Low compression

[Richard Farrant]

Originally Posted by Malcolm Towrie

The valley shows a depressing amount of sludge on all surfaces. And the oil pan has a good layer of black sludge on the bottom. We use a 15W40 diesel oil in most of our vehicles. This is a high detergent oil and I suspect it is bad news in this type of application as the detergent dissolves the stable sludge deposits and circulates it around the engine. We should be using a non-detergent oil on these old engines.

Hi Malcolm,
A mistake people often make is getting an old vehicle of unknown history, draining the oil and refilling with a detergent multigrade oil. Most of these old WW2 era engines (and earlier) had either no filter at all or a bypass filter, which was tapped of the oil gallery and returned to sump. Detergent oils keep dirt and carbon in suspension so with a full flow filter system it is quickly cleaned. The older mono-grade oils had no detergent in them and the sludge formed in the sump mainly, so it was flushed out on oil changes. If you get hold of an unknown engine and it has a bypass filter, or none at all, go for a non-detergent mono-grade as blended for vintage vehicles. Actually the Classic / Vintage mono-grades in the UK do now have a very slight detergency in them as I am informed by the oil companies, but they do not scour out all the old sludge like a multigrade does.
Hope I am not teaching you to suck eggs, but I am sure there will be readers who are not aware.
regards,
Richard