributing factor.
If the oil pressure surges at higher rpms, it may be the result of air being drawn into the pump for any of the just mentioned reasons or a sticking pressure relief valve. Sometimes an engine can run out of oil at higher rpm if the return passages in the head and block are restricted and donít allow the oil to drain back into the pan quickly enough.
Too much oil pressure
Though less common, too much oil pressure can also create problems. The most likely cause here is a stuck relief valve in the oil pump. If the pressure valve fails to open, pressure can reach dangerously high levels and may rupture the oil filter or force out oil gallery plugs. Replace the pump or valve (if separate) to cure this condition.
On 1995-í97 General Motors 2.3L and 2.4L engines, the OEM oil pump relief valve can stick causing pressure to shoot up to 125 psi or higher. GM says pressure should not exceed 85 psi at 3,000 rpms on these engines. GMís fix for the problem is a revised oil pump (p/n 24576315).
Other causes of excessive oil pressure can include a restriction or blockage in the main oil gallery, or a combination of tight bearing clearances, high rpm operation and using an oil viscosity thatís too thick.