Help. DB18 oil pressure

[Keith Ashworth]

Hi Dave,
All modern oils are unsuitable for our cars engines. I buy a special running in oil which is straight 30 with no additives. Modern oils have additives as a modern engine does not have to be "run in".
These additives prevent the rings on our old engines from bedding in.
Hope you sort your problems out.
Cheers
Keith

[ranald]

As mentioned in an earlier comment, I'd recommend checking oil is reaching the rocker shaft and valve gear. I've been following posts on the forum about engine oil and if I understand correctly, moving from 10 40 to 20 50 means the oil is heavier both when cold and when hot. I don't think this is what you intend. Have you thought about a mineral multigrade? Along with another forum member I've recently bought 15 40 mineral oil.

[grahamemmett]

Why are we experimenting by using anything other than the manufacturers recommended grade which is readily available? SAE 30 is available from Morris, Rock, Penrite, Comma and others over the counter or by mail order so why risk your engine by experimenting with a multi grade?

[ranald]

Graham, This the forum post which convinced me to replace a straight 40 with a mineral based 15 40. viewtopic.php?f=18&t=2222&start=10
All the best, Ranald

[grahamemmett]

Thanks for pointing me at this thread Ranald, I'd missed it.
Chris has provided very comprehensive research there and it seems it's a matter of personal choice of sticking with the manufacturer's recommendation (which he does advocate if your engine is in good condition) or moving to a mutligrade the choice of which depends on your use of the car.

[Dave Glyde]

Thanks again to everyone for their advice and help. I take the point that Daimler recommended SAE 30 but straight grade oils were all that was available then and I'm sure if multigrade oils had been available then they would have recommended it.

Update on my problem. I drained the oil from the engine to replace it with a mineral type. Less oil came out then I put in ! Confused ! Refilled with 20w50 mineral oil, started engine and oil poured out of the oil filter o ring seal. Stripped the filter unit and changed the o ring seal for another new one I had. Reassembled and no leak. No oil light even on tick over when warm.

The oil filter wasn't loose so why did the o ring leak ? This was a new one when I assembled the engine with a new filter. I had run the engine up to temperature several times and had driven around the block several times before I went for a test run. There were no oil leaks whatsoever from anywhere. The test run included a little bit of motorway driving at 50 mph. The question is could the oil pressure relief valve jam shut and cause the system to over pressurise thus causing the oil loss through the o ring?

I have striped the valve and was a bit of a kerfuffle to get the piston out but then it is a neat fit and nothing to get hold of. Has anyone experienced a jammed valve or am I barking up the wrong tree?

Luckily the engine is still smooth and quiet ( phew ) but I haven't got the nerve up yet to take it on a long run at any speed.

[Phillmore]

A relief valve jammed shut would cause full pressure when the oil is cold but should have no effect when the oil is hot. This could cause high pressure leaks when cold perhaps but would not be the cause of low oil pressure or leaks when warm as the valve would shut under spring pressure.

[Db n j]

Dave,

Happy it's looking better.

My guess is that oil type was not the reason for improvement, as it seems so dramatically different.

If the filter was sealed and not disturbed, then that suggests the original oil pressure was too low.

So, the improvement came after you removed and replaced the relief valve ?

If so, then it seems likely that the piston had stuck open, and is now free.

It sounds like you did this operation on the car ? Its easier to take the housing off the engine, which allows you to see the piston cylinder, and check for floating debris, you can work the spring against the piston and see if it binds at any point. My original unit was stuck open, which I had a real job to shift. Not ideal.

Given the cost, I'd be tempted to undo the 2 bolts and have a look. Otherwise it might stick again.

Btw your comment about less oil out than in, is explained by the oil collected in various pools, particularly the oil filter housing.

[Dave Glyde]

Thanks Andy and David. This morning I took a large quantity of tranquilisers and got the nerve up to take the car for a decent run for an hour or so including a few miles at 50mph on the motorway. Pleased to report all trouble free. No oil light on even at tickover and no sign of any oil leaks.

To answer your question David, yes I took the whole filter assembly off and stripped the relief valve down to check it was free and moving. The piston was difficult to get out and wouldn't come out under gravity or tapping the unit, so the piston may have become jammed or was just held in place by oil suction, and there is nothing to grip to get hold of it, finally got it out and inspected it, looked ok and worked it up and down the bore a few times.

I thought that what may have happened is if the relief valve is jammed closed so that it doesn't relieve the oil pressure from the pump, which as a gear pump at 50mph will produce a lot of oil flow and hence pressure as a positive displacement pump then the o ring oil seal is likely to be the weakest point and distort and leak. I lost several pints of oil and so believe the oil light came on due to lack of oil ! I was more then fortunate that I didn't destroy a brand new engine. That really would have meant a trip to Beachy Head for me - and the car.

Going on a 50 mile rally tomorrow with the local Daimler enthusiasts so fingers crossed.

[Phillmore]

Good luck Dave and enjoy your drive! It sounds as though it's sorted now. Daimlers all seem to show low oil pressure on tick over when warm so a flickering oil light when the oil is hot is quite normal. I think this is partly because our tick over revs are around half other classic car engines due to the fluid flywheel requirements which take up drive over 400rpm.