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MG MGB Technical - How to proceed - low oil pressure

74.5MGB with 18V engine. Odometer shows 35k but must be 135k. I bought the car late last summer at a good price mainly due to the condition of the body and have only put a few hundred miles on it as I sort things out.

My oil pressure is low. The gauge rises upon startup to 40-50 (but usually have to tap the glass to get it above 40) then, when the engine is warm will drop down to 25 at idle. (I've tested with an external mechanical gauge with he same results.)

Compression test gives results of 140, 135, 134, 130. Valves have just been adjusted to .015 and the engine runs good and strong although there is a brief amount of rattling at startup that goes away once the engine is running.

As I said, the engine runs very strong - peppy with lots of pickup and feels like it can cruise at 60 all day so I'm not sure what you guys might suggest. I've changed the oil using the proper Wix filter.

My first thought is to change the relief valve and spring since it is cheap. Other than that, if I should pull the oil pan what do I look for? I've done everything else on MGBs except engine work. Money is a concern so whatever I do will be me doing the work. I don't just want to replace parts so I guess I'm looking for some checklist to proceed (after replacing the relief valve). Thanks!
Greg Smela

25psi is not a bad idle pressure and is in fact within limits recommended in the manual. It's the running pressure that I would be worrying about which should be between 50 and 80 psi. However as a general rule of thumb I was always taught that oil pressure should be 10psi for every 1000 rpm and it would seem that uou just about meet that criteria. The rattling on start up is probably the rod bearings before they have sufficient oil. The compressions don't look too bad assuming that this is a low compression engine. I'd be inclined to take a few more miles out of this engine before pulling it down for a crank grind rebore and new pistons. Alternatively you could replace the bearing shells and re ring the pistons to extend the engine's life but you will only be able to replace the rod bearing shells as the main shells are not accessible until the engine is removed from the car and the crankshaft taken out. It's your call really.
Iain MacKintosh

Thank you Iain. Since I'd rather not pull the engine at this time is it even worth replacing the rod bearing shells?
Greg Smela

I wonder, at 135k, worn oil pump? Could be done with engine in place if so. May be worth the effort given the overall good compression figures.
Roger T

"oil pressure should be 10psi for every 1000 rpm"

I don't think that works for the MGB with the recommended levels of 50 to 80 running and 10 to 25 idling. Mine is at 60 for most of the rev range, 40 at idle unless stuck in traffic on a very warm day when it will gradually drop to 25 as the oil heats up. Greg's seems only slightly low to me, I'd say just drive it. However the rattle on start could be worn bearings as has been said. Bearings should be swimming in oil even when switched off hot and left for a couple of days. With larger clearances the tendency to rattle until oil pressure fills the gap and cushions better. The WSM recommends changing the shells at 30k or so for maximum service life. Mains are an engine out job, but big-ends are probably an in-situ. May be worth it given the good compressions.
PaulH Solihull

You didn't state your oil pressure at speed but sounds like it is okay. 25 psi idle as stated is within spec.

Ian is correct that likely your rod bearings are worn and causing the startup rattle. I think I remember discussion of bearing startup rattle in one of my old repair manuals. Usually attributed to rod bearings. I doubt the oil pump is the major cause. Main bearings wear less and are likely not the culprit. Worn cam bearing are a remote possibility. I doubt the relief valve or spring is the problem. Plus it is a bear to do from what has been posted in the past. Aftermarket springs are dicey quality wise.

You can drive for some time like this. Don't try to solve the problem with overly viscous oil. You need to get the oil flowing as fast as possible. When my car with an 18V was rattling I'd not pull the choke until I turned the engine over to start oil flow. (I later changed out the rod bearings).

If you have the Zenith Stromberg water choke you don't have that choice plus you wouldn't want to pump unburned fuel into the catalytic converter.

I'd be tempted to pull the pan and replace the rod bearing shells. If the overlay gets worn off and you are down to backing you can start scoring the crank journals.

I replaced my rod bearing with the engine in the car once. It's messy but doable. The pan didn't leak after I reinstalled it. Visual plus plastigage should tell you if they are worn.

Bob
65 Roadster
Robert McCoy

Thanks Paul. Robert, the pressure at speed is between 45-50. In fact, I'm only seeing 25 at idle or the 45-50 at speed, nothing else. I"m used to seeing 60s-70s in previous Bs I've owned which is why I'm worried but the comments here have settled me down a bit!

