Oil pressure

Technical MGB discussion
Dave Wheatley
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Oil pressure

Post by Dave Wheatley » Thu Apr 18, 2019 6:17 pm

I've done about 1000 miles since I got the car, and the oil pressure has always been 70psi hot running, and 50 psi on tickover. A couple of days ago, I noticed that the running pressure is down to 55ish psi, and tickover is still 50 psi both hot. Any ideas - no funny noises from the engine. Pressure relief valve?
It's one of those cars that as soon as you fix one problem, another starts up!
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

Vic Butler
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Re: Oil pressure

Post by Vic Butler » Thu Apr 18, 2019 7:04 pm

Check the flexible hose from the offside rear of the engine to the bulkhead where it connects to the pipe leading to the gauge. I checked mine last year and found that although it wasn't leaking it wouldn't have been far off and then all the oil is pumped out.
I bought a far superior one to the standard version from Stevson in Birmingham.
It's strange that the pressure has dropped in that way suddenly. It might be the relief valve, not the easiest job to replace.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Paul Scott
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Re: Oil pressure

Post by Paul Scott » Thu Apr 18, 2019 7:39 pm

Hi Dave,
Following your post with interest.

I'm not an expert as this is all new to me too. Can the Oil Filter be your issue? I have seen issues mentioned somewhere before about the valve / flap inside being incorrect. Looking at your posts Oil & Filter changed recently then you had oil issues.

Just a thought it maybe completley outside of the box.

Regards
Paul
1975 MGB Roadster
Webguru for MGB Register

Dave Wheatley
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Re: Oil pressure

Post by Dave Wheatley » Thu Apr 18, 2019 8:30 pm

Thanks Vic and Paul.

The pipe to the gauge looks OK, and is bone dry, so I'll leave that one just now.

Regarding the oil filter, my son who runs a Mk.3 Cortina, said exactly the same as Paul, so I've got another filter coming.
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

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Charles Farran
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Re: Oil pressure

Post by Charles Farran » Thu Apr 18, 2019 9:22 pm

Filter needs to be a GFE 121 or equivalent quality (same height i.e. top of filter should be at least 1/2 inch above oil pipe on filter mount ). I am not sure if this answers your smoke problem out the exhaust unless in the process of your last oil change some oil ways have become blocked/ crud dislodged. As it appears to have come on so quickly possibly not worn valve guides that can creat smoking without obvious oil consumption. Certainly easier to change the oil filter than the pressure valve which might have a tired spring.
Was this the first oil change you did following buying the car?
Cheers, Charles
1980 Roadster

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Peter Cresswell
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Re: Oil pressure

Post by Peter Cresswell » Thu Apr 18, 2019 10:05 pm

In common with the BMC A-Series engine, The B-Series as found in the MGB uses a bullet shaped oil pressure relief valve. This can lead to two problems: 1) it is prone to wear over high mileages, and may have been fitted since the engine was new despite rebuilds and overhauls, and 2) it is prone to getting bits of muck stuck behind it. The first gives rise to the oil pressure slowly reducing, and although many people assume the engine needs a rebuild, unless there are other symptoms to back that up it can be worth trying a new bullet shaped valve first. The second gives a sudden reduction in oil pressure which can be a few psi less or a total loss of pressure.
A small loss of pressure might rectify itself with the washing action of the oil over the seat and the valve.
The size of the debris is likely to be tiny and difficult to see, but a good squirt of brake should remove it. It can be metallic as there are lots of tiny fragments within an engine after a time. Frequent oil changes (every 3000 miles) with a new filter helps prevent this and although I haven't tried one yet, using a magnetic sump plug from a Mini may catch more of the debris.
It is relatively easy to clean/inspect/change if you can get to it. It is located on the left hand side of the engine (under the carbs and exhaust manifold) at the rear of the block, but I've never tried changing one in situ and access from underneath may be easier. There are two large headed plugs and the lower one has a domed head. This is the Oil Pressure Relief valve housing. Undo it slowly - the spring is quite strong so take care it doesn't all fly apart as you come to the end of the thread! The parts can't go into the engine! There are 5 pieces - the dome shaped nut, a sealing washer, the spring, the bullet shaped valve and inside that a packing piece (this may be stuck inside the bullet valve or missing!). Clean and inspect the bullet valve and replace if needed, but only use an OEM part 12H865 (OEM is without a Z suffix) as the aftermarket ones are of variable quality. You can try cleaning the housing in the block with a non-fuffy cloth or a light squirt of brake cleaner. Put is all back together when you are happy. I have found it is not usually necessary to change the spring and to do so may give too much oil pressure. 70psi is fine when running and 45 - 55 at tickover.
A common mod on Minis is to replace the bullet shaped valve with a ball bearing. As the bullet valves have the same part number for the Mini and MGB you can get a suitable ball bearing from Minispares if you want to go down this route, although I don't think there is much wrong with the original bullet valve if in good condition.
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Dave Wheatley
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Re: Oil pressure

