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Understanding Oil Pressure

Topics covered in this article: Types of Pumps, Understanding Oil Pressure, Low Pressure, Troubleshooting Low Pressure, Erratic Pressure, and To Much Oil Pressure.

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. 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. What's more, oil also serves as a hydraulic fluid inside hydraulic lifters.

With so many important jobs to do, it's obvious that the engine needs a good oil pump to deliver a steady supply of oil. But how do we know if the bearings and other moving parts are getting enough oil? Since there's no simple way of measuring oil flow inside an engine, we look at oil pressure as a means of gauging oil flow and lubrication.

Oddly enough, an oil pump doesn't create oil pressure. All the pump does is displace oil and push it into the oil galleys so it can flow to the bearings and upper valvetrain. What actually creates the oil pressure is the resistance the oil encounters as it circulates through the engine.

Types of Oil Pumps

There are three basic types of oil pumps:

  • Twin gear pumps: also called "external" pumps, use a pair of intermeshing gears to pump oil. One gear is driven by a shaft, and the second gear is driven by the first gear. The pump is usually driven by a shaft that connects to the crankshaft, camshaft or distributor shaft. Thus, the pump operates at half engine rpm. The pump gears turn in opposite directions. This traps oil between the gear teeth and carries it around the outside of each gear from the pickup tube inlet to the pump outlet. The tight clearances between the gears prevents the oil from flowing backwards to the inlet.

  • Rotor pumps: also called "gerotor" pumps, have an inner gear that turns inside an outer rotor. The inner gear has one less lobe than the outer rotor. The inner gear is also mounted slightly off-center to the outer rotor which forces the outer rotor to spin at about 80 percent of the speed of the inner gear. This creates a bellows-like pumping action that pulls oil from the inlet port and pushes it toward the outlet port. Close tolerances are required for good pumping efficiency. This type of pump may also be located in the crankcase.

  • Front cover pumps: also called "internal/external pumps" are usually located in the front engine cover. This is also a rotor style pump with an inner drive gear and outer rotor, but the inner gear is mounted directly on the crankshaft. The direct drive approach eliminates the need for a separate pump drive shaft. This type of pump turns at the same rpm as the engine, so it generates more pressure at idle and does a better job of sucking oil from the crankcase and getting it to the upper valvetrain in a hurry. That's why front cover pumps are used on many overhead cam engines. When this type of pump becomes worn, it isn't always necessary to replace the entire cover assembly - provided the pump housing inside the cover isn't worn or damaged.  A new drive gear can be mounted on the crankshaft and a new rotor installed in the cover to rejuvenate the pump.

With all three types of pumps, wear and damage are major concerns. Wear that increases internal clearances between the gears, rotor and housing will reduce the amount of oil the pump displaces and cause a drop in oil pressure and delivery volume. That's why high mileage oil pumps often have to be replaced. The oil pump is the only internal engine component that runs on unfiltered oil. The screen in the crankcase pickup tube will only keep out the big pieces of debris, not the micron sized particles that can scour away metal over a long period of time. Consequently, the oil pump can be easily damaged by debris that ends up in the crankcase. Pump seizure caused by debris as small as .010? can cause a complete engine failure by starving the engine for oil. "High volume" oil pumps are another variety common in the aftermarket.

High volume pumps typically have longer gear sets to displace more oil. A high volume oil pump may flow 20 to 25 percent more oil than a stock pump to increase oil pressure at idle and to compensate for increased bearing clearances and wear in a high mileage engine. But a high volume oil pump is no cure-all for worn bearings or sloppy bearing clearances. What about "high pressure" oil pumps. These have a stiffer relief valve that doesn't open until a higher pressure is reached (75 psi or higher). This type of pump can provide additional oil pressure at high rpm when the pump is working hard, but it won't have any effect on idle pressure when the pump is turning slowly. Most engine rebuilder's replace the oil pump because they don't want to jeopardize engine reliability by reusing an old pump. Another item that should always be replaced (but sometimes isn't) is the pickup tube and screen. Swishing an old pickup in some solvent may only loosen the crud inside that's accumulated over the years, allowing it to be sucked into the new pump as soon as the engine is started. Since there's no way to see inside some pickup tubes, the safest bet is to install a new one.

