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MG MGA - Oil Pressure Drop and RPM

I thought I'd just throw this up for the archives. It is common knowledge that a signficant drop in oil pressure at idle after the car is warmed up is a sign of potential problems with the oil pump, or worse, bearings.

However I wanted to point out that its very important to makes sure that you are looking for oil pressure changes at idle, but at a consistent idle rpm!

I just rebuilt my engine and was throughly irritated that my oil pressure was showing a drop down to 30 psi after a long fast run. I was envisioning that I had screwed up a bearing installation.

What I discovered was that oil pressure was dropping after a long fast run because my idle RPM's dropped after a long fast run. I replaced the ignition coil (long story): my idle rpms stabilized and my oil pressure stabilized.

Moral of the story, before you worry about oil pressure make sure you have a stable rpm at your comparison point.


T McCarthy

Hi Tysen - looks like need new bearings then - those are the symptoms I have. At tick over on start up I get around 50psi - running at 40 mph it goes up to 60psi until it reaches normal running temp when it drops down to around 45. On Sunday after running at 60 mph for about 20 mins the reading was 15psi at tick over ( 900 rpm)although it had been reading 40 psi at that speed. After a few minutes of running at 40 mph the oil pressure was back up to 45psi. Water temp all this time was a steady 190F. Looks like the oil is thinning but I might try slipping new big end bearings in to see if it makes a difference - my friend did just that on his MGB and got an immediate increase. The engine has done 88000 km - (55k miles) and is original as far as can tell.
Cam Cunningham


Before you go down the road on the bearings. Check on one thing.

Using a good quality electronic rpm instrument check the idle RPM and oil pressure at 5 minute increments for 25 minutes.

Also check the idle RPM and oil pressure prior to a hot run, then check the idle RPM and oil pressure after the hot run.

If idle rpm over time declines markedly or if idle rpm before the hot run and after the hot run declines or become bumpy then it may not be the bearings.

If idle rpm is the same before and after the hot run over over the 25 minutes, then you may have to explore the bearings.

My first clue was that after a 20 minute hot run I would pull up to a light and have to feather the gas pedal in order to keep the engine from stalling.

T McCarthy

My Mk 2 shows 80 psi at start up on fast idle when warm drops to 40 at 750 rpm still 80 at over 2500 revs but then gradually drops to 60 rpm when fully warmed up at motoring speeds -will stay at 60 all day but idle drops to 15-20 when hot- but a blip of the throttle sees it rise again instantly to 60. I consider this OK ? matches what the handbook says !!
P D Camp

This thread was discussed between 12/05/2009 and 13/05/2009

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