Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - low oil pressure

I have a question about oil pressure. I have a 1970 MG that was running at about70lbs oil pressure when the gauge would suddenly drop to 20lbs. It would then stay at that level and would increase when the car dropped to idle - almost the reverse of what one might expect.It then reverted to 70 lbs. It then dropped again. I ordered a new oil pressure switch, fitted it and found it made no difference. In fact, the car is only running now at a measured 10 lbs. Could an electrical ground problem cause this situation or are there any other solutions?

Mike, Try another oil filter. You don't say what filter you have but try a different brand. There have been several low oil pressure posts in the past year and a different oil filter solved some of the pressure problems. I would stay away from any filter made by Champion, STP and Walmart filters to name two.
I think the filters may be collapsing inside, Champion uses a simple plastic inner support tube that doesn't look very durable. I have cut lots different filters open for inspection recently and there are some differences in their construction.

Clifton Gordon


Sounds like a good reason to change the oil filter! I'll try it tomorrow and hope for the best. I'll post the results. Thanks for the advice. Cheers, Mike


Treat the car to a K&N HP 2004, you'll notice a difference.

I changed to the K&N HP2004 on my 73 MGB and noticed an immediate reduction in the time to register oil pressure on start up.

B.J. Quartermaine

I tried changing the oil filter - Oil pressure jumped up to 100 lbs then quickly dropped to zero. I cleaned all connections in the engine bay that seemed to be conected to the switch and still got a reading of zero.

Any other suggestions?


Mike- I believe the 1970 mgb roadster sees the oil pressure mechanically first to the switch, then electrically from there to the gauge. Try blowing out the line from the back of the switch to the block . Same to the gauge head, even though its new. The swings you are representing suggest a mobile blockade. Sounds like the filter is a dead horse. Next shot might want to be the byepass spring ( broken or fouled) and oil pressure valve side block- below the #4 exhaust. I'm interested in your findings as I had a heck of a scenario on my 1980 MGB, which finally came down to relapping the bullet shaped pressure valve to the block seat. Just unbelieveable afterall, but it fixed the damned low pressure for once and all! Good luck. Vic
vem myers

Mike. The first thing to do is to fit a positive (direct) gauge to the oil pressure outlet to the block and see what you really have. If the oil pressure is good, the electrical sending unit, oil pressure gauge, or a wiring problem is the cause of your readings.

If the positive gauge confirms the dash gauge, you need to look at the oil pressure relief valve on left side, lower rear, of your engine. A weak spring or a plunger which is jambing will cause what you are seeing.

Les Bengtson

Thanks guys for the advice. I don't presently have a gauge to measure pressure mechanically and I can purchase a new pressure relief kit including valve, spring etc for about $10. So I might fit the new kit and see what happens. Before I do that I might try to blow out the oil line as Vic suggested.

As a matter of interest - where would i purchase a mechanical pressure gauge and what sort of price am I looking at? I do have a compression tester and am assuming it would be similar.

mike thorne

You could use the gauge of the compression tester if you wish and provided you can make up an adaptor. I read this thread early on and whilst I didn't post I had exactly the same thoughts as Les Bengtson. I too am fairly sure it is the relief valve.
Iain MacKintosh


I was able to test the oil pressure with the compression tester on my Fiat Spider. I'll try to make up an adapter. The only down side is that it makes abit of a mess of the compression tester.
Thanks for the advice, Mike Thorne
M J Thorne

Mike, I suggested the oil filter because it's an easy change. It's strange the symptoms changed when you replaced the oil filter. I wonder if the filter you bought is bad? I believe I would put the old filter back on to see if the original problem returns. If it does I would try another oil filter. I know that sounds unreal, but it's possible. I would also bypass the cooler to rule out it causing the problem, highly unlikely. You'll understand why I'm suggesting the easy things if you do have to change the oil pressure relief valve with the engine in place. As Iain and Les mentioned, I think you will have to change it. I didn't find it a fun job when I had to do one a few years ago.

Clifton Gordon

Hi Mike,
You could use any gage. The simplest might be a well pump gage, which you should be able to get at most (small town?) hardware stores. They are not that expensive, and you won't mess up your compression tester. Also a compressed air gage should work, I went that route in my MGA. Do you have the large electric oil pressure sending unit? I know a guy that has a '69 B and this unit went bad on his with symptoms such as yours, low to 0 oil pressure reading, occurring suddenly.
I think they were only used for a few years and were known to be troublesome. The
best thing is to get a reading with a reliable gage and go from there.


