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MG MGB Technical - Low Oil Pressure

My 79 B with 147,000 miles has recently begun to show low oil pressure, dwon to 25 lb on the highway where previously it would run at 60lb. I'm sure it's due to wear on the engine although the car runs great and I use it as a daily driver.

What are are my chances of success if I pull the engine and replace the main, rod and cam bearings as well as the tappets and rings. This would skip a machine shop for a rebore as well as new pistons and possibly a machined crank.

I realize that the best answer can't be given until the engine is out and examined but I'd like a idea of what chance of success a partial rebuild such as this would have.

Thanks,

Jim
jm McHugh

Jim. I just got through with a similair project on my 62 B. Had the engine out for an OD transmission swap, and decided to replace mains, rod bearings, timing gear and chain,oil pump and reseal the engine. I did not do rings, tappets or cam bearings. Vehicle had 119K at this mini rebuild time. The crank was in spec, so it was just polished and standard shells used. Oil pressure greatly improved, but it was still at min. spec before the work, not near as low as yours is. If there are no unusual noises,etc., it can be done. I still wonder if I should have gone all the way with it though, but I am very satisfied at the moment. If you pull the head and pistons for just rings, I doubt you will be able to do without machine shop work though. With that many mile on the crank, you better do your measuring correctly. I would bet you need machining.
Emmanuel Kafant

Jim, Worth checking the pressure guage is OK and the relief valve hasn't stuck open if there are no other wear symptoms.

Oil consumption and a compression test would give an idea of bore wear, and whether or not it rattles at start up will tell you how worn the bottom end is. If all is well otherwise and it's not been serviced recently try an oil and filter change and see if new 20W/50 improves it.

If you do rebuild it check/change the oil pump. If the bores aren't too worn glaze bust them and fit new rings but check the rings grooves aren't worn in the pistons. Crank will be a case of inspection and measurement.

Rich

Jim, As I understand your post this problem happened rather quickly? Or is a gradual change that has taken months or a couple of years? I ask because a worn engine would normally cause a gradual change in oil pressure. Have you recently changed the oil and filter? You may want to the following thread for information about what can happen with oil filters. Strange oil pressure after oil change;
http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&mode=thread&access=&subject=71&source=T&thread=2005061415261521994

If you have not recently changed oil and filter it would be a good idea to change and see if that helps. If you still have the problem borrow or buy a mechanical oil pressure gauge and check your pressure with another gauge to be sure you don't have a gauge problem.

You say the engine runs well, I assume that means no recent knocking or any other unusual noise coming from the engine.

Before removing the engine you can also try a new oil pressure relief valve, that could help.

With 147k miles the engine probably needs more than you suggest if you want a good reliable engine for the next several years. It's best to check the wear of all internal engine components and replace or repair any part that is not in tolerance. Just replacing mains, big end and cam bearings may not fix your oil pressure problem.

Clifton

Clifton Gordon

Thanks for your comments. Clifton, The drop in pressure was first noticed in December when it went below 50. With winter running in Connecticut it stayed up around 50 on the highway but as soon as the weather warmed up it dropped to the current level on the highway. When I start it up it immediately runs up to 65-70 (the gauge on the 79 isn't calibrated very well). Actually the bigger drop came right after an oil change where the garage put in 10w-30. I changed this oil right away to 20w-50 but did not change the oil filter. That will be my next step.

Thanks,

Jim
jm McHugh

Jim,

First thing after a fresh oil change and filter should be replacement of the relief valve spring - and get an UPRATED spring! I recently replaced one with an uprated spring. Cold idle pressure went from about 50# to about 90#. It used to run slightly under 50# hot at speed; afterward it would never run less than 70-75#. This is not a "cheap fix"; the spring will not give you oil pressure that your engine doesn't have. If your bearings are bad, you'll still have low oil pressure. But a $5 spring is certainly worth trying before pulling the sump and rods, etc.

The only downside is that getting the new spring in with the engine in situ requires brute strength or a lot of time, patience and/or ingenuity. I did mine with the car on jackstands and it took me the better part of an afternoon. ' Think I ended up removing the exhaust.
' Suggest you find a lift and then line up a professional weight-lifter to put it on for you. ' Still easier than replacing bearings.

FWIW,
Allen
Allen Bachelder

I rebuilt an engine with about 135000 miles. The crank was within spec. I replaced main and con rod bearings. I put new gears in the oil pump (old housing though).
running 10-40 or 15-40 oil; when the engine is hot, (195 f thermostat) I'm holding 60+ lbs at 2500 rpms and above.
Barry
Barry Parkinson

Hats off to Clifton! Changed the oil filter from Mobil 0241 to Fram PH43 and immediately the OP went back to 65-70lb on the highway! I have always had luck with this filter and will make sure it's the one used from now on.

Thanks to all who responded,

Jim
jm McHugh

Well done. Quite frightening cheapo filters. There are many that use the same thread and gasket dimension and will physically fit. The result is many suppliers cross ref their stock to keep fewer and some manufacturers rationalise to make one part number do more than one application.

For example, looking down the Mann spec list shows tens of them that will fit but only 1 which has the correct valving.

Inside the can the byepass valve and non-return valve, paper and type are different so I'd only get a filter listed as the exact application from a filter manufacturer and not a branded one. Mobil will not make their own, that filter will have been catalogued for them to fit many cars. Those cars won't have a pressure guage just a light and no-one else will know there may be a problem.

Rich

This thread was discussed between 19/06/2005 and 20/06/2005

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