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MG MGA - Puzzled by low oil pressure

I have two cars that are showing strange oil pressure symptoms

One is a twin cam, the other is fitted with a balanced MGB 5-bearing engine. Both are displaying the same symptoms: Good oil pressure on starting, and the oil pressure stay good for the first 30 minutes of driving, then steadily reduces. On both cars the idle oil pressure at 750 RPM is around 5psi after forty minutes. Have done an oil change and filter change on both cars without any discernable improvement, using Castol Classic in one and Penrite Classic in the other to see if that has any influence but the problem remains.

Increasing the idle RPM improves pressure to 40 psi at 850 RPM on the B-engine. Compression is good and the engine has only 600km since overhaul, and oil pressure is 50-60 PSI at cruise.

Same for the 30 year old rebuild on the Twin Cam, which is also constantly fouling the plug on Cylinders 1, so more is amiss here....

Any ideas?
Dominic Clancy

Hi Dominic
I had exactly the same problem with my "new" 5-bearing 1950 cc engine a couple of years ago.
The oil pressure was fine after startup but then dropped to around 35 psi when the engine warmed up.

It would still increase up to 50 or so at high revs but would drop to 35 at cruising speeds.

I thought it had developed worn bearings and was beginning to think in terms of a engine rebuild but the engine had only done about 15000 miles.

Then I watched John Twists YouTube video about the oil pressure relief valve and the modification he does to it to increase oil pressure. He fits a washer beneath the assembly, instead of the thick spacer that normally comes with the kit.

My thoughts were that the OPRV would only affect the maximum pressure and not the general running pressure, but I thought it would be worth a try, before an engine strip down.

So I bought an new MGB OPRV from Moss, it was a bit awkward to get to and when I compared the old spring with the new one, it was a little bit shorter.
I fitted Johns washer and went for a drive to see what happened.

Well it worked a treat and the oil pressure now never drops below 60 psi at over 2000 rpm, only dropping lower than this at tickover.

Apparently, the MGB engines oil pressure should run about 5 to 10 psi higher than the MGAs and so, if the problem re-occurs, I may try fitting the thicker spacer

This is the link to Johns video clip.

It has to be worth a go Dominic.

I would imagine that the OPRV on a Twincam is slightly different but again, worth replacing for the same reasons.

Here is the link to John Twists YouTube clip

https://www.youtube.com/watch?v=ZMvqYVlm5jU

Hope it works for you.

Cheers
Colyn
Colyn Firth

These are a couple of pictures of the oprv that came out of the engine.
The biscuit shaped shim is the one that came with the new kit, I followed John Twists advice and fitted a thinner washer, which is less than half the thickness of the shim.

I found it relatively easy to get to the oprv but I realised that my car has the Twincam style access panels which make a big difference to the access problem.

Colyn





Colyn Firth

I reckon you could have worn oil pumps. Good oil pressure until oil heats up and becomes less viscous - pump efficiency decreases.
Mike
Mike Ellsmore

I wondered about oil viscosity too, but that shouldn't change once the oil is warm (after 10 minutes or so the cars are both at operating temp), and an engine with so few km after rebuild should not be suffering from a worn oil pump. I have increased the idle speed a smidgeon as a first step (700 RPM was quite low) and will wait to see what is reported back
Dominic Clancy

Dominic

I had similar issues with my 1958 MGA, say 18 years ago. After driving a bit, oil pressure dropped to around 5 to 10 at idle, after all was warmed up. The car was around 65,000 miles, plus or minus, at that time. I followed the solutions John Twist recommended. I pulled the pan and replaced the oil pump, rod bearings, and all but one thrust washer, that I could not get to with engine in the car and me on my back, looking up with oil dripping downward. I shimmed the new oil pump spring with a washer, as Twist's article recommended. That was about 30,000 miles and oil pressure is 70 psi plus and all has been well. It was a pain to do but almost twenty years later there has been no issues and the car is very dependable.
Bill Haglan

Does the oil heat up as quickly as the coolant?

