Oil pressure relief valve

[k. rogers]

My car now has a modified pre-war PRV, courtesy of Geoff Wills, which now means I can adjust the pressure. However, the pressure fluctuates quite dramatically, even when the engine is warm. Generally it maintains around 20lbs when motoring and 6 - 10 lbs at idle, but for some reason it goes right up to 40 - 60 lbs every now and again with the needle literally oscillating before settling back to 20 lbs again. Any ideas? All I can say is that the feed for the guage is coming off the first banjo at the front of the engine (so closer to the pump) instead of off the PRV as designed. I have plans to relocate it to come off the PRV as Geoff has incorporated a suitable outlet in the machining.

[Keith Clements]

Ken,
Is the gauge after the pump and before the relief valve or is it after the relief valve?
We had a similar issue in the Javelin when traversing the USA. We had the PRV out at least three times and polished it to get rid of any ridges in the piston. The issue went away when we changed the PRV. Like any piston in a sleeve it can stick in one position. It may be because it is worn rather than tight. The spring also may be catching or locking up. STP or the like might cure or mask it and save you a disassemble.
I do not know the construction but is the piston hollow( cylindrical) or solid? If the former, the spring may be catching on the lip, so chamfering the latter might help.
The bypass hole may have a burr and catch on the piston or spring.
If too much oil is getting past the piston that may equalise the pressure either side of the piston and thus lock it out. This is more likely to occur when the oil is thin and hot. Make sure the bypass hole is well open behind the piston when the piston is fully out so that any pressure is relieved, also that it is wide enough to evacuate the full output of the pump.
The oscillation may well be caused by a pressure wave (similar to hammer in domestic water systems). Whenever a valve closes such a wave is created. The spring and piston will react to this wave when it returns from coming back from the pump and can be in phase thus causing the oscillation. Putting a different spring in would alter the harmony. This is why some heads have double valve springs so as to reduce valve bounce at a certain speeds.

[k. rogers]

Thanks for all that, Keith. I am going to relocate it at the relief valve which is further from the pump than its current position in the hope that sorts it out, otherwise you have given me some leads in what to check for!

[Keith Clements]

Note the PRV on the JAV/JUP is within 2 cm of the pump. If it is longer than this you could be setting up a standing wave between the two. The short distance means any oscillation will be high frequency and low amplitude. A second spring inside the other may be the answer.
It could be the gauge is oscillating as it contains a sort of bellows to move the pointer. Usually a small gauge pipe feeding it reduces any oscillation because of friction in the narrow tube. Some systems have a reservoir (sort of like a silencer) to dampen the oscillation. You can often see this on the vacuum advance take off from the carb.

[Dhbangham]

I have a slightly different take on this valve issue having needed to sort it out on the roadside in the south of France. (just showing off here).
I was loosing lots of oil from the glands around the cable connection and screw thread. I looked long and hard at the piston and springs and decided that it was not a pressure valve but a stopper to make sure that the oil in the system did not drain when not running and to ensure a "head of pressure" ready when the engine is started. I think that the oil pressure is actually set by the size of the hold from the feed tube into the valve cylinder. When oil starts to build pressure the piston pops up and the fluid circulates. I therefore suggest that the spring strength is not very important and does not adjust the pressure.
Any thoughts?