An engine that exhibits low oil pressure at an idle or higher rpm has excessive clearances in some part of the engine supplied with oil. The oil pump does not supply pressure as much as volume. This is very similar to a garden hose insofar as when the hose has no nozzle or anything restricting its flow, it has little pressure. Place a nozzle on the hose to restrict the pressure and the result is obvious. The same applies with the oil pump; restrict the flow with tight clearances and the result is high pressure. Minimal oil pressure must be 20 to 25 at an idle and 45 to 50 at or above 2500 rpm.
Oil Galley And Route
The oil is picked up by the pump and sent through the oil galley to the crank and the camshaft and lifters or directly to the head in the case of an overhead cam. The oil enters the crankshaft through the main bearings and exits through the connecting rod bearings. The second level is a galley that supplies oil to the cam bearings and lifter galleys in a conventional engine or to the cam bearings in the head for overhead camshafts. Once the oil has risen to the cylinder head it drains through holes in the cylinder head where it returns to the oil sump.
Determining if the Gauge Reading is Accurate
If the engine displays lower-than-normal oil pressure at an idle and above, the first thing to test is the oil pressure sender. For this, a remote oil pressure gauge will be needed. Remove the oil-sending unit and install the remote gauge and it will either confirm the low oil pressure or show normal pressure, in which case the sender is bad.
Bottom End Search for Pressure Loss
With low oil pressure confirmed, the next step is to examine the bottom end in search of loose rod or main bearings, improperly installed oil pump or a cracked crankshaft. Bearing clearances can be checked using a Plasti-guage, which can be purchased at any auto parts store. All bearings on domestic vehicles have a tolerance of .001 to .003 of an inch. Foreign engines differ in that they have closer tolerances in the range of .0005 to .0010 of an inch. Their respective service manuals need to be consulted for assurances.
Oil Gallies and Top End
The oil gallies should be checked for blockages. Pull the plugs out of the passages and blow air through them. If the air pressure builds and does not pass freely, this could be the problem or a contributor. Look at the oil pressure relief valve. Make sure that it is free and not stuck partly open. If the problem has not yet been found, the pan must be installed and the engine prepared to start again. Pull the valve cover bolts but leave them on for the moment. Start the engine and let it run for a few moments. Remove the valve cover and observe the amount of oil at the rocker arms. If everything was in order in the bottom end, and there is very little oil to the rockers, the lifters or their bores may be defective. The lifters for broken springs or oversized bores. If the engine has an overhead cam the cover should not be removed with the engine running due to oil splash. Look through the oil filler and observe the oil flow. If unable due to a baffle, prepare the cover to come off quickly and once the engine has run for a few moments, shut it off and pull off the valve cover. Note the amount of oil on the cam and flowing into the return holes.
Lifters and Bores
On a conventional engine, if the oil flow was insufficient, the intake manifold must be removed and the lifter bores examined and measured. The lifters should also be examined. If no problem is found, in all likelihood the cam bearings are bad, requiring new bearings.
Foreign Engines with Crank-Mounted Oil Pumps
A foreign engine with a crankshaft nose-installed oil pump must be inspected thoroughly for gasket problems, pressure relief as well as rotor-to-housing clearance or warping. The oil filter is often overlooked as a culprit. Make sure the engine has the correct oil filter. If an oil filter with the dual inner and outer element is used, it can reduce oil pressure due to the fact that the oil filter mounting tube is too long.
Fuel Saturation, Oil Coolers and Remote Filters
Check the oil for fuel saturation due to a fuel problem such as a stuck-open injector, flooding carburetor due to a float level error or needle and seat, just to mention a few. Fuel in the oil seriously compromises viscosity and will drop oil pressure. On a vehicle with remote oil filters, the hose may be collapsed, or if it has an oil cooler, the lines could be collapsed or crimped.
Cracked Crankshaft
Another cause of low oil pressure after a rebuild is a cracked crankshaft. If you replaced the crankshaft in the engine rebuild without having it turned, you may not see a microscopic crack. Generally, if the crankshaft is cracked, oil pressure is fair to good while the engine is cold. Oil pressure drops considerably once the engine comes up to operating temperature.