I think I will look into replacing the rod bearing shells. Never done it before so a good learning experience.
Greg Smela

Have you changed oil & filter?
Filters are a can of worms, and there are many erroneous listings out there. The rattle on start is a sign that the anti drain back in the filter is not working, or not in the filter you have. And there are quite a few reports of filters that reduce pressure by significant amounts.

I have posted the following previously, and there is plenty more on the subject in archives:

This might save an engine, so listen up!
I have been puzzled by repeated mention of oil pressure drops related to filter changes, and now have come across one possible cause. A long distance customer (79 MGB) called to say that he had taken his car to have the oil & filter changed, and on driving away had 10psi, instead of its normal 60, so he went back, gingerly. They admitted that they had put a modern 5W-20 or somesuch in, rather than the 20W-50 requested. They changed the oil and said it was now OK, so he drove off, NOT looking at the gauge, since it was "OK". Fortunately he was in the city and driving gently, because when he did look at it, it still had 10psi.
He called me, and then sent it the 100 miles up here via AAA.
The very small filter was a dead giveaway, since it was about the same height as the length of the standpipe. On examination, the standpipe had been in contact with an internal support in the filter, in fact bending it. The standpipe was effectively 98% blocked; if he had made one heavy acceleration run, he would most certainly have scattered the engine. Other filters may have similar but less drastic effects if the standpipe is a bit less restricted than this one was. A difference of 1/8" in the internal construction could give normal performance at low speed and total disaster at full throttle. I would recommend that any filter be examined to be certain that all internal components are at a minimum a full diameter away from the standpipe when fully tightened. I have noted that many parts listings list things like "MG, all, 71-80". This filter screws on to an A-series and may in fact be correct for that app, but it certainly isn't for a B. Be really careful if you let anyone but yourself change filters. I have long hated standpipes that could reasonably be expected to allow the possibility of total blockage under abnormal conditions, and have taken to cutting notches in the end of them so that even in a case like this it is impossible to block them completely. The same thing applies to oil and fuel pickup pipes, as in Magnette, where if the sump gets mashed it can block the pickup.

FRM
FR Millmore

Whilst I am aware that the running pressure should be 50-80 as mentioned in my post 10psi per 1000 rpm is just about enough for safety. However much less than this must certainly be investigated to avoid serious engine damage. I you wish to replace the rod bearings then that will be no bad thing and it may just give that little bit of extra oil pressure you need to bring you within factory specs
Iain MacKintosh

Sorry, I neglected to mention that I have changed oil (20w/50 and filter. When I got the car it had a K&N filter which I replaced with a Wix that *supposedly* was the proper application.
Greg Smela

Enjoy the car!

Our GT is going strong for the last 20000miles with a hot oil pressure of max 10psi......
cold it goes to 45psi when driven and hot it reaches 35psi.

As it runs great consumes hardly any oil and makes no funny noises I can't bring myself to pull the engine.

But I expect to have to do the clutch in the next year so I'll do the shells and pump then in an effort to improve things.
But I'll postpone it as long as I can (better to be driving)
Onno K

According to my manual the oil pump can also be removed with the engine in place. It recommends changing big-end bearings every 30,000-50,000 miles. But this was written for three main engines with a supplement for the later engines at the back. I expect the 18V bearings last longer as the rods are lighter and lubricants are much better.

As mentioned, if you don't thrash the engine and keep revs at a decent level you aren't going to have imminent failure.

Of course when you open up a high mileage engine where do you stop?

All the best.
Bob
Robert McCoy

Ok, every owner must make their own call.

However, given the relationship between bearing condition (mentioned by Iain prior to my post), pump condition and oil pressure, AND 135k run, if I were going to the trouble of opening the sump for bearings, I think I would take the opportunity to have a look at the pump at the same time.

I'd hate to button it up with only bearings done and then discover there was little or nil change for the better.
Roger T

If it really has 35000 on it, it should not need any bearings; if 135000, then it needs all of them, and a cam & tappets, and lots of other stuff. Oil pumps were never a fail point on these engines, but new replacement pumps are frequently at less than ideal internal clearances, and commonly now have bad flashing in passages - examine carefully.

FRM
FR Millmore

FRM, for what it is worth, I spoke with John Twist yesterday afternoon and he recommended an oil pump rebuild kit from Engle Imports in Kalamazoo, Michigan rather than a new oil pump for the reason you mentioned. I think I'll be pulling the pan to look at what I've got. Thanks all for the comments, very helpful.
Greg Smela

Working on looking at the cheapest first.