My 'modern' will display oil temp if I scroll through some of the menus, but I've never really monitored it.
Dave O'Neill 2

This may have been posted twice as I am having some problems submitting a reply to this post.

When I removed the original oprv I noticed that its spring was about 1/8" shorter than the new one. (The new spring has a free length of 3" iirc.)
So I assume that the old spring had weakened a little bit.

It is now over 2 years since I replaced the oprv and the oil pressure has stayed up in the high 60s so far.
And the pressure doesn't drop noticeably when the engine is hot.

So I would suggest you try this before you do anything else.
Cheers
Colyn

Colyn Firth

Couple of comments.

Firstly oil certainly heats up a lot slower than the water. On my Golf it takes at least 3 times as long for the oil to get to running temp. Also you can see it on your A by comparing time to get to running temp compared with time to get to running oil temp.

Interesting Colyn I spent a lot of time looking at low oil pressure on my B engine which incidentally had been rebuilt by the same guy who did yours. No names. I tried all sorts of things including the oprv and oil pump but no significant change. In the end I was reassured that my pressure was ok even if low by the book and only to worry if it changes. Perhaps I will try oprv again after what you say although I don’t fancy fighting with refitting it again!

Paul
Paul Dean

I don’t believe it is an oil pressure relief valve (oprv) problem if the pressure drops after 30 mins running unless the piston is scuffed and jams. This issue is solved by replacing with 9/16” ball bearing. Too much oil pressure just robs engine power and causes wear of oil pump /distributor drive gears.
Dominic, you say after 30 mins it steadily reduces, does it keep falling or does it level out at 5 psi at idle? Assume it is fine when driving >50psi? I am of the Barney school - set the idle at 1000 rpm!
Cheers
Mike
Mike Ellsmore

Have you asked what your friend Chris Betson thinks about the oil pressure issue Dominic?
I would be really interested to hear his opinion.

Colyn
Colyn Firth

Neither of these engines are in my car, the oil pressure just gradually declines as operation temperature goes from 70C to 80C, 35-40 PSI on cruise, 5 PSI on 700 RPM idle
Dominic Clancy

Hi Colyn,

Difficult to diagnose issues over the ether, but it could be a sticky OPV or a bit of dirt caught on the seat of the OPV.

If the pressure hot is 40 psi at 850 rpm then it is unlikely to be oil pump or bearings.

Adding a thinner washer than the one supplied in your photo would reduce the OPV blow off pressure, not increase it. If it did so on yours then I suspect it was the seat sealing that improved.
Chris at Octarine Services

I go for dirt on the RV seat
Art Pearse

Chris,
On the B, oil pressure at 70C is a good 60-ish PSI , but when 80C is reached on cruise is 40-50 ish PSI at 2800 RPM, which is marginally acceptable for the newly rebuilt engine. As soon as the car is allowed to idle, only 5 PSI at 700 RPM, but rises quite dramatically with the small increase in RPM to 850 RPM at idle to 35-40 PSI, but hardly changes from there over the cruising range. When cooled it's fine with good oil pressure until 80C is reached, then the oil pressure drops again. I have checked the temperatures with an IR thermometer gun and believe the twin gauge in the B to be reading correctly. The TC has just had a new twin gauge which I tested before it was fitted (to ensure the pressure gauge wasn't the issue.)

I think the TC has a valve problem, as it is emitting blue smoke on the overrun. Not a job for me.....
Dominic Clancy

Dominic, I would pull the OPV and check it for free movement and seating.

It only takes a little swarf from the newly rebuilt engine to get through the pump and either get trapped in the top oilway to the OPR or get stuck in the seat.

I had a similar issue on a freshly built XPAG in a TC - 10 miles after first start up and the oil pressure dropped right off - very scary as I was driving it at the time. A clean out of the pump relief valve restored normal pressure and huge relief for me!
Chris at Octarine Services

Thanks to Chris for the tips. The B should be back to me at the weekend so I will have a look then.
Dominic Clancy

This thread was discussed between 10/05/2020 and 13/05/2020

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