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MG MGB Technical - Oil pressure

I have a 67 mark 1 B which I have recently purchased. When I start up at cold and let it run at idle oil pressure is 40lb.Raise it to 2000rpm and oil pressure drops to 20lb but regains 40psi when dropped back to idle. After it warmms up it holds 40psi approx at 3500rpm.It seems to it is starving for oil when it is cold. I am running 20/50. Is it possible that the sump straineris blocked or is the oil pump tired.Haved talked to anmg specialist here in NZ and he could shed no light on the problem although apart from this he thought the engine is quite healthy.
R J Pemberton

As it is recently purchased I would change the oil and filter and get a second gauge to verify the readings.

It sounds a bit low at 40 psi at 3500. IIRC there is a pressure relief valve on the back top of the block. If you remove this it is a bu**er to get back but the spring can get tired. Flushing oil might de-crud the strainer (check the sump does not have a big dent in it as this can affect the strainer). After that I guess it is sump off time
Michael Beswick

Thanks for comment Michael. I forgot to say I have changed the oil and filter.Also a new relief valve and spring have been fitted. The old spring was 3/8in shorter than the new one. The job wasnt to bad once I completely removed the manifolds from the car and marked the spot were the cap would start threading in
Rod
R J Pemberton

! check the height of the oil filter against the length go the filter housing tube. It has been known for the tube to be far too close to the base of a short filter, partially starving the system. A situation which improves marginally as the oil warms up.
2. Also the oil pressure relief valve spring often has a shim which gets lost or is not noticed on re-assmbly.
Allan Reeling

Does sound like starvation. Many years ago there was a case of starvation at high speed, that turned out to be the pickup-strainer too close to the sump, and he was getting cavitation. The person involved did some measurements on oil flow rate, and found the pump shifts a huge amount, most of which goes through the relief valve. Has the floor of the sump been pushed up? Maybe someone jacked it up there without spreading the load.
paulh4

! check the height of the oil filter against the length of the filter housing tube. It has been known for the tube to be far too close to the base of a short filter, partially starving the system. A situation which improves marginally as the oil warms up.
2. Also the oil pressure relief valve spring often has a shim which gets lost or is not noticed on re-assmbly.
Allan Reeling

A '67 shouldn't have a spin-on filter, unless it's been changed.
Dave O'Neill 2

I have fitted another head with large inlet valves and hardened seats. During the fitting I noticed a lot of coke on the old head and around the tappets under the side plates. Wondering if this is blocking the strainer. Oil pressure problems were there before the head change. Oil pressure has improved with the fitting of a new relief valve assembly when hot.
R J Pemberton

Do you have an oil cooler? If so it may be partially blocked.

Symptoms are low oil pressure , when cold, increasing as the engine warms up.

Herb
H J Adler

Rod
Has it had a spin on oil filter conversion
It's a known fault of some conversions, as the base doesn't have a filter bypass valve and you really have to make sure you have a filter with an internal bypass
The problem is that the filter gets full pressure but won't allow enough flow when the oil is cold, causing low oil pressure from the filter onwards until the oil gets warm and thins enough to flow through the filter properly
I fitted two op gauges on my racer just to see what was happening
Cold it had 100psi before and 10psi after the filter
Warm both gauges had 70psi
Fitted a filter with a bypass and had
cold 90 before and 85 after the filter
hot was still both 70
hope this is usefull to you

willy

Also with the op relief valve, they have a .1" (hundred though)spacer inside the cap to preload the spring---------is it there, they get overlooked ---
William Revit

Thanks Herb and Willy. I will check the oil cooler. Also Willy I am still on standard oil filter. The reason is it has a big capacity even though it is a b.... to fit. I purchased a new oil relief spring and valve. I didnt see any shim washer. When I had the old relief valve in I could see the oil pressure guage pulsing as it opened and shut. It has done this since the new one has been fitted
Rod
R J Pemberton

Oil gauges were modified with a sintered iron plug at one point to try and control fluttering, but it still can happen depending on the exact characteristics of your oil, pressure, relief valve, temperature and so on.
paulh4

Rod
If you are going to drive it hard, it's fairly normal practice to double up on the relief valve shim as well giving a total thickness of .2" (two hundred thou)
willy
William Revit

Update. I put engine flush in the oil last night and let the motor idle for 40 min after which I drained and refilled with good oil. I now have 40psi pressure cold and at idle. The pressure doesnt drop when I increase engine speed. I am now off to get a new oil filter as the old one I am sure is block. My theory is that the oil pump was scavenging due to a restriction on the intake side. I suspect coke around the sump strainer as other parts of the engine seemed
To be coated with the stuff.
Rod
R J Pemberton

This thread was discussed between 08/06/2017 and 14/06/2017

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