O-435-1,-11 OIL RELIEF VALVES

[Sam Taber]

A quick look into  O-435 oil Relief valves-

Most engines have one high pressure relief valve  which as the name implies, will prevent an over pressurization of the oil in the engine in case of cold weather starts.  It will stop the oil pressure at a top limit  and bleed off oil as needed to keep the pressure at or below the set limit.   On our 0-435s it is known as the Auxiliary ( AUX) valve  ( also know was High Press Relief valve) and it is mounted at the front end of the right engine case .

The AUX valve works using by a poppet valve held closed by a spring. The pressure oil is pushing against the valve head trying to open it, but the valve won’t open because the spring is holding the valve plunger  closed against the valve seat.  When the oil pressure pushes harder than the spring tension, the valve plunger will be pushed off the valve seat and oil can then flow past the plunger and return to the oil sump. Once enough oil has been release and the pressure drops below the spring tension, the valve plunger will close against the seat once again. This is standard in most piston engines.   Depending on the  “over “pressure  and oil flow amount, this valve can be open a split second and then close again or it may stay off the seat for some time in order to regulate the High pressure limit. Normal operation will have the valve off the seat as long as needed to lower pressure.

This pressure relief valve regulates the HIGH oil pressure on most engines.

The O-435 in our L-5s has two oil pressure valves instead of the normal single valve installed in most engines. In addition to the High pressure valve ,like the one listed above, O-435-1 and -11 have a low pressure valve as well.   Oil is picked up out of the oil pan (sump) and drawn to the oil pump and then to the oil pressure screen.

Pressure oil exiting the oil screen cavity, will travel down stream to the Main relief valve (also known as the low pressure relief valve ). This valve is located in the oil pan bottom ,Rt side.  The function of this valve is to bleed off oil and send it to the oil  cooler and then on to the oil sump.  This valve does not control the high oil pressure relief. It controls the low end of the oil pressure range.    This Low Press valve is designed to open at aprox   50-70 PSI. It only senses oil pressure ,not temperature.

On the O-435-1 the Minimum oil pressure in cruise is 50 PSI. Taxi back RPM or idle allows oil pressure as low as 25 PSI. Normal oil pressure runs between 65-80 PSI

This  Main pressure valve is controlled by a spring #65703and is not adjustable.  Note: I had tried to raise this pressure on my engine by installing a washer under the spring the way many older Lycoming engines do on their relief valve. ( who doesn’t’t like higher oil pressure/:)  What happed was the increased spring tension did increase the pressure  about 10 PSI but the relief valve now had a stronger  action and it created a pressure spike. If this conditions happens it can cause a valve to “chatter” open /close very quickly. This chatter will often create a wiggle or pulse in the oil pressure gauge indication.  The fix is remove the washers and just run the spring.  If the problem still occurs the valve could be sticking or the spring is too long.  Now, if someone has stretched the spring in the past , you will need to replace the spring with a proper one.

If this Main pressure valve was to become stuck <span style=”text-decoration: underline;”>closed</span> or obstructed from in some form,  this would create a high oil pressure situation and then the ‘Auxilary” or “High” pressure relief valve would open to keep from possibly blowing a gasket.

If the oil pressure is reading High at the cockpit gauge , this spring may be too strong.

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