Re: '34 718 Std 8, 2/4 Coupe, Engine & Drive Train Diagram(s)

Posted by Owen_Dyneto On 2016/5/22 22:22:59

Given the right combination of conditions (engine clearances, oil viscosity and temperature, engine speed) the oil pump on your engine can produce oil pressures that are harmful, possible damage includes perforation of the oil filter, the oil cooler heat exchanger, fracture of the main bearing oil manifold, and even rupture of an oil gallery in the cast aluminum crankcase. To prevent this possibility the oil pump output is passed thru a pressure regulator (relief valve) to keep the maximum pressure within acceptable limits (55-60 psi maximum). The regulator is simply a piston within a bore, loaded against a spring whose tension is adjustable by the screw located underneath the large acorn nut on the relief valve. If the pressure overcomes the tension of the spring, the piston moves to uncover a passage whereby a portion of the flow is dumped back into the crankcase, thus reducing the pressure.

Remember, the valve adjustment sets a "not to exceed" pressure. Highest pressure normally occurs with a cold engine on start-up, and with increased engine speed. If under these conditions your gauge shows more than 60 psi you should adjust by loosening the adjusting screw, thus lessening the spring pressure on the piston.

Normal pressures vary a bit over the life of that engine but are typically at least 3 psi at hot idle, and 35 or more psi at road speed with the correct viscosity oil for the ambient temperatures (see owner's manual). If your pressures are much less than this, it could be from just a worn and tired engine but also could be from either a broken or mis-adjusted relief valve. If by increasing the spring tension adjustment (tightening the screw) your pressures increase, the valve is incorrectly adjusted - continue adjusting until your max pressure (cold engine, high engine speed) is regulated to the 55-60 psi max. Problems with the relief valve itself are pretty much unheard of but you can remove the valve assembly itself to inspect and make sure the piston moves freely and the spring is unbroken. You don't remove the entire unit from the engine block, just the valve assembly unit accessed beneath the large acorn nut.