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MG MGB Technical - Meandering Oil Pressure

OK - here's a new one for me.

My 68B has had terrific oil pressure - 50-55psi at idle, 60-65 psi running at any speed.

Over the last few drives, the oil pressure starts out as normal, then after about 10-15 miles begins to drop from the normal 60-65 psi to 25 psi. This is most notable when driving in 4th at about 40-45 MPH. If I downshift, it comes right back up, and stays there until I shift back into 4th. If I stop and let it idle, it comes back up to 50psi and stays there.

To make it stranger, we took the B out for a drive yesterday and the pressure behaved as described. Stopped for lunch, and then drove home. The oil pressure on the drive home was 55-65 psi, and didn't cycle at all - it did what it used to do all the time - pegged.

It is a 68B with 61,000 miles, but only 6,500 miles on the bottom of the engine, and about 2,500 miles on the head. I threw a rod, so the block is a rebuilt 5 main block. The head is fully redone with hardened valve seats, new valves, springs, cam etc.

Any ideas - this makes no sense to me.

Thanks!

Rob
Robert Hardy

Rob,

Am I correct that you have an electric gauge? If so, you can do a temporary (or permanent) installation of a mechanical gauge to see if the electric gauge is telling the truth.

Charley
C R Huff

It could also be a sticking pressure relief valve, if a mechanical gauge shows the same thing. Hopefully it isn't the oil pickup coming loose!
Paul Hunt 2010

Robert-
The 1968-1971 models used an electric oil pressure gauge (BMC Part # BHA 4687) powered by an electric transmitter unit (BMC Part # ?, Moss Motors Part # 131-580) that was mounted behind the alternator. This electric transmitter unit is both expensive and is prone to failure in use. This transmitter unit failure is usually characterized by the oil pressure dropping to a low reading, sometimes zero, while driving. When you put a mechanical direct pressure reading gauge on the engine, you will find the problem to be the transmitter unit. MG came to recognize this problem, and chose to return to a mechanical direct pressure gauge system on the North American Market models in August of 1971. Fortunately, this later system, complete with a same-size gauge (BMC Part # BHA 5091 or BHA 5092, Lbs and Kg, respectively), can be readily installed on the 1968-1971 models. However, since your reading returns to normal, I'd opt for first cleaning and then reconnecting all of the connections with CRC QD Electronic Cleaner.
Stephen Strange

You may have overfilled the sump and generated foam by the big ends thrashing the oil. Had this happen on my J**tt*. Then you can be pumping air.
Art Pearse

Robert:
Our friend Stephen Strange is spot on (very correct). But you mentioned something very important. 6500 miles on the short block and 2500 miles on the head. Did you have this MG anomaly before the engine work? If not, then one can hardly attribute the fault to the oil sending unit. I would suspect too much clearance between the cam bearings and the cam shaft. A 1968 B huh? Did the machine shop use that special reamer to ream out the bearings after installation? Did they even replace the cam bearings? This was a common problem with the MGA engines and many US machine shops did not have that special reamer which probably costs $75.00 to $100. bucks (2010 USD).They would opt out for "mic-ing" the cam journals and then reaming the bearings to suit the level of clearance they felt was "adequate". Too much clearance would give very high oil pressure readings on a cold engine and then down to almost nothing once the engine warmed up.
(I speak from experience on this one).
R Murray

This thread was discussed between 05/04/2010 and 09/04/2010

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