I unscrewwed the gauge off the top of my compression tester and screwed it into position in place of the the oil pressure switch to test pressure mecanicaly. Good news - it recorded 65-70 lbs - what i was used to seeing upon start up when cold. I guess that means my oil pressure is fine but the problem is electrical.
Any guesses where I should start,
keeping in mind I have already tried installing a new oil pressure switch. Could the new switch be bad? Also, the new switch is not the same size as the original and is a delco type supplied by an import parts supplier in Canada. Any other thoughts?
Thanks for all the help so far! At least it seems my engine is probably not going to sieze up anytime soon!! Touch wood!
mike thorne

I do have the larger oil pressure sending unit/switch. After discovering my oil pressure was okay I thought I would checking the sending unit/switch again. I took it off blew it out with air, gave it a shot of wd40 and screwed it back on and started the motor. Low and behold a reading of 50 lbs!! I then thought i would try cleaning it(the sending unit) more completely and again screwed it off - only this time a small amount of oil leaked from the opening. Is this normal? this is the first time that I've take the switch off and oil came out. THE PROBLEM SEEMS TO BE THE SENDING UNIT. SHOULD I TRY TO REPLACE THE SENDING UNIT WITH A DIRECT REPLACEMENT(SAME TYPE,SIZE) ? ANY THOUGHTS WELCOME.

mike thorne

Mike, To use your electric gauge I believe you will have to use the correct sender. In the Moss catalog they list for $136.95. I tried a generic sender on a Jensen Interceptor a few years ago and it wouldn't work.

I would convert it to a mechanical gauge. If you have a used parts place near you that has MGB parts they may have everything you need. eBay is another source. Parts you will need are; Oil pressure gauge from a 72-76 MGB. You can use your existing flex hose. The Moss parts you will need; 435-515, leather washer; 435-475 pipe assembly; 435-530, connector, pipe to flex line. All these parts are shown on page 94 of current catalog. You may be able to find these parts, except for the leather washer, from a used source. If you find used parts the connector with the anti-run-on valve switch will work. You can remove the anti-run-on valve switch and plug the hole. I see a distance tube is shown, I did a conversion a few years ago and don't recall any problem using the existing hardware from the old gauge. If you don't object to non standard parts you should be able to adapt a generic plastic or copper line and fittings to the flex hose or the block fitting and run it to the gauge. You would still need a gauge.

Clifton Gordon

Oil coming out of the sender fitting after you disconnect it shouldn't be a problem. Changing to the later gage is one way to go. The later gages are the Bourdon Tube type, and are nearly unbreakable. Sometimes a piece of dirt in the copper tube that connects it to the engine oil gallery will cause a problem, but I've never experienced that. The later gage looks identical to the one you have now, only the connections will be different.
If you want to stay original, get the correct sender from Moss and you should be OK. Good Luck

Mike- Per my suggestion above on examining the pressure byepass valve. It comes apart ez, but can be tough to re-thread that 'ol retaining nut ( 1" I believe). Assuming you've inspected the spring ( I recall the stock spring is 3", and the high pressure spring a bit longer and stiffer), disc between the spring and valve ,bullet valve, and removed any debris found, the replace of the nut is best done, I've found, with a scrap 2 x 4 on the fender/chassis and a flat bar ( Wonder Bar) levering the nut to the threads to get it going. Take a close look at the valve where it seals to the block seat. If the surface is buggerred, you'll get byepass and oil pressure swings like you've reported. Vic
vem myers

As Clifton suggested, get the setup from a 72 to 76 car. I have had several of the elctrical units go south. They are expensive and a new one may or may not last very long. I have had the exact same symptoms as you. Both my 70B and 69CGT have been converted to the 72 to 76 mechnical system. I have never had a pressure issue since. IMHO it is the best way to go.

Ron Smith

I have had a similar problem with oil pressure drop on my recently rebuilt MGB GT race car.

Indicated oil pressure cold is only 4 bar 60 PSI) , and after a few laps it goes down to 3 bar (45 PSI). The Pressue guage is a VDO electrical unit, and I run a 13 row oil cooler with Castrol Futron.

I have spaced up the pressure relief spring, but this has not had any positive effect.

initially I thought the cause could have been air entrainment from oil surge, but the car has a dropped and baffled sump.

Any comment from racers with B engines would be welcome.
Franco Resca

45psi at what revs? The book quotes a hot idle of 17 to 25 and running of 50 to 80. It also quotes the relief valve operates at 70psi, so unless you at least reach this packing the spring will have no effect. I have a Celica with electric sender and this registers about mid-way cold, and barely off the end-stop at hot idle, which concerned me. The handbook is no consolation as it shows a huge range of both hot idle and hot running positions. I temporarily hooked up a mechanical gauge and found plenty of pressure both hot and cold, running and idle, the electric sender is giving a very non-linear gauge movement, or at least not registering until there are 10 to 15psi in the system. It also is very slow to rise, whereas the mechanical gauge instantly registers. Try a mechanical gauge.

Some years ago Barney Gaylord was experiencing oil pressure loss at high rpms which he eventually discovered (with a masterly bit of diagnosis) was due to the pickup being too close to the sump bottom and simply being unable to pick up the volume of oil the pump was demanding.
Paul Hunt 2

This thread was discussed between 12/11/2005 and 26/11/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.