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MG MGB Technical - Quick question about oil pressure

hi,
I know there are tons of shreads about this but could't get a clear figure of the oil pressure that we should get.
Mine is running when cold ar around 50 and when a bit hot then it can drop to 30-40 and at 30 I can feel it's not that right (probably because she's overheating..).
thanks!
Alexia
Alexia Krug

If those are at idle that would be OK. Should jump by around 20 psi at 3k revs. If those are at revs then the engine is a bit worn and may need to start thinking about the possible causes and fixes. I always think of 30-50 psi at idle and 50-80 psi at 3k revs.

Iain
I D Cameron

OK Iain, well these figures are when the engine is running, at idle I've got 50psi most of the time and then it's dropping to 35-40 when/after running for a while. I never got 50-80 !!!?? but maybe I have to try to check so as my engine has 28 000 miles (don't know if had done already 100 000 but as it has been on the island since 1990 after being in UK)and hadn't not been driven the last couple of years before I bought it....thanks
Alexia

The oil pressure that is important is when the engine is hot and at full operating temperature so the cold pressure is largely irrelevant except for providing some diagnostic information. From memory the BMC manual states 20/40 at idle say 800 rpm and 50/70 running which seems to be 2500rpm + We make a huge fuss about this but its not nearly as important and certainly not down to the last 5psi. Every engine will be different, at different stages of its service life and the clearances when new will all have been different as well so pressures will vary. Only a very few cars had oil pressure guages and most had a warning light which must stay out. That came on at 8psi so as long as it stayed out even if the pressure was just 10psi then everyone was happy. What the running pressure was was a complete unknown.

A just about acceptable figure would be 15psi approx when idling and an absolute minimum of 40 when running at speed. The running figure is the important figure here and 40psi would apply to any car. 30psi is too low to maintain a reliable oil film under load and you should investigate the cause and rectify it sooner rather than later.

Back to your cold figure, 50psi seems about right for a cold idle but how long does it take for the gauge to fall to zero when switched off? An engine with sound bearings will take in the region of 10 - 15 seconds. If it falls pretty well immediately the bearing clearances could well be excessive.
Iain MacKintosh

Alexia:

Your pressure sounds low enough to require attention. However, don't do anything expensive until you check the pressure with another gauge. It could be that the culprit is your gauge.

Jack
J.C. Weidner

Iain, Jack, when switched off my gauge fall to 0 in 1 second ! and when running the idle is not very stable...
and then if i make it checked and if i really have low pressure (and i'm worried also about a tipping sound which could be a O-ring broken .... will investigate this with a mecanic) what would be the procedure? thanks a lot
Alexia

I'm not sure about this "tipping" sound and presume you mean ticking. Is it there all the time, when idling, under load at speed or when off load and at what rpms do you hear it. Neither am I sure what you mean by O ring. I think you mean piston ring. Anyway that's another matter and nothing to do with the oil pressure or lack of it and is only a general indicator of engine wear. When you say the pressure drops in 1 sec, is this a cold engine ? It's a quick drop even if hot but when cold it should take some time to drop. There is a slight chance that the pressure relief valve is stuck open or the oil pump is just generally knackered but it does sound as if your engine has reached rebuild time. Check these points first and by the way can you tell us what your oil consumption is please.
Iain MacKintosh

sorry Iain as I'm french so sometimes my english goes wrong ... well yes i meant ticking and it's all the time even when I just start the engine. About the oil pressure yes it's dropping when cold straight away and when really hot well it's taking (I can't really remember should be quick as well)... about the oil consumption I don't know really but I check nearly every 2 weeks and it doesn't seem to consume anything I still have the oil level required. Please don't tell me it's the engine rebuild time !!
Alexia

