Oil Pressure Diagnostics

by Roger Moment

Originally published in the Austin-Healey Magazine, January 1989

Low oil pressure gauge readings indicate potential for the possibility of serious engine repairs, ranging from renewing the rocker shaft and bushes to replacement of bearings (and the usually associated regrind of the crankshaft and other major work). Lower-than-expected pressure on a rebuilt engine, however, raises other questions, such as whether the crank and bearing sizes were indeed correctly matched, or the possibility that some other unknown gremlin lurks within. There is a way to use basic fluid physics and the gauge readings to isolate the problem in either case, and perhaps avoid wasted time and expense from an unnecessary engine tear-down.

The Lubrication System

Fluids are incompressible, and in an engine lubrication system there is an interrelationship among four basic elements: 1) flow (or output from the oil pump), 2) oil viscosity, 3) leaks (such as from wom bearings or bushings) and 4) oil pressure. A simple way to visualize the oiling system is shown in Figure 1. The pump takes oil from the sump and sends it into a long tube, which represents the passageways throughout the engine, including the crank, oil filter, rocker shaft, etc. The tube has a number of "leaks" in it representing the bearing clearances which, because they are not zero, allow some seepage back into the sump as the oil performs its lubricating functions.

As the pump forces oil into the tube, pressure is zero until the rate of flow in is greater than the leakage rate out. From that point on, the pressure will build up in proportion to the pump output. Note that as the pressure builds the flow rate through the "leaks" still increases too. The thinner the oil, the faster it will flow through the leaks, so as temperature rises, the lower the steady state pressure will be, for a constant flow rate from the pump into the tube.

Output from the oil pump goes up as the pump speed increases, so flow into the pipe will increase with engine speed. The size of the "leak" openings will stay essentially constant with temperature, so the rate of oil flow from them will depend inversely on viscosity, which goes down as temperature goes up. (While multi-viscosity oils reduce the amount of this decrease, nevertheless your oil still gets thinner and flows faster through bearing clearances as the engine temperature rises.)

Using this rather simple model, one would expect oil pressure to go up with engine RPM, and the oil pressure gauge to react to engine speed accordingly, Now to establish better control over engine oil pressure, a relief valve is incorporated into the system, located just below the oil filter on the engine block, which opens the passageway to allow more oil to "leak" from our tube in case the pressure builds too high. This valve is simple in operation-a spring-loaded cone presses into a hole drilled into the oilway. When the pressure gets to a pre-determined level, it overcomes the spring pressure and pushes the cone back from its seat, allowing oil to leak out and drain back into the sump. The higher the oil pressure, the farther back the valve cone is pushed, and the more oil "leaks" back into the sump.

A subtle aspect of this design on 3000 engines is shown in Figure 2. The cone sits like a piston in a cylinder, and has a short section of reduced diameter machined on the outside, as shown. When the cone moves back, oil passes past the face, and then past this step to drain back through the side hole into the sump. This means that there is some restriction around the valve cone which also will limit the oil flow, even after it passes through the valve seat, and this can affect the measured oil pressure, as we shall see. This shows why it is important for the relief valve in 3000s to have this machined step on the cone, for without it there is less room for oil to pass, and this will prevent the valve from doing its job properly, resulting in very high pressures, particularly when the engine is cold. The 100-4 valve doesn't have this relieved feature but oil flow past the valve cone is accommodated by location of the drain-back hole.

Operation of the System

So the system consists of: 1) the oil pump, whose output goes up with engine RPM, 2) the passageways, with "leaks" through which the oil will flow faster as temperature rises, and 3) the relief valve, which is set to allow excess oil to escape from the system once the pressure reaches a set level. Using that description, let us now see what the oil pressure should read under a variety of situations.

Figure 3 is a plot of oil pressure vs engine RPM. At idle the pressure is low, because the pump output at low RPM is not great enough to overcome all the bearing clearance leaks. As engine speed is increased, the oil pressure will correspondingly rise until it is great enough to force the relief valve open. The oil pressure will remain essentially constant, rising very slightly as engine speed is increased, up to a point where the flow rate through the relief valve is so high that it cannot be fully accommodated, because of the limited clearances, at which time the oil pressure will again continue to rise noticeably with RPM.

When an engine is cold, the relief valve will open at the preset pressure, but because of the clearances through this valve, and the high oil viscosity, it is possible for the controlled pressure to read higher than it will when the oil is hot. This explains why a cold engine may show 40 psi on idle and 70 psi at speed, but these pressures will drop to 20 psi and 50 psi for similar RPMs after it has reached stable operating temperature.