Rattle at start up can just be wear on the cam sprocket, chain and tensioner. The tensioner has a spring to hold it against the chain and then hydraulic pressure tightens things up. Wear here causes the chain to rattle a bit until oil pressure has built up. Poor filters with inefficient anti drain back valves cause the same rattle! I've found Mann to be the best.
As for the oil pressure, no harm in changing the relief valve spring, or doing what the factory did and putting a shim in or replacing what is probably a pretty worn valve with a ball type and seating it again.
Then use the car until such a time as you overhaul the engine completely.
Allan Reeling

Allan-
The chain tensioner has a ratchet mechanism which keeps the tensioner in the furthest out position it ever gets to, within the limits of the ratchet. This leaves much less chain slack than is required for rattle, so that is not the source of noise on startup.
The rattle noise is caused by con rod bearings with no oil in them; either the filter has drained or the bearings are defunct, or both.

FRM
FR Millmore

FRM, am I correct in assuming that if the bearings were completely shot the noise would continue? As it is the rattling stops after a very few seconds. I guess what I'm asking is if there are degrees of bearing wear that show as noise? I know next to nothing about internal engine stuff and am trying to learn. Thanks.
Greg Smela

Greg-
Bearing clearance is around .001" new. That amount of clearance will stay full of oil for long periods of time at rest, which will keep the oil passages pretty full. If the filter is holding oil as it should, then oil pressure will refill the bearings as soon as the car starts.

If the bearings are worn to say .003 clearance, oil will drain out of the passages, but if the filter is full then pressure will build quickly = no noise. If the filter drains out, then the engine has to run long enough to refill it before any oil gets to the bearings, and you get the rattle on start. You are somewhere in this range now; it is either fairly worn or the filter is bad. Every time it rattles, it is hammering the bearings and making things worse.

Once wear increases the bearing clearance to a level where the pump cannot maintain pressure at a given engine speed, it will rattle all the time. This is subject to loading on the engine, such that light loads are more likely to give the noise than heavy loads; you can accelerate without noise, but steady speed on a flat or slight downhill will rattle; we call this "on the float". Obviously, at this point both bearing wear and pump wear are contributing.

Eventually, wear will give clearances so great that the pump cannot ever keep up, and it rattles all the time, until pistons start hitting the head when the bearings fail completely.

Your reported 25 psi hot idle vs 40-50 max pressure is anomalous. I would expect either lower hot idle or higher max pressure; I could believe 5-10 hot idle and 40 max. Please clarify that/if the 40-50 is max pressure at 3000+ rpm hot.

I would double triple quad check that you have the correct filter, and fit a new one regardless. People have reported 10-20psi drops after fitting some filters, in addition to the problems I mentioned above. The correct filters are about 3 5/8 diameter, and the wrongly listed ones about 2 1/2". Lots of discussion re filters in the archives.

If that doesn't correct it, then I'd go for the OP relief valve, before tearing into the engine.

FRM
FR Millmore

Thanks for that very clear explanation. Yes, the hot idle is a steady 25 while the pressure at 3000 is between 45-50. It was also that way before I changed the filter (which I will do again this weekend).

I have read all that was written about the filters which is why I chose the Wix but, who knows? I'll let you know what I find.
Greg Smela

FRM,
There you go, I always assumed the spring/ratchet was too course an arrangement to make fine adjustments, and the hydraulic pressure did just that.
Allan Reeling

If the filter change does not improve things:
I'd say the idle pressure indicates that both the pump and bearings are not very bad, but the low max may be an indication that the relief spring is dubious. Any pump/bearing combo that can give 25 at idle should easily exceed 50 at 3000. It is also possible that a leaky relief valve is letting the gallery between the pump and filter drain, accounting for the initial rattle. Replacement springs have been reported as being highly variable and suspect. WSM gives a spring load of 16lb at 2 5/32" length. I have not bought any in recent years, so am not up on safe suppliers here.

FRM
FR Millmore

FRM makes a solid case for checking the relief valve spring. He knows far more about these cars than I'll every know. Also his advice regarding using the right inverted filters as is spot on there have been bearing rattle problems attributed to faulty or lacking check valves especially on Frams. I'm not sure how easy it is to get the pressure relief valve spring re-installed after you remove it. I'd definitely go after these first.

FWIW I checked on bearing rattle on another forum archive. Two posters reported the same oil pressure readings as you did. 25 psi at idle and around 50 hot around 3000 rpm. They had the startup rattle. Both found worn rod bearings upon opening up the engine. If I went to the trouble of changing bearings I'd also check the oil pump. There rotor clearances given in some manuals.

