low oil pressure mgb

[andrew.searston]

hi everyone
i have sudden loss of oil pressure on my mgb gt. the gauge was reading 50 or above and now only reads 25-30
would this indicate oil pump???
i have replaced oil and filter recently and oil level is fine
the only things i have found is the rocker cover leaking a little but would be unlikely to cause this problem.
its a 1978 mgb gt with an oil cooler

[PSL184]

First job would be to strip and clean the relief valve.... Is the gauge electric or a wet line ?

[bmcecosse]

Is that the running pressure - when hot? If so - don't run it! You say oil change - what oil did you use ? Hopefully good old 20W50 and not anything thinner..... It will have a relief valve somewhere - but I'm not familiar with the B series engine....but that's what to check first.

[andrew.searston]

that is the pressure when hot, hope it hasnt done much damage
i used 20w50 oil but will be replacing the filter and oil to be on the safe side
i wouldnt have thought it would be the oil pump as i thought it would have no pressure at all if it suddenly broke???
does anyone know where this valve is on the engine??

[Alex'n'Ane]

Is around 20-25 when idling hot after a long run worryingly low? My engine (1098) idles at this, and at full throttle at the end of a very long run it only reads about 30. And yes i run 20/50, but hasn't been changed recently. It has been like this for quite a few months and a couple thousand miles without any issues at all.

[Longdog]

At full throttle the engine should achieve 60psi which is the point of blow by. Tickover I would have said at least 40psi. My rebuilt A series 1380 does 55psi on tick over and 70psi at over 1500 rpm but that is a new engine. I would have thought your readings are on the low side. Still they are well above the terminal orange light 9psi.

[bmcecosse]

30 psi when running at 'full throttle' is very poor - DON'T DO IT !! You will wreck the engine...... Idling pressure is basically unimportant - although it's obviously desirable if the oil light stays off. In top gear a 'good' engine will have AT LEAST 10 psi per 10 mph when hot -up to the relief pressure of 60/70 psi depending if the spring has been 'adjusted' in some way.

[Alex'n'Ane]

30 psi when running at 'full throttle' is very poor - DON'T DO IT !! You will wreck the engine......

Out of interest, can i ask what damage this would do to the engine?

[andrew.searston]

starve the big end and mains of oil

[bmcecosse]

Exactly - it already indicates the bearing clearances are excessive - and so the protective oil film wedge will not be retained - and the bearings will 'run'..... Basic stuff really! In addition - there will not be sufficient oil getting up to the cam shaft bearings - and even less getting up to the rocker shaft - since it's all pouring out the worn mains and big-ends...... There WILL be trouble ahead...

[andrew.searston]

just swopped my oil gauge over and was reading about 30-40, just gone out to it now and its reading 50 at idle, then i swopped the gauge back to the original and its reading 50 from cold start and goes up when revved.
do you think it might have been the relief valve stuck open???
ill take it out tommorow anyway clean it up and then change the oil and filter

[bmcecosse]

Cold oil pressure means nothing - it's the pressure after 10 miles of hard driving that matters......

[andrew.searston]

ok will take it for a drive and will report back tomorrow

[beero]

Pressure relief valve is held in by a domed nut , back of the block, below the manifold, circled in this photo.
Take it out, inspect it and measure the free length of the spring, should be 3 inch, any less, renew it.[frame][/frame]

[bmcecosse]

Excellent - so, same parts as the A series - just in a different place!

[andrew.searston]

thanks for the pic, when i undo it should the spring just pull out???
and will anything drop inside the engine when i remove the spring??

[bmcecosse]

The spring will come out easily - the bucket inside can't fall into the engine - to get it out - push a bit of dowel into it - the handle of a valve grinding tool fits nicely.

[andrew.searston]

im guessing its the bucket and spring that cruds up and causes it to stick or would it be down to a weak spring throughout time?

[bmcecosse]

Could be rubbish/swarf jamming the bucket. The spring can lose a little bit of pressure over the years - but not by any important amount.

[andrew.searston]

here a pic of the bucket, very scored
but the bucket come out easily with the spring[frame][/frame]