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MG MGB Technical - Oil pressure inconsistency

6,000 miles in from fitting a new engine to my '67 MGB, I've suddenly started getting very varied oil pressure readings from the guage. At normal motorway speed, it will be registering 60psi as usual, but at idle it can drop as low as 15psi. At around 2500rpm it registers 40psi. If I sit and give the engine a good rev it can fly up at 100psi. I've done an oil change and replaced the filter - any suggestions as to why this should suddenly be happening?
Thanks in advance of any help.
Martin
Martin Port

Before tearing anything apart get a known accurate oil pressure gauge and check the oil pressure, your gauge could be ready to die. Could also be a bad oil pressure relief valve.
John H

The guage is relatively new and had been working fine since I put it in. From memory, the oil pressure relief valve is a 'challenging' one to get to, but I think I may even have a spare one in the workshop. If I access the current valve though, what am I looking for, or should I just put another one straight in as a matter of course?
Martin Port

Does sound like pressure relief problems - sticking in the bore. This may be only when hot with differential rates of expansion, look for rub-marks on the piston and relieve them a bit with fine wet and dry. Before replacing the valve and spring just fit the cap on its own a half-turn, mark the block, then slowly undo the cap until the threads just clear each other, then put a matchmark on the cap. Makes it much easier to get the cap started when you are fighting the spring - position the cap matchmark just ahead of the block one, press down, turn the matchmarks past each other, and you should be away.
Paul Hunt 2

Good advice Paul - thanks. From experience is this job easier done from beneath the car or by removing the carbs and air filters and attacking it from the side? If I can get away with it I'd prefer not to remove the manifold.
Martin Port

I just had a simular problem with my pressure gauge. Engine with only 500 miles after rebuild.

Did the oil change, head retorque, valve lash, timing and started it up. Things looked good and took her for a ride.

About 25 minutes into the run, the oil pressure started to fluxuate somewhat wildly between 45 and 80 PSI. Before, my oil pressure was rock solid between 70 and 80 on my gauge depending on idle or revs etc. (I have the dual gaude direct feed oil pressure gauge and temp like on the 67s ) I'm not sure what the true pressure is.

I went back home to look. Did not notice any thing unusual until about 10 minutes later when I had a small puddle of oil under the car. That was a little odd as no engine leaks yet after rebuild. I thought she might just be marking her territory.

I took her for another run the next day and the same thing. Pressure fluxuations, bring her home and a small puddle of oil. I felt around the oil connections to the block (that was just above the puddle)It had oil on it. Got out the talc and put some on the block and stainless steel oil line. Started the engine and oil flowed from a small hole in the oil pressure gauge braded line.

So take a look maybe you have developed a small leak.

Cris
Cris DeYoung

This thread was discussed on 25/07/2006

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