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MG MGB Technical - Oil Pressure Query '67MGB

My 2100cc engine is 17,000 miles old and has consistently pumped 65psi oil pressure when driving and 25-35psi at idle using 20W/50 Valvoline XLD oil.
Last week I changed the oil and filter and have just completed a 600 mile trip in temperatures of 35-40 degrees celsius. I have a 92 degree thermostat as the engine runs sweetest at this temperature and uses no oil.
My concern is that the oil pressure dropped from 65psi to about 57psi at 3500rpm but would rise to 65psi when the revs dropped to 2500. I can only put this down to the particularly hot weather. My immediate thought is to replace the oil pump with a new unit, as this was not done at the time of the engine rebuild, with the objective of having a normal pressure close to 80psi thus providing a greater comfort zone for hot weather touring.
Any comments would be greatly appreciated.
Peter M
Peter Malkin

Peter,

When you said slowing from 3500 to 2500 made the oil pressure go up, do you mean immediately or after cruising at 2500 for a while? If you mean after cruising for a while, it is probably temp related. If you mean immediately, that’s odd and I don’t know what would cause it.

Charley
C R Huff

I don't think that this is anything necessarily to worry about. The cause could be cavitation where the pump is not capable of achieving a suffiently high flow at the higher revs and so sucks some air. A new pump however just might put this right but if you do get one check the clearances out very carefully and correct the rotor end clearance if necessary by lapping in the end of the pump body to achieve minimum clearance.
Iain MacKintosh

It's as likely to be the relief valve as much as anything, these are well known to oscillate at certain combinations of revs and viscosity i.e. temperature, Abingdon changed the gauge to include a sintered plug to control this but it is still visible. Every engine is different, doing it at different times, or not at all, and quite likely altering when you change the oil. Fiddling with the relief valve may stop it, or may just move it to a different set of circumstances. Mine has been doing it for 20 years, I can't even remember under what circumstances.

However many years ago I came across a case where the strainer was too close to the floor of the sump and restricted the flow of oil so much it cut oil pressure all together, but that was at maximum revs. Unlikely to suddenly have suddenly started happening unless you have clouted the sump, unless you have gone for a thicker oil last change.
Paul Hunt

If the oil filter has been changed to a spin on it could be a restriction in the new filter. There have been reports of the standpipe being too close to the top of the filter causing a pressure drop. RAY '67 Tourer
rjm RAY

Hi Peter,

One other thing it could be is that you overfilled the sump and at the higher revs the oil is being churned into a froth. I've had this happen years ago, on a VW Golf.

Herb
H J Adler

Thankyou all for your assistance.
Charley - the pressure increases immediately the revs drop - not temperature related.
Iain and Paul - I'm inclined towards the relief valve prognosis and to some extent the cavitation theory as the oil would have thinned considerably due to the excessive heat.
Paul - The sump has not been dinged at all. This is the first time that I have had this happen in 10 years/17000 miles.
Ray - Filter arrangement 10 years old.
Herb - Oil level spot on.
Again many thanks - I'll go for a drive again when temperatures return to normal.
Peter M
Peter Malkin

I think this is normal behaviour of a worn oil pump - it is only capable of flowing a certain amount of oil but the oil "usage" at the bearings increases as the revs rise. So at the point where the bearings are throwing oil faster than the pump can deliver then the pressure starts to drop. When hot the delivery from the pump is lower still and the usage from the bearings is greater, so the pressure drop happens at lower rpm than when the engine & oil are cold.
Chris at Octarine Services

Hi All

An oil change shouldn't precipitate the oil pump wear problem, it is likely however, to dislodge some debris whick could cause the valve not to seat the same as it did, maybe change spring and fit new high quality OE valve if you can find one or fit ball bearing which gives even more interesting oscillations, but works well.

Peter
peter burgess

Still, the easiest and cheapest thing to try is to replace the oil filter with a taller unit. This will eliminate it from the equation. RAY
rjm RAY

Chris - I've ordered a new pump which arrives tomorrow.
I'll be happier once I fit it as is another potential problem removed from the equation.
Peter - I assume that you are referring to the relief valve. I intend adressing that issue also.
Ray - The oil filter I used is identical to all those fitted since the rebuild.
Many thanks for all your assistance - will report results.
Peter M
Peter Malkin

Hi Peter

Please let me know the make of the oil pump ie what box it comes in.

Peter
peter burgess

And check the end clearance of the rotors in both old & new pumps. I've had a lot of new pumps with more clearance than spec. I set them to low limit.

FRM
FR Millmore

Peter Burgess
The pump has markings on the box as follows;
OP44 BLMC OIL PUMP B SER 5 MAIN BRG
Similar To: POL 702
Date: 17.03.09
FRM
Haven't measured clearance yet - what should I look for?
Many thanks
Peter M
Peter Malkin

Peter-
It's in the book, with pictures, better than I can write. The endfloat is all you can change, and I set it down to .002-.003. You can actually set it at .001 as long as it is not tight, but it is difficult to check this closely without bolting the pump to the block and ensuring it is still free with the bolts tight. I've had new pumps in excess of .015! And I've had worn pumps where the other numbers were too big, but setting the endfloat snug is the most important bit as far as pump function goes.

FRM
FR Millmore

FRM
Thanks for the info - I have just downloaded the full service manual (AKD3259 14th Edition) giving me the specs.
New pump measurements are; end float<.003"
outer rotor clearance<.010"
between rotors<.006"
All appears within spec.
I will measure my old pump once I get it out.
I have packed the new one with vaseline and lok tighted the two securing bolts.
Peter M

Peter Malkin

Hi Peter

Please tell me what box the pump came in.... very important.

Peter
peter burgess

Peter
The box is white cardboard having dimensions of 22cmx10.4cmx9.5cm. It is stapled down one side.
Hope this helps.
Peter M
Peter Malkin

Hi peter

Do you have a make on the box? Can you find out who made it? Some pump drives have a problem with the pin securing the rotor to the shaft.

Peter
peter burgess

Peter
There is no brand on the box - only the markings mentioned. I bought it from Heritage MG, probably the pre-eminent MG suppliers in Oz, and they told me that they sell "hundreds of them without a problem". I have never had cause to doubt them and their service is first class. Short of sourcing a nos item I probably don't have any other choice.
Any other Oz members have any experience with this pump?
Peter M
Peter Malkin

This thread was discussed between 15/11/2009 and 20/11/2009

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