Oil pressure.

[Norton]

Hi.
1964 3.4 «S», MOD.
Starting from cold oil pressure is sky high even at idle, needle off scale in high end. As engine warms up pressure indicates more normal. When hot and idling off scale other end, indicates really nothing, 0 psi. At 3000 rpm a little above 30psi.
Running on a 20/50 «classic oil». ( have read Glyn Ruck’s oil threads, going to put in something else next time )

I suspect the oil pressure relief valve, could it be stuck partially open? If working properly should not get that high pressure when cold and nothing when hot. Owned this car year and half now, last oil change I dismantled the pressure relief valve, it looked ok. I am used to the behaviour of my -52 Chevy. Were oil press is high when cold, deacreases as engine warms never goes down to 0 and not very much variation with rpm once above idle.

Or very worn oil pump? Malfunction pressure sensor or indicator?

Thanks for any input.

Rgds Harald

[Glyn Ruck]

This is going to be trial & error to a degree. The symptoms are not typical of a stuck pressure relief valve unless it is shot & requires complete replacement.

Nevertheless I would remove it and check it's not fouled up.

My major culprit here would be the pressure sender unit on top of the oil filter housing ~ most likely. and/or gauge.

So replace the oil pressure sensor, pull & clean the pressure relief valve & try & borrow a known good gauge.

If that does not work we will talk further but I don't suspect we will have to talk further.

[Norton]

Thanks Glyn.
I dismantled the pressure relief valve and it looked as it should. And I think there is not much that can go wrong there. Maybe spring tension, not very likely unless someone fitted wrong spring.

Didn’t do anything else and have just completed a 70 miles drive on a dual carriage road. Here I was able to hold constant RPM over prolonged time and monitor pressure. Pressure indicates very high when cold, deacreases as temp rise. When engine definitely warm indicates about 32 at 1500 rpm, 35 at 2000 rpm, 38 at 2500, close to 40 at 3000. A bit more above 3000. Restartng warm engine around 12 psi. All quite normal. But it is a little inconsistant. With relation to the numbers above there are a few psi’s variation. Probably within limits for nearly 60 year old instruments. Was a bit jumpy when I saw pressure dropped towards zero last time I drove it. Engine sopped needle goes quite a bit below zero, whatever fhat means..
So I will leave it as is for now.
Harald.

[John Quilter]

Your figures are quite consistent with mine except I get about 20 PSI at idle when hot. And this is all after a very comprehensive engine overhaul. To increase your confidence you might temporarily rig up a known good mechanical gauge and see what you get. Idle pressure is likely quite dependent on idle speed as this can vary between cars according to how it is set.

[Norton]

Thanks, my mind is at rest..
Traveling at lower rpm last trip, I have definitely higher ( indicated ) oil pressure at 1800 rpm than at 2200 rpm. This is the case whether I deacrease down to 2200 or increase up to 2200. Same story with pressure at 1800. But again, it is almost 60 years old. Maybe been like this all the time. Plan to take the sender unit off during the cold months ahead and have a look. And see if I can get hold of an other press indicator.
Harald

[Glyn Ruck]

You have nothing to worry about. The best check of sufficient oil pressure is to remove the oil filler cap hot & if there is plenty of flow at the cams you are fine.

[Norton]

Thanks, I’ll do that.
Harald