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MG MGB Technical - Another Oil Pressure Question

I am wondering whether the behaviour of my oil pressure is a symptom of the gauge or the engine.
Basically when started from cold having been sat in the garage overnight, the needle takes several seconds to display any pressure. When it does show pressure it speedily reaches around 55psi and stays there. Since the cold weather started (1-2 Celsius) it drops down to 40psi as soon as I start driving down the road and this lasts for around 10 minutes until the engine starts to get warm. It then picks back up to 55psi and stays up there all the time I am on the move. At idle when warm, pressure varies between 25 and 45psi dependent on how hot the engine is.
Admittedly the engine is old, but it isn't burning masses of oil and I think the pressure during running is not worryingly low.
My gauge is a little damaged in the sense that the glass did fall out at some point, and on replacement the dial it is a little off-centre as I couldn't get it to sit quite right. So there may be some pressure on the pointer shaft that may affect the readings. Are the gauges fragile in the sense that I may have damaged the return spring in some way. Can they lose accuracy over time? If the gauge connecting pipe becomes blocked does that cause unusual readings?
I guess my initial lack of oil pressure after a period of standing may be an engine issue. I have read that the oil filter is a key part of this. I am using a unipart filter, and I think that the engine has been modified from the old cartridge to the newer type (Standard 1800 '71 B)

Any comments or advice are appreciated.

Thanks,

Phil


Phil B

It certainly shouldn't take 'several seconds' for the oil pressure to register overnight, maybe the drain-back valve on your oil filter has failed or is non-existant, not all filters have them, Unipart should. 55 immediately is OK although possibly 5psi on the low side, as is 25-45 when hot. However I have not heard of it dropping then rising again during warm-up. There is a known situation with the pressure relief valve which can flutter under certain conditions giving a slight drop and a rise again as you raise the revs which is not a problem, I suppose yours could be doing the same with temperature changes. Along with what else you say I'd get a second opinion on the gauge by trying another one. If that also shows a several second delay in oil pressure overnight I'd change the filter. There is no return spring as such, the gauge contains a bent tube that tries to straighten under pressure so moving a pointer, returning to its original shape when the pressure is released. A completely blocked pipe would show no reading of course, a badly blocked pipe would show a slow rise (not the same as a long time to show any pressure at all) if full of oil but probably little difference if it contained air. You can check the tube by disconnecting the gauge and running the engine, you should get a decent trickle. I'd blow the oil out of the tube before reconnecting the gauge as it being full of oil will make it slower acting then full of air.
Paul Hunt

USA 71 MGB's have an electric oil pressure gauge, are 71 UK cars fitted with a mechanical gauge? Clifton
Clifton Gordon

UK cars had mechanical oil gauges throughout, the temp changed from mechanical to electric in 76.
Paul Hunt

Phil

The early standard V8's suffered badly from delayed oil pressure registering on the gauge, and that's why the take off was moved from the remote filter housing to the oil pump. The funny thing is that fluid is not compressible so the logic of delay is that there is air somewhere in the system. Are you using 20/50???
The early fall to 40 psi and return to 55 when hot sort of implies that there's a viscosity effect in there somewhere.
Change the oil & filter, & try a more modern grade if you're on 20/50.
You can generally hear oil starvation on start up by a mechanical clatter or knock which disappears instantaneously as the oil gets there. If you don't sense that I wouldn't worry about it.
Dave
Dave Wellings

"logic of delay is that there is air somewhere in the system"

Can't see that, 60psi at one end of a column of air is 60psi at the other end. But a restriction in the pipe will limit how quickly oil can pass through it, as opposed to air, due to its much higher viscosity especially when cold. Get a long enough pipe full of oil and you can have 60psi of pressure at one end and nothing at the other, you would need amuch longer pipe for the same thing to happen in air. That is why pipelines from reservoirs to tap have to have a certain amount of 'fall' to deliver any pressure, if they were at the same height you would get very little out of the tap. The capilliary tube is also very narrow, and especially long on the V8. Mine is on the oil pump but is still slow to rise (and equally slow to fall when cold) but not slow in *starting* to rise, that is the critical difference. A long time before it *starts* to rise is potentially bad.
Paul Hunt

Phil

as Dave Wellings says, if you are not getting knocking on start up I wdn't worry - interestingly, recently I overhauled an engine including a complete cleanout of all the galleries, and now get the same symptoms as u despite several filter changes - I have concluded that what happens now is that the oil drains from the galleries, and thus the pump has to build pressure in them before it shows on the gauge, whereas before the galleries and the bleeds to mains and cam bearings were so gunged up that the galleries stayed full !