You mention overheating, in what way? A mechanical fan car should indicate around N pretty-well all the time, only going a bit above normal when idling for a long time in very warm conditions, which AFAIK we aren't getting in the UK at the moment. An electric fan car will rise to about mid-way between N and H before the fans come on, then drop about half-way back to N before they cut out again. Rubber bumper cars also seem to run a bit warmer on the gauge than chrome bumper cars, for a number of reasons. But you should be able to get the gauge right up to H before experiencing any problems, basically if it isn't steaming or losing coolant it isn't overheating, a rising gauge is just a warning. However if it *is* running hotter than it should this will cause lower oil pressure at idle, at least. As for 'ticking' the MGB is renowned for noisy tappets, even when everything is good. I have had one noisy tappet (or something) for over 15 years and 40k miles. I've tried lots of things to find out what it is to no avail, so I ignore it while everything else is OK, it is a 'feature'.
Paul Hunt

well Paul, about overheating I conclude to this as my oil pressure was decreasing slightly when driving it more and more and I saw on the MGB Workshop book that could be one of the reasons fo the drop in oil pressure. My gauge is not rising dramatically ( I don't know yet if I can trust my gauges for oil and water temp.). Mine is a chrome bumper and that would be great if this noise is not something bad.
I have to do some proper tests this we about my oil pressure I guess.
Alexia

Do you have the electric type of oil pressure guage? That gauge will drop to zero as the key is turned off. Another factor that was not mentioned is the oil pressure relief valve, when my car goes above N, the valve opens and the oil pressure will drop 15 to 20 lbs. Once the engine cools the valve closes and pressure increases. Engine re-built with a little over 1,000 miles, oil pressure at idle 50 at 2500rpm 70 cool, oil pressure at N no change, oil pressure above N, idle 35 at 2500rpm 55. That is with an inacurate electric gauge. Oil usage would be more of a concern with a worn engine.
Lee Miller

Forget the oil relief valve part, I dont know what I was thinking when typing.
Lee Miller

I was getting confused as well but we all have these senior moments. Your gauge may not be accurate but we knoe that it does read up to 50 lbs so at least we have something to compare with. The readings are still far too low when hot. Lee's point about the electric gauge is a very valid one so please tell us if it is electric or a hydraulic type. If it is hydraulic then we are back to the relief valve, oil pump or it's bottom end rebuild time at least. Sorry Alexia
Iain MacKintosh

ok first apparently I have a mechanical pressure as "('68 to '71) with an electric oil pressure and a '72 to '80 mechanical oil pressure gauge ) and have found on internet : The MGAs and MGB s should run down the road with 60- 80 psi of oil pressure. Readings below 60 are cause for concern, as oil pressure is an indication of the condition of the engine. We can help you raise your oil pressure by following this gradual list:
Change the oil and filter. The oil is sometimes contaminated with gasoline which cleans out the inside of the engine quite nicely - but it's no lubricant.
Change the oil pressure relief valve. This valve, located at the bottom left side of the engine block, controls the upper limit of the oil pressure. It's easily removed, but often requires a strong wrist to replace. Change the rod bearings and oil pump. The pump makes the oil pressure and the bearings use it. Both the pump and bearings wear - and they are located in exactly the same part of the engine.

but no rebuild yet Iain !!!! :)
ok thanks a lot for your senior moments which helped me a lot!
Alexia

Agreed that the oil and filter do affect the situation but not much unless the car has been run on choke for about 20 years and the oil not changed. The oil filter has a bypass fitted which will open if the filter is becoming restricted so this really wont alter the situation. Your pressures are far too low to be cured by these means so it's relief valve, oil pump check and crank and bearings in that order. There's a bit more to it than just the rod (big end) bearings. The main bearings will also need attention and I'm pretty sure that just the replacement of the bearing shells by themselves will not rectify the situation. The crank journals will need inspection and measurement and whilst the centre three mains can be inspected with the crank in the engine they are very difficult to measure. You can hewever measure the clearance using Plastigage.

