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MG MGB Technical - Low oil pressure in a 3-main engine

My 1957 MGA coupe has a 3-main, 1798cc, engine. It was thoroughly rebuilt by a trusted MG mechanic about 500 miles ago. Rings, bearings, oil pump, etc . were replaced. So, it’s a low mileage engine that should be in good condition. I don’t suspect worn bearings

The car was out of service for about 2 years while I changed to a 5-speed transmission and installed air conditioning. It is all back together now, and it starts and runs smoothly through all gears.

The problem is the oil pressure. When the car is started cold, pressure quickly climbs to 40 psi. When the engine is hot, it will maintain 40 psi at about 1800 rpm, it never exceeds 40 psi. At idle (1100rpm) the pressure falls to 20 psi and lower as the engine gets to 190-200 degrees F. It has a big cooling fan that keeps the temperature about there even if the AC is on.

Note: I have tested the car’s oil pressure gauge with another new reliable one, and they measure the same: maximum of 40 psi. The car has a new oil filter, new hoses to the oil cooler, and a fresh fill of Castrol 20W-50 oil. I have not flushed the oil cooler.

I can’t recall what the MGA’s oil pressure was before its 2 year sleep, but I’m sure that I’d remember if the pressure was as low as it is now.

Thanks in advance for you thoughts on my problem!
M.E. Whalen

Sounds a little bit like the pressure relief valve has stuck--Might need to get it out for a clean and fit a new spring which may have been overlooked during the overhaul-the spring should be 3" long ,if it's failed/given up and any shorter than 3" it needs replacing--
Also, seeing it has an oil cooler it'd be a good idea to fit a packer between the end of the relief valve spring and it's retaining cap--the spacer measures 12.5mm dia. and 2.5mm thick---later mgb's had these and the really late us spec ones without a cooler had 2

190-200F is fairly hot --what thermostat are you running, you're living on the edge there a bit

willy
William Revit

When checking the relief valve spring look for signs of wear on the spring wire, the sides can wear significantly flat, which reduces the wire sectional area and therefore the strength of the spring. Quite common in engines needing overhaul and not always replaced as it should be.
Paul Walbran

Thanks to everyone for their very helpful comments. I replaced the oil pressure relief valve spring and piston. I used the up-rated spring, which is slightly longer than 3.1 inches.

The oil pressure jumped to 60 psi upon starting. As the engine heated up at idle, the pressure fell to 30 psi but climbed back to 60 psi with throttle.

I took the car out for a drive on a fairly mild day. It was sunny but only about 85 degrees F. The car started up as described above.

I drove about 1.5 miles at speeds of 25-40 mph. The electric cooling fam came on and maintained the engine temperature at 190 F. After a mile or so, the oil pressure fell to a maximum of about 50 psi with throttle and 28 psi at idle.

Out on the highway, I drove 2 miles at 60 mph and in 5th gear. Engine temp steady at 190 degrees F. Oil pressure was around 45-50 psi with throttle. At idle, it fell to 15 psi.

On the 2 miles back, again at 60 mph, engine at 190 degrees F, oil pressure fell to about 30 psi with throttle and 5 psi at idle. I still have a problem.
M.E. Whalen

Sounds better-but
It has the symptoms of running the wrong oil, you're not running synthetic by any chance are you-
What oil have you got in it.
William Revit

As the hot idle pressure hasn't changed with the uprated spring it looks like there just isn't enough pressure in the system.

Castrol 20W/50 should be OK, so not enough back-pressure from the bearings, or not enough flow from the pump i.e. worn or faulty gasket sucking in air.

Assuming a new filter after the rebuild what make, and try replacing it anyway.

If you could put a pressure gauge in the line from the block to the filter that might reveal something, the standard gauge takes it's feed after having been through the filter and run back along the block feeding all the bearings, despite being right next to the filter feed.

What fitting do you have in the block on that filter feed pipe? There is a connection between those two ports as cast and machined and on early engines some oil was allowed to bleed from the pump direct to the galleries. Later engines used adapter 1H922 which is described as 'full flow' i.e. all the oil went via the filter.
paulh4

This thread was discussed between 13/07/2023 and 07/08/2023

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