Post by Dave Wheatley » Fri Apr 19, 2019 10:24 am

Charles, yes it was the first filter change after acquiring the car. I ran the car last October for a few hundred miles with no problems, layed it up in November, changed the oil and filter during the lay up, recommissioned this month. The oil pressure was fine until a couple of days ago, when it went down (tickover still 50 psi but running down from 70 to 55 psi). Dislodged crud is a distinct possibility, as I suspect that the car had not had an oil change for some years (less than 2,000 miles in 5 years). Recorded miles 27,000!

Peter, thanks for the description of the relief valve. It does seem likely that crud could have kept the valve open slightly. I can't get to my car lift just now, so whether or not to risk using the car as it is?
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

ChargedAutoGT
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Re: Oil pressure

Post by ChargedAutoGT » Fri Apr 19, 2019 11:13 am

The problem is not removing the relief valve, it's getting it back. access is limited and the spring is strong. ideally 2 people, one above and one below. john twist has a vid of a technique but to be honest it's more common sense than anything really. i also have only removed it with engine out. so those comments are anecdotal....

the drop is rather unusual with no immediate thoughts i'm afraid. i have been told that when oil gets past it that the pressure drops a bit as it's not doing it's job as well, but i recall 5psi is more the expectation and your oil is fairly new. what year is your car? the chrome bumper gauges do have an odd behavior sometimes - pulsing around the 55 mark for no apparent reason which disappears. many people have reported that and i have had it myself. perhaps not the gauge itself but the way it's designed/plumbed in.

55psi hot is an oil pressure many would be proud of, but of course the drop is the conundrum. i would have no worries about driving the car but make regular under bonnet checks & notes on numbers so any trends will be noticed. Peter's suggestion may well turn out to be the case - but if it is the cause you may be lucky and find crud makes it's way out again on it's own.

a small enhancement which some consider worthwhile (and i have bought bits to put one in) is a low oil pressure warning light. a T piece in line with gauge with sensor and warning light somewhere. only costs ~£15 in bits and would warn of a catastrophic failure. One guy in the MGOC forums changed oil. Fortunately he idled he car in his garage before noticing a huge pool under the car. filter on cross-threads. but he could just have easily driven down the road.

the other thing to note is that 70psi is about as high as you would ever want it. too high an oil pressure can cause accelerated wear.

G

Dave Wheatley
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Re: Oil pressure

Post by Dave Wheatley » Fri Apr 19, 2019 11:26 am

Ta for the reassurance about using the car, and food for thought re refitting the pressure relief valve.

I'll see what happens when I replace the filter, and another oil change and maybe a dose of flushing oil first.

Marvellous place this!

D.
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

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Charles Farran
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Re: Oil pressure

Post by Charles Farran » Fri Apr 19, 2019 1:13 pm

Dave,
Graham referred to a guy idling his car in the garage & then finding it had dumped oil due to a crossed thread when putting the oil filter on. (You only do it once - i should know & am possibly the guy Graham mentioned)! Luckily i realised pretty quickly when the oil pressure guage started to rise & then dropped like a stone. It took sometime to clean up probably 1/2 gallon + off the garage floor.When i have changed the filter after initial start up i can always top up by 1/2 pint or so once the filer has been recharged with oil & then check levels again after the first run.
Cheers, Charles
1980 Roadster

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