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 engine's 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 oil's 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 oil's 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 pump's 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 isn't 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) don't pump enough oil to overcome the relief valve spring. The relief valve generally only comes into play at higher rpm's when the pump's 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 the engine returns to idle or a lower rpm. Most manufacturers recommend a minimum of 10 psi of oil pressure for every 1,000 rpm of engine speed. Using these numbers, most stock engines can get by with 50 to 60 psi of oil pressure. But some engines have different requirements, so always follow the manufacturer's recommendations.

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 won't come on until the oil pressure is really low, like 4 to 5 psi. If the warning isn't 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 a long silence after the bearings seize. Low oil pressure is bad news because it usually means the engine isn't getting enough oil, and that can lead to serious trouble very quickly for the rod and main bearings, and upper valvetrain components (especially overhead camshafts). For this reason, any indication of low oil pressure should not be ignored and should be diagnosed immediately. Low oil pressure can be caused by a variety of things:

  • Excessive main and rod bearing clearances.
  • Worn main and rod bearings - the most common cause in high mileage engines, or engines that have been poorly maintained (infrequent oil and filter changes).
  • Worn crankshaft journals.
  • Worn camshaft bearings or excessive bearing clearances.
  • Assembled rod, main or cam bearing clearances that are too great.
  • Worn oil pump or excessive clearances inside the pump.
  • Oil pump relief valve stuck open.
  • Low oil level in the oil pan - usually due to oil leaks or oil burning (worn valve guides, valve guide seals, piston rings, cylinders).
  • Too much oil in the oil pan - causes aeration and foaming.
  • Plugged oil filter.
  • Obstructions in oil pump pickup screen.
  • Loose fitting oil pickup tube.
  • Improperly installed oil pump pickup (too high in oil pan).
  • Leaks between the oil pump and engine block.
  • Cracks in oil pump housing.
  • Cracks or leaks in oil galleries or gallery plugs.
  • Oil viscosity too thin for hot weather.
  • Oil viscosity too thick for cold weather.

If a freshly built engine has unusually low oil pressure, it probably means there's too much clearance in the bearings. Only about .001 of extra clearance in the main bearings can reduce oil pressure up to 20 percent! Sloppy bearing clearances also create more oil splash inside the crankcase, which can overwhelm the rings and cause the engine to use oil. Too much bearing clearance will also increase engine noise and pounding, which will eventually lead to bearing fatigue and failure.

Troubleshooting Low Oil Pressure

To catch oil pressure problems before the engine goes out the door, many engine builders will spin the engine to make sure it is developing oil pressure, compression and vacuum. It's also a good way to make sure the oil system is fully primed before the engine is started. 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, the faulty gaskets or seals will have to be replaced. Common installation mistakes that can cause gaskets to leak or fail prematurely include over tightening the fasteners and crushing the gaskets, using sealers or RTV on molded rubber gaskets (makes them slip out of place), reusing valve covers and oil pans with damaged or warped sealing lips and using cheap, ill-fitting gaskets or ones that can't hold up. 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 okay, 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, there's probably a short to ground in the warning lamp circuit. Likewise, if there's 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? You just replaced a sending unit 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 you'll 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 pump's 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 in a newly built engine appear to be worn or damaged, you may be dealing with an oil contamination problem (leftover machining residue inside the block or oil pan). Don't install any new bearings until you've carefully inspected the oil filter and inside of the oil pan for contaminants. Also, you should re-measure the crankshaft journals with a micrometer to check for dimensional accuracy and damage. 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 engine's 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 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 rpm's, 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 don't allow the oil to drain back into the pan quickly enough.

To 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. 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 that's too thick.

Article written by: Larry Carley, Engine Builder, November 2001




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