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Slightly low oil pressure


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Can anyone out there help with a slightly low oil pressure and where it might be going?

 

The engine was rebuilt in 2012 since when it has not travelled many miles. It has a reground crank 30 thou undersize

 

Initially when I checked the oil pump it seemed all within spec so rather than replace an original item with a questionable replacement I kept it.

 

When I started to use the car the oil pressure was on the low side when the oil was hot so I bought a new pump from the TR Shop along with a set of big end shells and fitted them. When I took the old shells out they weren't showing signs of wear and the crank was good but the pressure did go up a bit. However, it is still marginally on the low side.

 

I have just got back from a weekend at Spa Classic 6 hour which involved approximately 800 miles of mainly motorway driving. During that time the pressure at idle (750rpm) was around 20 psi but only about 60 psi at 2000 rpm, rising to 70 psi at 2500 rpm.

 

There are no untoward noises coming from the bottom end of the engine and the oil consumption was/is negligible.

 

As I said at the beginning, has anyone got any ideas on where the oil pressure could be going?

 

It might also be worth adding that the pressure seems to take a while to build up on the gauge when the engine is started from cold.

 

Rgds Ian

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Those pressures your quoting are fine, I wouldnt worry about that, its not the pressure anyway that matters its the volume.1920`s Racing Bentleys used to run 8lb pressure.

Stuart.

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Hi Ian,

The pressures sound pretty well on the ball to me. Your tick over pressure of around 20psi is what I get on my 4A & what most others seem to think is ok & your 2500 rpm figure also looks good & similar to mine. Not so sure why it might take a while to build up as mine starts quite high (80 - 85) & then drops back when the oil warms up after a few miles but I take it easy to try & keep it lower in those few miles. On my 4 the tick over pressure is around 50psi & @ around 3000rpm up to around 70psi. Pretty steady @ 70 - 75 above 3000.

What oil are you using?

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Hi Ian,

following Stuarts advice - don;t panic.

 

As for disappearing - the pump output , both pressure and volume is RPM dependent

When cold the pressure, for any given rpm, will be higher than when it is hot. The oil thins and the metal expands.

 

On tick over with cold oil I get 50psi easy. When hot it drops to 20psi.

 

Roger

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Ian,



Oil change always ups my pressure for a couple of thousand miles then starts to fall away. Have you actually checked the oil pressure with a better quality gauge than the original Smiths? My pressure dropped about 7 lbs when I swapped gauges, but which gauge was correct - if either?



A slow oil gauge at startup can be caused by air in the capillary tube, readings will be OK just a little damped.



I heard gauges described by a workshop as for the worried well, the pressure light being a better indicator for the driver.



I would not worry with the readings you have given which are similar to my own.



Alan

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Those pressures your quoting are fine, I wouldnt worry about that, its not the pressure anyway that matters its the volume.1920`s Racing Bentleys used to run 8lb pressure.

Stuart.

Yes Stuart that Bentley pressure is quite surprising but they did OK!

If the pressure is unduly high on our cars does it not force oil out of all those TR orifices?!

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What oil are you using, Ian?

Once, I fell victim the the blandishments of the oil companies and filled a new engine with a "synthetic", 5-40W. The oil pressures were so low that i drained it and chucked it, never mind the cost and refilled with the real stuff, whihc restored normal service.

JOhn

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[quote name="Ian Vincent" post="599746" timestamp="1537286288

 

It might also be worth adding that the pressure seems to take a while to build up on the gauge when the engine is started from cold.

 

Rgds Ian

 

Hi Ian,

It is normal that it takes a moment to build pressure.

The standard oil filter which is hozizontal will drain back to the sump after a while.

If you install the modern oil filter (with adapter) you can hang the filter almost vertical, and the correct filters have a non return valve that prevents draining back.

Waldi

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Crankshaft end float letting it leak out ?

 

Mick Richards

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Where from Mick? The thrust washers?

 

Rgds Ian

 

"When I took the old shells out they weren't showing signs of wear and the crank was good but the pressure did go up a bit. However, it is still marginally on the low side."

 

Yep.

 

Mick Richards

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Yes

 

Harvey S. Maitland

But measure the end float first, deflect the crankshaft against a DTI ( clock) to check the range. However as suggested above it wouldn't bother me running an engine at the pressures you have recorded.

Flow...flow...flow counts for more as long as the pressure is not excessively low, I ran my road car for a couple of thousand miles with well under those pressures before I overcame my laziness inertia and stripped the oil filter head to remove a chunk of carbon that if a Diamond would have made a nice ring ! No damage to crank bearings.

 

Mick Richards

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It is also quite possible to loose oil pressure due to worn camshaft bearings..

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Those pressures are fine. I get pressures like that, on a still standard-size 280k mile crank and original pump.

I found that blow by from worn bores can drop the psi, especially after a long drag uphill.

Cant see worn tw dropping oil pressure, arent they splash lubricated?

Peter

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Can anyone out there help with a slightly low oil pressure and where it might be going?

 

I have just got back from a weekend at Spa Classic 6 hour which involved approximately 800 miles of mainly motorway driving. During that time the pressure at idle (750rpm) was around 20 psi but only about 60 psi at 2000 rpm, rising to 70 psi at 2500 rpm.

 

 

Rgds Ian

 

 

 

 

If I am to believe my odometer reading, my 6 has 108,000 miles clocked up and the above readings mirror the situation with my engine so I am encouraged by these posts - thanks

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On 9/18/2018 at 5:42 PM, john.r.davies said:

What oil are you using, Ian?

Once, I fell victim the the blandishments of the oil companies and filled a new engine with a "synthetic", 5-40W. The oil pressures were so low that i drained it and chucked it, never mind the cost and refilled with the real stuff, whihc restored normal service.

JOhn

If anyone interested in updating their expertise on 'oil technology' see below

August 31, 2018 issue of Car Mechanics

 

Regards

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