Oil Pressure Relief Valve Problem

[pantmawr45]

1964 series 1 3.8 FHC and
Hi all,
I have been struggling with low oil pressure getting 15-18psi on idle and 30-35psi at 2,500 rpm when hot. If I remove the OPRV hose and run the engine on idle, hot or cold, oil gushes out of the OPRV (into a container). This suggested to me that the OPRV was opening well below 20psi so I put a new spring and valve in and there was no difference. Next I removed the oil filter housing and had the valve seat machined by about 0.5 mm and replaced it all along with a new gasket (the right way round) - still no difference, oil was gushing out of the OPRV on idle!
It has always been like this since I purchased the car not that long ago and the previous owner sugested that it had always been like this and that it was just the dial giving a low reading. The engine was rebuilt in 2011 with new bearings and oil pump etc and has done less than 8,000 miles since.
Am I right in assuming that if oil is exiting the OPRV in this way, it must be faulty or is there another cause?
I am a bit lost as to what to do next, any suggestions would be very much appreciated.

[mgcjag]

First thing to do is test oil pressure with a wet guage

The electrical senders and guages are notorious for incorrect readings...you could take off the filter houseing and have a good look at it...is it correct.....has it been damaged modified....test the valve opening pressure with compressed air...Steve

[MarekH]

...This suggested to me that the OPRV was opening well below 20psi ...

No it doesn't.

I have both a wet gauge and and the standard gauge on my car and they read not too much different to what you describe.... only the mechanical gauge caps out at 70psi immediately after startup and it idles at ~50psi when hot. Steve's advice s good. Cross check with a mechanical gauge.

kind regards
Marek

[malcolm]

Yes, I had a mechanical guage fitted because the normal sender was so misleading. It was a dream to see 70 on start up, 40 at idle and 50 plus at anything above 2000 revs - and to know it was correct (Checked against a workshop pro guage and read identical)

[pantmawr45]

Hi, quick update.

I decided to check my sender and guage with compressed air so I connected my air line to the sender unit and gave it exactly 60psi after connecting it via an extension wire and an earth. The results were no different to when the engine was running at around 2,500rpm hot, around 28psi. If I increased it to 80psi the result was the same.

This to me suggested it was the gauge and or sender at fault


Air line feed to the sender unit set up


28psi reading when the air pressure was either 60psi or 80psi

I have now fitted a mechanical guage from Quality Classics in Ludlow for £175. Great bit of kit looks very similar to the original, was fairly easy to fit and is now giving me an accurate reading, 30psi at idle and 55psi at 2,500 rpm when hot.


New mechanical oil pressure gauge and sender from Classic Quality in Ludlow

I must say that I was always, rightly or wrongly, under the impression that the OPRV remains shut until the pressure hits a certain limit and at all other times remains closed but it appears that it opens as soon as the engine starts and progressively continues to open as the revs increase. This is why at idle, if you disconnect the OPRV hose, oil gushes out even at idle, and is normal. It is only fully open at high revs when at maximum pressure.

So *clamping the OPRV hose at idle to see if the pressure increases will give you an indication if the OPRV is working correctly - if there is an increase in the reading on the gauge it would indicate that the valve and or spring is at fault and if there is no increase it would suggest it is the gauge and or the sender at fault.

*Don't clamp the hose completely as this will result in a split hose and oil everywhere!

This is my understanding, please correct me if I am wrong!

Many thanks for your help and comments.

[mgcjag]

The relief valve is purely mechanical...it opens as far as im aware around 45-50 psi....so on a cold engine start oil will be thicker so pressure higher....so typically the valve will open....Steve

[MarekH]

It is only fully open at high revs when at maximum pressure.

It's a mechanical spring - it won't care about revs.

My impression is that it takes 60-70psi to push t open. That can be immediately at idle thanks to thicker cold oil or later with hotter (thinner) oil if enough is being pumped.

It's good to see the mechanical gauge working. If it is T'd into the oil gallery, then it gives you the reassurance that the pressure is being correctly measured where it needs to be measured. The old "rule of thumb" is that you are hoping for 10psi per 1000rpm.

kind regards
Marek

[pantmawr45]

I appreciate that the OPRV is mechanical and "care's not about revs" but I suppose the point I was trying to make is that if, as I've read in several places that it opens at 50, 60 or even 70psi, why does mine open at idle when hot (30psi), and when cold (45psi)?

At higher revs the oil pump will be pumping a higher volume of oil and therefore at a higher pressure. Is this correct or am I being a bit stupid?

Should I be concerned about mine opening at 30psi even though I am getting good pressure in all instances?

Cheers Jon

[mgcjag]

As I see it the valve only works on pressure....however hot/cold expansion could affect a valve operating correctly......it shouldn't be opening at 30psi....Steve...ps I would be setting up some pressure testing with the air line to check it