Oil Pressure Drop After a Period at High Load

Engine is a stock 2010 SOHC Subaru Forester but with non-standard oil cooler (Laminova heat exchanger), sandwich block with relief valve and filter.

Pressure sits at about 75 psi until after about 1 1/2 minutes at full throttle then fluctuates and reduces to around 40 psi. Recovers quickly after load is reduced to around 70 - 80%.

Oil sump is full - further oil is foamed by crank and exits rocker cover breathers. No indication of increasing oil temperature. This is the second Forester SOHC EJ253 with AVLS to behave in this way. Other EJ25s with the same non-standard oil path do not have the problem.

Unsure whether to increase oil pump size as problem may be in cavitation - perhaps larger sump ? Any thoughts would be much appreciated.

Not an engine I've had much, if any, do do with, so generalisations only.

First thing I would be thinking is the oil getting too hot, how were you measuring the temperature?

Second thing would be the oil viscosity and type? Viscosity may fall off faster over the spec' temperature for some than others

Are all the coolers identically plumbed and do they use a thermostat? I was wondering if a sticking thermostat might cause the temperature, and hence pressure variations, or if a thermostat controlled oil path wasn't fully bled and some air was still being bled through?

You mention AVLS - is this issue restricted to that specific engine variant?

I can't see them using a different oil pump (don't know, though) and if they're the same, you would expect consistent behaviour.

Generally, the main difference a larger sump makes is to reduce the rate of temperature change - it would take longer to reach the higher temperature, and longer to lose it, but given time it would do so.

Hello Rupert,

I'd definitely suggest monitoring the temperature and trying a more robust oil if your cooling capacity is marginal.

I have seen oil pressure dropouts due cavitation previously and it its been cause by a number of factors.

You don't mention the speed of the engine, raising the rev limit with a stock oil pump could cause cavitation in which case it best to replace the pump with something designed for a higher speed; the same effects can be had by gearing down the oil pump drive.

Other factors such as lateral load and oil pick up location could be causing oil cavitation, in any case increasing the capacity of the sum will help. you could also increase the oil pressure to give it some more headroom by changing or shimming the PRV spring but this will take some careful consideration with regard to bearing clearances and turbo oil feeds etc.

other solutions to cavitation could be to install an air/oil separator type swirl pot that will allow the air to escape; this is most effective on the scavenge or pick up side but can still be an advantage anywhere in the system.

Its also worth monitoring crankcase pressure as if this becomes positive it will effectively fill the sump with blow by gas which could quite easily make its way into the oil system.

if you're planning on running at high rpm a dry sump system would be the best solution but will require an active AOS or centrifuge if speeds regularly exceed 9000rpm

Thanks for the advice.

The engine oil appears to be staying reasonably cool (190 F) with the temperature probe on the sandwich block which feeds the heat exchanger.

Pressure drop occurs anywhere in the 4000 - 6000 RPM range. Maximum engine load is 700 and there is no pressure drop at 550 or below.

The AVLS is just a two position lobe for one of the intake valves (higher lift and longer duration when on - triggered at quite low settings). However the cam is adjusted by a solenoid supplying oil pressure to a plunger so there may be some oil flow involved.

It seems weird that it always takes about 90 seconds from when full power is applied for the pressure to start dropping and recovery is immediate when the load is reduced.

Could it be a drain-back problem?

Have you tried holding the engine at high rpm, without loading it? That would help identify if it was a level/pump/cavitation problem as it should give a similar drop - if it's steady, might be time to look elsewhere?

What is the coolant temperature doing, and how is the exchanger plumbed into the cooling circuit? Heat transfer is from hot to cold, could there be a case where the oil is actually picking heat up from the coolant?

The coolant temperature increases from 90 to 97 C at full power and reduces to around 93 C running at 75% power.

The heat exchanger is fed by coolant from the radiator en route to the water pump. Oil is fed from the port before the oil filter and (I think) then flows through the filter to the pump and engine. There is a spring loaded ball relief valve between the ports to the heat exchanger which is meant to open if the pressure differential becomes excessive.

Good idea with high RPM and lower load. From what I have seen load is the determining factor but I will definitely check. Thanks.

Checked oil pressure at 75 kpa and 5800 RPM - steady at 75 psi and no drop after extended running.

Pressure drop must be caused by high load but unsure why as this engine and an identical one have exhibited same behaviour from new.

I am also getting lower EGT's from one cylinder (as load increases) and a few misses after a while at full load (despite having re-gapped plugs from 1.1 to .7 mm which seemed to work for a while).

Ignition is by 4 separate LS2 or LS3 coils so might try swapping circuits to see if the lower EGTs can be moved to another cylinder.