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MG MGB Technical - erratic oil pressure readings

Can anyone suggest a reason for some odd oil pressure readings I have been getting for my 66 BGT.

When cold the oil pressure is near to 80 psi. Once the car has warmed up the oil pressure reading is around 65 psi at about 3000 rpm in 4th overdrive. Occassionally the needle on the gauge will flick down to 45ish and then a second or so later flick back up to 60-65. It might do this half a dozen times and then be fine for a while.

Two other observations. On a motorway journey this started happening so I slowed down to come off at a service station. With RPM down to around 2500 the oil pressure went up to over 70 for a number of seconds before dropping to below 60 as the cars speed dropped.

Back on the motorway I increased crusing speed to 3500 rpm 4th overdrive and the needle stayed solid at just below 70 psi but then started with the occasional flick down to 60 psi before returning back to around 70.

Any idea what is going on?

Other info - the engine is standard apart from an oil thermostat and a new oil pressure release spring and valve that I fitted last year

Thanks
Richard

in my very inexperienced head i'm thinking maybe you've got a sticking release valve?

i'm sure the guys here will know.
d buck

I'd go along with that. Mine stuck so fast that the oil pressure went over 100psi and stuffed the gauge.

I took the oil pressure relief valve out and swapped it for a new one, but there are still some burrs on the valve seat so its still a bit 'flickery'. It's a sod to do with the engine in, but it can be done!

I think it'll only be a problem if the pressure drops rather than increases, but you might lose a bit of oil through some seals....

Dave.
Dave Smith

i have a problem at the moment, that in traffic the oil pressures dies, slowly sinks back and i have to blip the thorttle to keep it in check [60 at tickover]

i don't think it is the pump on mine as my engine has had a new pump only 5000 miles ago. i'm somewhat lost with this problem
d buck

Some fluttering at a particular combination of temperature, pressure and flow is 'normal' and differs from car to car. The factory fitted a sintered iron plug to the gauge to try and smooth these out quite early on but it still happens. Whilst changing or working on the relief valve may stop it happening, it is just as likely to move it to another combination of said factors. But what Richard describes *does* sound like a relief valve problem, or possibly a bypass valve problem in the oil filter. You could also temporarily fit another oil gauge to eliminate that.

One tip with the relief valve - while it is off, with the spring out, refit the cap and mark both cap and block exactly where the threads start to engage, by marking the block and slowly unscrewing the cap, then marking the cap. Makes life a lot easier when you come to refit it *with* the spring, as you won't be trying to compress the spring and turn the cap without any idea when or where the threads are going to pick up.
Paul Hunt 2

Thanks all for your replies.

If I understand this correctly, whilst cruising with oil pressure at a steady 65 psi the oil pressure valve is closed. It then opens due to a combination of circumstances and then, in my case, sticks momentarily, causing the oil pressure to fall quickly to around 45 psi. It then closes and oil pressure returns (it actually flicks back) to "normal". This seems to be a slightly more exaggerated version of the flickering Paul describes (presumably when the value is opening/closing very quickly i.e. not sticking).

What I cannot yet understand is why the valve should be operating at all - if it is there to prevent the oil pressure getting too high (presumably under cold starting conditions) surely it must be closed all the time when the oil is at the correct operating temperature and the engine rpm is not too high (unless of course my gauge is systematically under-reporting the pressure!)

d buck - 60 psi at idle seems very high - the manual suggests between 10-25 psi

Paul, it is ineresting what you say about the by-pass valve in the filter. The car has its original GB series engine and still runs the downward pointing canister with felt element. I am not clear where this by-pass valve is then - the manual states it opens at 13-17 psi.

After very little use for 18 months its good to get the old car out on the road again!

Once again, thanks for all your input
Richard Atkinson

I hate to admit it, but I've had this problem when running the engine too low on oil.

j
JohnB

Richard,

Under normal conditions the oil pump is capable of producing a flow that will generate a pressure higher than that which will open the OPR valve, the flow varies proportionately with engine speed - the valve is normally open all the time to a varying degree. The aim is for the valve to regulate the pressure to around 70 psi max, but with poor sealing this varies as the oil heats up and leaks past the seat. This figure also decreases over time as the spring weakens with age.

If the valve sticks while the flow is great then as the flow reduces with engine speed the pressure will drop. If the valve sticks when the flow is lower then as the flow increases, so will the pressure.

Early OE valves were hard chrome plated to resist galling and scoring (remember that the oil is unfiltered when it goes thru the valve) replacement valves are usually just soft steel and are prone to this type of wear and to sticking occasionally.

One alternative is to replace the valve with a ball bearing of the same diameter as the valve - but in this case the spring needs to be shortened to suit.

Chris at Octarine Services

Thanks Chris, very helpful. Any idea what might be causing the symptoms I describe?

JohnB thanks for the suggestion. The oil level is ok.
Richard

really? i read a thread on here a while ago and i thought it was normal.

i see the best part of 80psi at 4000rpm
d buck

Checking the workshop manual i it also goes on to say "New engines with new oil will give considerably higher readings at low speeds" which I think if I remember correctly yours is - so sounds good.
Richard Atkinson

ah that explains it had an oil change very recent.

phew i thought this forum had thrown up another issue with my car
d buck

This thread was discussed between 21/02/2006 and 23/02/2006

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