Oil Pressure

by Norman Nock, British Car Specialists

Originally published in the Austin-Healey Magazine, July 1994


Big Healey engines in good condition should have 60+ lbs at 3000 RPM and at an idle of 600 RPM the pressure will drop to about 35-40 Ibs. If your idle oil pressure drops to below 20 Ibs, you could have a problem, even if you have 60 Ibs of pressure at 3000 RPM. If the pressure drops rapidly with only a slight decrease of RPM you will have to do something about it now.

Oil pressure can be increased by various ways, even on an old worn-out engine. If you would like to see 90 Ibs at 3000 RPM, it's easy to do—all you have to is add some very thick 90W differential oil to the engine. If you have any reason to suspect someone's doing this to boost the oil pressure, remove the dipstick. The correct type of engine oil will quickly drip off the dipstick. Smell it, differential oil has a very distinctive odor.


If there's regular engine oil in the car and you have a low idle oil pressure look for the obvious. Is your gauge telling you the truth? Have a quality professional test gauge hooked up in parallel with your own gauge on your car. Now go out on a good long road test and see if you really do have low oil pressure or is it only your gauge giving you an incorrect reading. (If we only had an idiot light). If you have checked the oil pressure gauge and it is giving you a correct reading - now what do you do?


High Oil Pressure


Don't think that high pressure is good and higher pressure is better. The problem with oil pressure that's too high (75 lbs and above using a professional test gauge), is that the quantity of oil coming from the rod bearings is of such great quantity that the piston rings can't stop the oil getting into the combustion chamber. The rocker arm bushes and shaft are usually worn on most Big Healeys. The increase in flow of oil from the rocker arms can cause oil to pass down the valve guides and into the combustion chamber.



The proverbial oil leak from the rear of the crankshaft will become a bigger oil leak. Diagnosing high oil pressure is easy when it is known that as the engine RPM is increased so is the speed of the oil pump. The oil pressure will increase in proportion to RPM, and to control maximum pressure a relief valve (Figure 1) is placed in the oil gallery between the oil

pump and the oil filter. This relief valve can be found below the oil filter held in with a cap nut #4. Remove this cap nut #4, spring #2 and release valve #1. The usual cause of high oil pressure is this valve. Look for a spacer that could have been fitted into the release valve #1 or cap nut #4 to increase spring pressure. If nothing is found, replace spring #2 and replace release valve #1 new parts. If release valve #1 is too long, it can block the return of oil to the oil pan. This is unusual but possible.


Low Oil Pressure At Idle (600 RPM)


If you are sure you have low oil pressure, 20 Ibs or lower at 600 RPM, you could have the start of a serious problem that should be diagnosed as soon as possible.


Change your engine oil, remove the oil filter and make sure that the necessary spring plate seal and washer are fitted below the oil filter as per the picture shown in the factory service manual and parts book. Replace the oil filter and recheck your pressure again. If at this time you still have low oil pressure, the cause has to be found.


The diagnosis starts with the easiest first. Check your engine temperature with a thermometer placed into the top of the radiator to check the accuracy of the gauge on the panel. If running temperature is over 200 degrees F, this will cause the oil to become too thin for efficient lubrication of the bearings and will show up as low oil pressure at idle. If temperature is 165 to 185 degrees F, remove the valve cover and run the engine at 2000 RPM. If you have oil spurting up from the rocker arms and splattering all over the carbs, you have found one of the many reasons for low oil pressure. A new rebuilt rocker arm assembly could increase your oil pressure up to 5 Ibs, providing there is nothing else wrong in the lubricating system.


If your car is equipped with an oil cooler this can cause low oil pressure. The friction due to the flow of oil that is now going through extra pipes, hoses and tubes will cause the relief valve to lift on its seat. Fitting a stronger spring in the relief valve will solve the problem, but what we are looking for is low idle oil pressure, so remove your oil cooler lines and return to factory original to diagnose further.


There is no where else to look outside the engine. Now is the time to remove the oil pan and find the cause of your low oil pressure at idle. With the oil pan removed a professional shop will use a specially made steel container that is half full of oil and holds a steady pressure of 20 Ibs. This tool is joined to the oil gallery via a flexible pipe and oil under pressure is applied to the engine's lubricating system. The cause of low oil pressure will now be able to be observed from the various bearings within the engine. Steady streams of oil from any of the bearings will show excessive clearance at that point and the cause of your low pressure. You probably don't have this bearing leak detector so you will have to press on without it.


Remove the oil pump, screen and oil feed pipe (100-4), inspect the oil screen to see if the oil pan has squashed it. If this has happened, it is possible that the pickup on the oil pump is restricting the flow of oil to the pump causing low or no oil pressure. If your oil pan is damaged, it should be removed and repaired. Damage to the oil pan can be seen by a visual inspection by just looking under the car. Strip and clean the oil pump. If there are any swirl marks on the inside of the pump body that can be felt or marks on the impeller, the pump should be replaced. Checking the pump clearance with feeler gauges can only measure the high stops, (like the wear spot on ignition points). If you need to check the condition of a worn oil pump, immerse the lower body in clean solvent and spin the drive gear by hand. Now try it again with a new oil pump and compare.


The oil pipe on the 100-4 from pump to block should be inspected for cracks at the point of contact with the nut.


Remove one rod bearing. If the bearing surface is in good condition, it will have a flat dull grey color and very smooth. If the surface of the bearing is very rough or is showing a bronze color, you have found the reason of your low oil pressure. If the crankshaft is not smooth when you scratch it with your finger- nail, there is only one fix and that is regrind the crankshaft. The same inspection can be made on the main bearing but only remove one cap to inspect the half shell. Do not take off more than one at a time, and do not try to remove the top half shell. You might not get it back again.


Mechanics have used fine emery cloth in the past to give a finish to a worn crank journal, but don't do it. You will not solve your problem only prolong the inevitable. The bearings on the camshaft can't be inspected at this time, but if the crankshaft journals are worn, expect to find worn cam bearings.


Oil Changes


If you have invested money to rebuild your engine or the engine you now have is in good condition, change the oil regularly. The only way to get carbon and other harmful pollutants out of the oil is to drain it. The expense of an engine job compared to the price of an oil change says "change your oil regularly." I change my oil every 1500 miles and the filter every 3000 miles.


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

10 views0 comments

Recent Posts

See All

by Norman Nock, British Car Specialists Originally published in the Austin-Healey Magazine, January 1994 Worn rocker arms have been a problem with six- cylinder Austin-Healeys. This problem shows up i

By Bob Krapp Originally published in the Austin-Healey Magazine When checking individual valve timing on a 100-4, the service manual (pages D4-"Timing" and D26 "Timing Marks") says to set the valve ga

by Kevin Faughnan Originally published in the Austin-Healey Magazine, December 1973 All of our Healeys have reached the point where mechanical repairs are a fact of life. Generally one of the first jo