It is a long shot but you may want to check the pressure gauge. I had one bind at about 50 psi. I took off the bezel and glass, cleaned it up as best as possible then rotated the mechanism in the case to eliminate the bind. Been working ever since.

Bob

Robert McCoy

Bob may be on to something. Checking out all the ez stuff before pulling the pan makes sense. I've seen oil needles hitting the face insert about in the vertical position and sticking. But then you say you've done a second independent gauge and gotten the same readings.
The existing PR spring can be reused with a second 1/4 inch thick shim . The higher pressure replacements from Moss at 3.25" have not been a problem in my experience. In either case, shimming or replacing, the reinstall of the spring cap to block threads is facilitated using a scrap of 2x4 against the chassis under a Wonderbar, levering the cap into the threads to get her started. EZ Peezy and less broken knuckle.
The pan off is not that big a job if you have a lift. If not, I've done it a few times from a creeper, and it will take significantly longer. Use a 1/4 inch drive and extension on a cordless drill to quickly down the pan screws. Use a razor scraper to clean the pan/block surface, then gray Hylomar as a really good goop. The rod shell replacement is thankfully straightforward, and can be completed in a couple hours (assuming the rod journals are okie dokie). With sparkplugs out, you can rotate the engine to encourage the release of the rodcaps. You can even check the new bearings with plastigauge in sitio to determine if you're in the 1-2.5 k" range.
Gotta go with FRM on the 25 # at idle observation, and it is prolly not a worn oil pump. However my energetic friend, if you're there, drop the pump, check the drive as I've been stunned at the wear in these replaceables, then confirm the max 0.005" clear between the cap and rotor, and no more than 0.006" rotor to cylinder body. Consider that $100 for a new heart transplant may be a best buy. And if you want to pinch a final penny, replacement cylinders and rotors are available. If you wanna get fancy check out Sean Brown,FlowSpeed,Oregon :
www.flowspeed.com/street-pump.htm
on "flowing" a stock MGB oil pump for higher output.
Bottomline, after a second new oil filter, and perhaps a gauge double check, and spring shim or replace, it is "owner's call". On my supercharger, I had the same problem and at the end of the day it was rod bearings ( complicated with a scored, lost lapped pressure relief valve face). I got to it before journals were scored, and replaced the shells insitio with a bit of finesse, a lift to work under, and the best tri-metal shells ( Vandervell) I could find. I'm sure FRM or Basil Adams could find you some. Best of luck in getting back on the road again. Cheers, Vem
vem myers

Thanks again for all the good information. An oil change with yet another filter offered no improvement. I've been hampered by a severe lack of time to work on this but plan on changing the spring and valve this weekend. I'll let you know what I find.
Greg Smela

Well, I thought I'd give an update. I finally got the time to replace the relief spring and valve. (As an aside, I really lucked out - it only took me about 20 minutes to get it out and replaced. I was able to replace the spring and valve by laying on my back with my feet facing the rear of the car and doing it "by feel" instead of by sticking my head under the car to see what I was doing. I was able to get better leverage with my hand against the nut & spring and it went right in.)

I started the car and the pressure came up about 5 lbs higher than before. I don't think that is enough to say for sure that the new valve & spring helped so I will run the car over the next several days to see if the pressure is consistent.

But... I did notice immediately the engine does not have the rattle at start-up. At least it doesn't after starting it a few times from cold. I'll also keep an ear on that.

So, I think I will still pull the oil pan when I get some time and look at the bearings and oil pump. (I just found a recently calibrated Brown & Sharpe dial caliper in like-new condition for $25 so I'll be good to go!)

In the meantime I will drive the car and get a better feel for it. Thanks again for the comments.
Greg Smela

Greg,

You say the pressure has come up 5psi so that means you are 30 at idle and 55 running which is more than enough. I wouldn't worry further about it that engine has a lot of miles left in it. Still it would be no bad thing to change the big end shells at some time in the future but there is no immediate rush.
Iain MacKintosh

When all else fails, consider "piston slap". Wear on the piston(s) can leave it (them) loose and this rattle disappears as the pistons heat up and expand.
Not heard of this recently but it was quite common in the 50's and 60's. Don't know about earlier than that as I had no interest in cars as I was too young!
Nobody stripped an engine because of piston slap.

the rattle could also be the little end wear, but this gets louder as the engine rotation speed increases.
AG Martin

This thread was discussed between 02/04/2012 and 27/04/2012

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