I still think it's rebuild time but will be delighted to be proved wrong.
Iain MacKintosh

The tapping sounds like tappets which is common and no real problem. Worth adjusting carefully. The rockers can wear at the point of contact with the valve tip so when a broad feeler guage is used it measures incorrectly. Cut a 15 thou feeler to make it no wider than the valve tip.

The tapping sound itself may be amplified by a rocker cover that is tightened too far or a gasket that is too thin or has compressed over time. (The stud nuts are sleeved and will only tighten so far but it is not necessary to tighten them that far with a new gasket and rubber seals.) The thin pressed steel cover then contacts the head directly, metal to metal, and increases the sounds from the top of the engine. Not many people pick up on that. Replace stud nut rubber "top hats" and cover gasket and do not overtighten.

Change the oil, use 20/50, and filter. Many healthy engines show 50-60 psi hot but only above 3000 rpm. At 30 mph expect no more than 30 or 40 psi hot. Iain's point about ignorance being bliss is spot on. Does it rattle at start up? Does it burn oil? If not and if it runs OK I'd leave well alone. You can't exactly thrash it on Guernsey anyway can you :-) Rich.



Rich

well Rich it doesn't rattle at start, it doesn't seem to burn oil and it runs ok but I haven't pushed really the engine as .... well I'm on guernsey meaning no more than 50 mph !!!! and not highways! many steep roads and straight curves (a bit like a racing training circuit) :) and yes I can't really trash then
Alexia

oh I forgot it's interesting that you talked about the roker cover as I am planning to change it and the gasket as well as i'm sure it hasn't been changed since 72 .... hope it can improve a bit the state of the engine!
Alexia

Oops, maybe a lot of other things havent been changed since 72 either !! How many miles has the engine done?
Iain MacKintosh

well only 29 000 miles on the "compteur" but before the car arrived on Guernsey so in 1990 I have no history of her past life in UK ... yes I have done a lot on the bodyshell and teh interior during the winter so now I'm doing the mechanics...
Alexia

Er, It sounds like you need to do a tune-up, filter and oil change and then see what comes.

When you are driving on the narrow twisties, does the oil pressure ever suddenly drop after (usually) a hard left-hand turn? I mean, does the needle "twitch" toward the low side? If it does it may mean you are a little low on oil and the pump is cavitating as it draws air (when the oil sloshes to one side). Less oil in the sump will tend to get hotter and run "thinner" as well.

An old rule of thumb is you need 1psi of oil pressure per 100 rpm. This falls off at the low end and is a little excessive at the high end but is a pretty good rule for the middle rpms where we tend to do most of our running.

Give a good look to that oil when you drain it too. If you can, cutting open the filter will show you what it has been catching as well.

G'Luck

Mike!
mike!

ok ok ! well apparently since my exhaust has been changed last we and I changed my thermostat housing ... the oil pressure seems better! at idle and running the engine in the parking or small distances I have now more than 50psi sometimes...
hum! and as I have one of the headlamps not working I can't drive around the island in circle after work to check my oil pressure evolution... maybe I've been to woorried...to be continued! thanks a lot anyway for all your help, I really don't know how I will do without this website...
Alexia

Alexia, do not panic. My 1979 ex-California car has been used as a daily driver for the last eight years with similar pressures to yours. Drive it sensibly, keep the oil clean and it will probably give you another ten years. "If it aint broke, dont fix it."
Happy MGB'ing ma chere.
Greg

I don't think anyone has suggested this...and it may be too obvious...

If your oil level is very low, the oil pressure will drop. Make sure your oil is topped off to the full mark, then check again.
Frank Sperling

incredible !! I changed my rocker cover as well yesterday and oil pressure went to 60 when accelerating at 2500rpm .... and the needle didn't fall to 0 when I turned off the engine but took few seconds.... improvement improvement !! thanks guys for having listened to me! now it seems all right!
Alexia

This thread was discussed between 16/05/2005 and 24/05/2005

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