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MG MGB Technical - MGB Oil Pressure Gauge

I have a 1966 Mk1 mgb roadster which I have been restoring slowly and have just had the engine fully rebuilt and fitted it. When I start the engine the Oil Pressure Gauge reads 70-75 and when I rev the engine it goes up to 80 then back to 70 on tic over. I have been told it should be 25-30 on tic over but mine stays on 70 and was told not to drive the car till it is sorted. Can someone help fix the problem as it is the only thing stopping me from driving it and I am missing this lovely weather.
J B FOWLE

In my experience, the oil pressure in a good MGB engine is around 70 psi running hot, with about 40 psi idling hot. It should not be able to get above 75 psi even when cold because the pressure relief valve should blow open and restrict the maximum pressure. There's a good description of how it all works on this John Twist video http://www.youtube.com/watch?v=a7mVVvWnR34

If your pressure is too high the two things you need to question are (A) is the gauge accurate? and (B) is the relief valve stuck closed? I think I would start with the pressure relief valve and renew the spring and plunger, cost about £5. Having said that getting the thing screwed back into the block is not an easy task as you are trying to engage the threads while pushing hard against a strong spring.

Paul Hunt has this advice on his web site http://www.mgb-stuff.org.uk/enginetext.htm#relief

Mike Howlett

We tend to get high pressure on a new engine. If it doesn't settle to less than 75/80 max we shorten the relief valve spring to bring pressure down. We tend to get high oil pressure at idle with the ball bearing we use.
Pressure tends to be higher on a new engine and then reduce with use. Why will 70 at idle cause anything more than a loss of power to provide the pressure? I cannot see why you have been told not to drive it.
Peter
Peter Burgess Tuning

JB,

I think that the 25-30 figure is a minimum rather than a maximum. You did not say if this figure is after being fully warmed up, but since you say you are afraid to drive it, I suspect you are giving figures for cold oil. It should come down when fully warmed up after driving for maybe 12 miles or so. What oil are you running; 20-50?

Charley
C R Huff

I don't think I've ever heard of anyone being concerned by high oil pressure before. And who on earth told you not to drive it? On the basis of that don't ask them anything again, and ignore anything they may say.

Cold start and running will be 60 to 80. 25 to 40 is a typical hot idle, although the WSM says it can be as low as 10 to 25. From a cold start it will only gradually drop to your normal idle figure. Hot running should be about 60 to 80 i.e. a little less than cold idle.
paulh4

Hi, Many thanks for your replies. I have had the car running on 2 or 3 occasions since and the temperature has been around 70-75. When the car is really hot it goes down to 60. I leave the car running and it doesn't go below 60. Everywhere I look it states it should ne 25-30. I will check the pressure relief valve but it should be ok because a new spring was put in it when the engine was built.
J B FOWLE

The oil pressure should not be 25/30 at idle. It should be not less than!
Peter
Peter Burgess Tuning

I would be VERY disappointed if one of my engines had an oil pressure as low as 30 psi at hot idle.

50 to 70 psi is the normal range for a fresh rebuild and it should only vary by 10 - 20 psi from cold to hot.

Chris at Octarine Services

As far as hot idling goes, there is 'idling' and 'idling'.

As far as mine are concerned the roadster idles at about 50 initially, but if left long enough will gradually drop, especially in hot weather, but I've never seen it below 25.

For the V8 its initially about 20, but can go below 10. as Roger Parker has said about the V8 - "What hot idle pressure?".

I put this drop down to lack of airflow through the oil cooler, especially with the underslung cooler on the V8. But also because at a standstill hot air from the engine compartment can only exit downwards, and a significant portion of this exits forward to be recycled through the radiator and cooler when at a standstill. This isn't theory, I have sen it happening from the heat haze and measured it. If there is a slight following breeze the effect increases greatly. Some years ago I was on a Bath to Bournemouth run in hot weather that included an interview at the end. This involved waiting a queue for a significant length of time, people started panicking about temperature and pressure, and the interviews had to be abandoned.

There is no point in checking the pressure relief valve from JBs figures, that would normally only be done if the pressure were low, or excessively high, and his are neither.
paulh4

One word of advice. Check the pressure gauge against another gauge. even a cheap modern one.

Oil pressure gauges can get little particles in them that can affect the reading.

70/40 is what I get in my rebuilt BGT with about 3000 miles on it.
Bruce Cunha

This thread was discussed between 05/08/2018 and 22/08/2018

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