Now we can look at different pressure gauge behaviors, and Interpret them in terms of what might be the cause. According to the shop manuals, the control oil pressure point for the 100-4 and 100-6 engines is 55-60 psi, with an idle pressure of 25-30 psi. The 3000 cars use a gear-type oil pump, which has a lower output flow, so the control pressure is also indicated lower at 50 psi at speed with 20 psi at idle.

If the oil pressure remains very low after the engine reaches operating temperature (pressure at idle below 10 psi and at running speed below 20 psi), the engine may simply not have enough oil.

First check the sump level, then check the oil filter assembly to make sure it has all the correct parts. People have unknowingly blocked off flow through the filter by incorrectly assembling it, and this cuts off oil flow, resulting in very low measured pressure.

If, after the engine reaches operating temperature, the oil pressure rises and falls with engine RPM in the operating range of engine speeds, then the control valve pressure is not being reached, and pressure is really being determined by flow from the pump working against losses through bearing clearances. In this case, one would expect to see pressures lower than 50 psi. Excessive bearing clearances, a worn oil pump or a non-sealing relief valve are indicated.

Here is a simple chart for troubleshooting oil pressure problems:



Idle pressure below 10 psi and running pressure under 20 psi

Sump oil level. Also check that oil filter assembly has all correct parts and is correctly assembled.

Idle pressure below 15 psi and driving pressure varying from 20 to 40 psi with engine RPM in range from 2000-3500 RPM. Control pressure not reached.

Oil pump clearances—see shop manual for specifications. Rocker shaft wear, engine bearings. Use of a heavier oil might help, but don't expect miracles.

Idle pressure of 15 psi and control pressure reached around 2500 PRM or higher.

Same as above, but wear should not be as great. Heavier oil might be needed.

Idle pressure of 20 psi and control pressure reached at 1500 RPM, but value of this pressure is less than 50 psi.

Oil control valve spring. Check that control valve cone is not getting hung up on a burr, and that valve seat edge is smooth, assuring proper sealing.

It is easiest to first check the relief valve to see that the correct parts are in place and the valve cone seating area is not damaged. Next check the rocker shaft for wear, and renew it and the rocker bushings if necessary, before doing the rest of the engine. You can also check engine bearing clearances by draining the sump, dropping the pan, and using plastigage to check rod bearing clearances. The oil pump can be removed at this time and checked for clearances against the specifications listed in the shop manual.

There are a number of variations of this low oil pressure condition, depending on the RPM where the control valve appears to start operating-as indicated by essentially constant pressure as engine RPM is increased. An engine in good condition should reach steady oil pressure by 1500 RPM. In the worst case, the pressure might increase with engine speed from near nothing at idle to 50 psi at 4000 RPM, and indicate only 30 psi or so at normal highway speeds. Intermediate degrees of wear would result in reaching the control pressure at an engine speed somewhere between 2000and 4000 RPM.

But what does it mean if the engine maintains a controlled oil pressure of less than 50 psi over a range of engine RPMs, but the pressure can be made to increase only if engine speed is raised to the higher end of the operating range (such as 4000 RPM or more)? The controlled oil pressure over a range of RPM indicates that the relief valve is operating, and doing its job of limiting pressure. The oil pump is putting out more flow than is "leaking" through the bearing clearances. If the indicated pressure is less than the specification, then either the gauge could be reading low OR the relief valve spring is weak, allowing the valve to open at too Iowa pressure. The increase at higher RPMs results from pump flow being too high for even the relief valve to accommodate without presenting some restriction, as discussed above. There are a number of specialists who can check the gauge, but remember that to remove it will require draining the cooling system and carefully taking out the temperature gauge sending unit and connecting tube. New replacement relief valve springs are available from a number of sources, and the stiffness can also be increased by inserting shims inside the cap-nut that holds the relief valve spring in place.

If the pressure reads well over 55 psi (say 65-80 psi) after the engine has warmed up, the problem could be 1) the gauge reads incorrectly high, 2) the relief valve spring is too stiff, or 3) the relief valve doesn't have the required machine step (in the case of 3000 engines), oil pressure could be well over 80 psi and capable of blowing out gaskets, galley plugs, or other seals.

With an understanding of the engine lubrication system, components, and basic principles of fluids and pressures, it is possible to use pressure gauge readings (as a function of engine RPM and operating temperature) to diagnose probable sources of trouble and save unnecessary repair expenses.

Roger Moment of Boulder, Colorado is an active member and officer of the Rocky Mountain Austin-Healey Club. an expert on the technical facets of the Healeys, a member of the concours committee, and the owner of two pristine Healeys, a 100-4 BN2 and a 3000 B17 which have been featured in these pages. His article continues with a commentary by Geoff Healey on oil pressure in the land speed record cars.

Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.


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