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MG MGB Technical - Oil Pressure / Bearing Noise

1972 B is rattling at bottom end when initially started, also above about 2,000rpm.

Please can someone let me know what is normal for oil pressure? I am suspecting the oil pump, mine takes a time to build when started then sits at 55psi almost regardless of engine speed.

Thanks, Andrew
Andrew Walton

When hot, does oil pressure stay at 55 psi when idling? Seems very good - even my recently recondition engine drops back to 45 psi at idle when hot. I would check oil gauge calibration, adjust tappets and if still concermed about rattle - pull the sump off and check big ends and mains.
Mike Ellsmore

Started hot, or after being left overnight, pressure rise should be near instantaneous. Left longer it can take longer to start rising, but should still be rapid once started.

I've lost the first two pages of the general data out of my Workshop manual but Haynes quotes hot idle as 10 to 25 psi and hot running at 3000 as 50 to 80 psi.

Mine hot runs at the upper end of that, hot idles at about 40, unless stuck in traffic on a hot day when it might drop to 25.

Whilst 55 at idle might be classed as 'good' the fact it doesn't rise when revved does indicate problems. Also if it only rises slowly when started it is probably starving the bearings of oil. Could be the pump, or or pump bolts not tight i.e. sucking in air past the gasket, or the pressure relief valve. But the first thing to do is put another gauge on the line and confirm your gauge is correct.
Paul Hunt

Andrew,

The time lag to build pressure may be caused by a poor oil filter if it is the inverted type. Poor filters, or ones with no check valve will empty after shutdown and you won't get oil pressure till they fill.

Charley
C R Huff

Thanks for comments. Been out today and watched more closely. Oil pressure rises quite quickly, a few seconds. Then at tickover it sits at around 52psi. When driving at 2500rpm it rises to 60psi.

The bearing noise is there above 2,000rpm and when initially started up. When driving at 2,000rpm I can hear the 'clatter' and can make it stop and start by accelerating and decelerating - i.e. load and no load.

Is it likely to be a blocked oilway? Gauge pressure high, but bearings starved?

Also, is it worth taking the oil pressure relief valve out? I understand it is tricky to replace it.

Not keen to pull the engine, but will do if necessary. Sump off with engine in car sounds hard work!

Have checked with another gauge - same result or thereabouts.

Thanks,
Andrew
Andrew Walton

I don't see any benefit in removing relief valve in your case. It can be done, just needs strong hands and a suitable spanner to turn the nut while one hand puts pressure on the nut against the spring.you also need the car on hoist so you access from underneath.
I would be checking all the adjustments on rattly bits before pulling the engine.
Mike
Mike Ellsmore

Check the gauge.

A few seconds - how many? Have you tried counting seconds while watching the gauge? Five is a long time going by my engine. I say again, is the delay after being left overnight or less? If so something is not right. Even if left a week or more whilst it might take 2 or 3 seconds to start rising it should then rise rapidly, and that's with an inverted filter.

If it's a blocked oilway after the gauge you wouldn't know it from the gauge as it will just look like 'normal' bearing restrictions.

Worn bearings usually result in low oil pressure as they offer less restriction. But it could equally be a restriction or leakage between the pump and the gauge, delaying rise time and reducing pressure, and that has taken the bearings.

What oil viscosity are you using? A thinner grade such as 10W/40 will allow worn bearings to make more noise than 20W/50.

How old is the oil? If it's done several thousand miles you can have an oil analysis done which will indicate what substances are circulating, and hence where they might have come from.

I don't believe taking the sump off a 4-cylinder is that difficult, but you might have to jack the sump up using a piece of timber to spread the load in order to access the front bolts. With that off you can check the clearances on all the bearings with Plastiguage.
Paul Hunt

Andrew,
For the time being, you can forget about oil pressure and the causes of the 'fault'. Sound is a very good way to diagnose although it can be difficult to describe the different sounds. For what it is worth, the big and little ends each produce a distinctive tap - normally under acceleration but also decceleration. The mains which are loose rumble and this is easily heard when idling over a reflective surface such as concrete. The valve gear rattles at idle - the camshaft can move around when the bearings are worn, the followers will rattle if their surfaces are dished but you can often hear this on acceleration. The chain stretches with wear and can rattle, particularly if the tensioner's buffer is worn. The chain should be replaced at regular intervals. Perhaps significantly, the chain stops rattling on acceleration.
In your shoes, I would try to isolate the sound using the old lags' screwdriver trick (only be very careful around the fanbelt/alternator). If that is inconclusive, jack the car very securely so that you can listen to the sump area. If you are still worried, drain the oil and look for white metal in the residue.
Finally, when you have isolated the source of the noise, there may be questions to answer about lubrication. It's not too difficult to remove the sump, a white metal layer at the bottom is pretty conclusive!
Roger W

While this page relates to an MGA it does give a couple of tips on what spanners to use to get the sump off your MGB with the engine in place.
http://mgaguru.com/mgtech/engine/of106.htm
Mike
Mike Ellsmore

Other sources of rattle can be the front pulley on the crankshaft breaking up, and the water pump / fan (if you have one). If it is bearings in the engine I would definitely remove the engine for the work. It's not so hard if you can get the use of a crane or suspend a hoist from a beam. If the crank needs a regrind you must remove the engine anyway.
Mike Howlett

Many a person has thought they've got run bearings only to discover it was flogged out bushes in the fan mounting. Worth a check!