Can't be sure about this but it seems the only logical explanation - altho I have had on other cars the same experience as paul with filter draindown being the explanation - but in those cases a clear rattle/ knock was evident on cold start

Chris
chrsi

Chris-
Your deduction that "the galleries and the bleeds to mains and cam bearings were so gunged up that the galleries stayed full" implies that the gung was functioning as an anti-backflow valve. If this were to be the case, then the gung would prevent oil from reaching those main and cam bearings!

Phil-
I doubt that there's a problem with your oil pressure gauge. I suspect that you may have more than one problem. You said that "when started from cold having been sat in the garage overnight, the needle takes several seconds to display any pressure." This sounds like the classic symptom of a case of a bad backflow valve in your oil filter. The oil drains out of the filter and the oil pump has to fill it and flow onwards to the bearing supply gallery. During this period it is purging the system of air and the gauge will read little if any pressure. This accounts for your delay in oil pressure reading. You should hear a knocking or rattling from the engine immediately upon firing.

The fact that you achieve 55 psi oil pressure at idle, then observe it vary thereafter could be accounted for by crud between the piston of oil pressure relief valve and its seat. The function of this relief valve is to regulate the oil pressure inside the engine. If your pressure is fluctuating after warmup, it's your most likely suspect.

So, what to do? First, replace the oil filter with one that has a reputation for high quality (I don't know if the K&N oil filter, K&N Part # HP2004, is available in the UK). That should remedy the slow oil pressure buildup at start up. Your bearings will thank you. Second, you can either remove the oil pressure relief valve and clean it and its seat (a real swine of a job), or you can run the risk inherent in flushing the engine. Before considering the second option, remove your rocker arm cover and examine the top of the head. If you see lots of crud, flushing is very risky. During flushing crud may be flushed into a passage and block it, starving the bearing surfaces that it feeds. If you see crud, don't flush the engine. Instead, clean the oil pressure relief valve.
Steve S.

Phil, whilst all of the above may be true, I have an MGA that has a 1600cc B series engine that displays similar traits to yours. Because I do not like spending too much cash I tend to make everything myself and the oil pressure capiliary pipe is no exception. It is very small in diameter and during the winter the car is in a very cold garage, thus when I start the engine the oil pressure takes time to rise and only reaches 40 psi even when revved, Then after a while the pressure will increase to 60 or so. The reason for this is very simple, I am using Duckhams 20/50 oil and when it is cold it becomes very heavy and sticky, thus the pressure drop across the oil pipe/capiliary is very high. Although the oil in circulation within the engine may warm pretty quickly the oil pipe itself does not and relies upon the car being warm to make it flow better at which time ofcourse the pressure will increase at the guage. I do not have the problem of oil running back out of the filter because I am using the early MGB filter system with the filter in an upright position. The main point I suppose is to listen to the engine upon start up, if the oil pressure is not there instantly then the engine would rattle until pressure was established.
Bob

I would echo Bob's comments. When *cold* my 66 BGT only reaches about 45-50psi on start up before rising to 60psi when warm (at around 3000rpm). There isn't this delay during the warmer summer months.

I use the early paper filter element in a downward pointing housing - I changed from the upward pointing housing fitted by the PO because the time taken to get any oil pressure reading was about 15s. It now takes about 5-7s.

I would like to try fitting an oil thermostat to speed up the time it takes for the oil to reach operating temperature in the winter (around 20 miles average driving I was advised)
Richard Atkinson

Thanks for all the comments and advice.

If I remember correctly, this is the second Unipart filter purchased from an MG garage that I have used which gave similar behaviour. Perhaps it is related to the anti-return mechanism not behaving properly, although I haven't heard the engine clatter excessively on startup. This seems to be a cheap and simple item to replace and see what the results are. I take the point that 20/50 is probably quite viscous when cold which could be adding to the problem. If I remember correctly in the summer months the speed at which pressure built up was quicker.

So I shall try the filter and then look at comparing the pressure against another gauge to see if mine is damaged in any way. If I still have problems then, it looks like I will have to do some more investigation.


Phil B

This thread was discussed between 12/12/2002 and 16/12/2002

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