V8-250 oil pressure relief valve

[DaimlerDave]

Hi all. I have decided to try and improve my hot low idle oil pressure which is about 10-15 psi at 600 RPM. I have changed the big end bearings and I am replacing the oil pump relief valve. I was surprised to find the existing valve has a small hole through the centre which would allow oil to pass through even when closed under spring pressure. The new David Manners one does not have such hole. Has anyone found the same or has a theory. Dave

[John-B]

I had my engine restored by Phil Glennerster and I've been surprised that the idle oil pressure is 30 psi when hot, 45 when cold. I don't know what he did, but it's about 45 when revving hard.

Phil has retired, but I expect he would be prepared to talk to you. Tel. 0118 9340927 (day) or 01344 884071 (evenings)

[Ian Slade]

If you have new bearings and the crank is/was within tolerance playing about with the relief valve is a fudge, you need a new or fully refurbished oil pump, often the problem is a worn casing or cover and the oil bypasses within the pump over the gears, a loss of 5-10 psi on an engine with 30-40k on with a good pump is acceptable.

[Fossil]

During the rebuild of my engine I was intrigued to find a small 1/4" thick nut under the oil pump relief valve spring. I remember being told when I bought the car in 1981 that the engine had an "uprated" oil pump, and this was I think the secret modification! However I believe that this simple trick will boost the oil pressure by around 10 psi at idle, and it comes highly recommended, so without wishing to reveal my sources or precipitate another long discussion about oil pumps and pressures, this seemed to me to be a potentially beneficial thing to do. So I put the little nut in to the refurbished pump from BT and put it back in the engine. I've also done the same thing to my Century engine, because the relief valve on that is conveniently accessible on the outside of the engine block (how clever is that?).

The SP engine, having done only a tad over 1000 miles since the rebuild, now idles at over 40 psi hot, while the Century with a factory recon engine fitted in the 70s an unknown number of miles ago, idles at 30 to 35 psi hot, at least 10 psi more than before the mod.

I'm thinking mainly about long queues of traffic in hot English summer weather, which I do remember experiencing "back in the day" but certainly not lately, when a few extra pounds pressure of oil might be nice for ones bearings, perhaps?

Cheers

Geoff

[Phillmore]

I'm probably missing something here so forgive me but I thought that the relief valve was set to "release" high pressure when the oil is cold and thick. When hot and thin and with a standard relief valve spring it shouldn't be doing anything except keeping the valve in place. I don't know what pressure the spring is set to open the valve but it wouldn't be in the 10 to 30 range I would have thought?

[silverdart]

After many thousands of miles, the oil pump gears may have worn so that the delivery of the pump is diminished.
This simple mod. to the relief spring pressure, restores and improves the delivery.
Russ Carpenter told me many years ago that he fits a 3/16 in. washer behind the spring.
This certainly works for me.

Dave.

[Phillmore]

I still think the relief spring pressure is only relevant at high initial pressures. If it's relieving pressure at low hot tick over pressures the spring is too weak anyway. I'm not sure what Daimler relief spring pressure settings are but on my A90 Westminster it is 55 psi. Cold oil pressure can be in excess of 60 psi so the spring is set to relieve potentially damaging pressure in the system on cold start up. Once the pressure is below 55 psi the valve will remain shut however low the pressure gets and fitting a stronger spring or packing the spring won't make any difference.

That's how I understand the system anyway but maybe I'm missing something?

[Phillmore]

Going back to the original post I don't know the purpose of the hole in the relief valve but if it goes all the way through I would think it would be the cause of the low oil pressure unless it is a secondary oil pressure bypass system.

[A.N.Other]

philmore, could it be that the thinner warm oil escapes through the valve seat due to an old spring being slightly shortened in length.
The spacer may increase the pressure of the valve on the valve seat and thus not allow seepage which in turn creates a higher gauge reading.
Only a guess as I am no expert.

[Phillmore]

Yes Colin it could be that, in which case a new valve rather than spring is probably what's needed, but I'm no expert either. In theory the valve should seal on its seat perfectly and not let any oil past until the oil pressure pushes it off its seat against spring pressure.