Is the 52psi at idle on a cold or hot engine? (Hot means after at least 5-6 miles to get the oil warm) 52 on a hot engine would indicate bearing problems are unlikely, but if it drops away when the oil gets hot it is a different story.

That the pressure doesn't rise much above could indicate the relief valve cutting in a bit early. To check where the relieve valve cuts in, slowly bring the engine speed up from idel and note the point at which the gauge starts to flicker a bit. That's when the valve is just starting to open - there's enough pressure to drive it off its seat, but not enough to hold it there once oil has bled off so the valve closes a bit and gets into a repeat loop until the flow rises more.

As the oil gets hotter and thinner, the cut-in speed increases. The lower the speed the valve starts to operate the less likely it is that a bearing has run.
Paul Walbran

I had a similar problem with my 1977 roadster.....it was a blocked oil cooler in my case. Being in cool Scotland, I completely removed it. Hot idle is now 30psi and 55/60 on the open road...70/75 cold start instant pressure

Before, like you, pressure barely moved. You should see a good fluctuation....
M F L Sherrit

Thanks for all comments.

Have spent a bit more time and tried to be methodical !

1) - After 5 days of non-use, crank car, fires and 3 secs later oil pressure rises (builds, but not slow) to 60psi.

2) - Warm engine sits at 52-55psi on tickover.

3) - Car at 2500 - 300 rpm oil pressure back to 60psi.

4) - Have run engine without fan belt to eliminate water pump / fan, alternator - no difference in noise.

5) Noise is above 2000rpm and sounds like top end. It is a clatter not unlike nails being shaken. Tappets all adjusted and no noisy ones.

6) Oil cooler is only 2 years old and new oil 1000 miles ago.

7) - Tried squirting oil on rocker gear through filler cap, not easy to hit moving parts, but no change noticed.

Any ideas for next non-invasive test??

Thanks, Andrew
Andrew Walton

Your oil gauge readings sound more normal now, although three seconds after five days still sounds a bit long to me.

A clatter like nails being shaken which could be valve gear doesn't really go with the sound coming and going as you accelerate and decelerate. If the noise comes and goes but the engine speed doesn't really change, it can't be valve gear, but has to be something affected by the changing combustion pressure. This could be bearings, but mains normally rumble, and big-ends usually make a deep knocking sound. That leaves little-ends, or the pistons in the bores.

I've had a similar hot tapping from the top of my V8 for years, when the engine is cruising or idling, it quietens when accelerating or decelerating. I can also make it come and go by making small changes of the throttle where the engine speed (when driving) hasn't changed. But even more weirdly if I take it on a 300 or so mile trip it stops happening, for several days or even weeks at more usual journey distances, then gradually starts coming back again. New cam and followers no change, then I discovered the right-bank pistons were all facing the wrong way and thought that must be it as the con-rod is not centralised on the gudgeon pin, but corrected that and it still does it.

I still think you should have an oil analysis done, that will tell you what microscopic particles are circulating and hence what might be wearing.
Paul Hunt

A quick thought, "clatter not unlike nails being shaken" could it be pinking caused by too much vacuumed advance?

This would explain the noise going when accelerating as the vacuum decreases and when decelerating as the fuel/air charge is insufficient to cause pinking.

I had a similar issue due to a wrong vac advance unit giving far to much advance on light throttle, albeit it only caused a problem when it was really hot.

It's a simple one to test, remove and plug the vac advance pipe and take it for a drive

Bob
R.A Davis

Good point, but it's more likely to be centrifugal advance causing pinking on acceleration, as on deceleration the cylinder pressures and negligible and it's increasing cylinder pressure that causes it. Check the timing in any event - total advance at 2000 rpm, and with vacuum disabled as described.
Paul Hunt

I misread the OP’s earlier post “can make it stop and start by accelerating and decelerating” I’d assumed it stopped if accelerating or decelerating, only occurring if maintaining constant speed, these were the symptoms I had caused by the vacuum advance unit.

Reading it again suggests it happens on acceleration and not on deceleration. I agree it would be more likely centrifugal advance or just general over advance.
R.A Davis

I misread the OP’s earlier post “can make it stop and start by accelerating and decelerating” I’d assumed it stopped if accelerating or decelerating, only occurring if maintaining constant speed, these were the symptoms I had caused by the vacuume advance unit.

Reading it again suggests it happens on acceleration and not on deceleration. I agree it would be more likely centrifugal advance or just general over advance.
R.A Davis

This thread was discussed between 26/06/2014 and 17/07/2014

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