OIL THICKENING IN LIGHT WEIGHT DIESEL ENGINES
by Lou Achterburg, Castrol Technical Services Manager
From time to time we encounter instances of excessive oil thickening in light weight diesel engines. While the factors which contribute to this condition are many and varied the obvious culprit in the minds of motorists is the oil itself, which is in reality the least likely cause of the condition.
High performance engine oils marketed by the major reputable oil companies have been developed to the point that in heavy duty diesel highway engines 40,000km service intervals are not unusual and the oil is still in reasonably good condition.
This exact same lubricant, usually an sae 15w/40 meeting at least api cf4/sg, can not go to 5,000km in a light diesel engine without significant thickening and deterioration.
OPERATING CONDITIONS ? WARM UP IS CRITICAL
The major difference in oil performance is due not only to differences in engine design but mostly due to operating condition. A diesel engine does not achieve combustion efficiency until it reaches operating temperature which usually takes 20 to 30 minutes of running. During this warm up period incomplete combustion deposits excessive quantities of carbon and soot on the cylinder walls where it is collected and absorbed by the dispersant additive in the oil. Frequent stop-start, short trips and periods of idling also increase the moisture contamination of the engine oil.
HOW IT HAPPENS
As most light weight diesels are not spectacular performers, most people tend to drive them hard through the gears, which leads to over fuelling and further contamination of the oil. The combustion by-products from a petrol engine are volatile and can be driven off once the engine is run at full operating temperature of a period of time.
However, the combustion by-products form a diesel engine cannot be driven off by engine temperature and as such are totally accumulative.
THE EFFECTS ON YOUR ENGINE
This build up of combustion residue eventually leads to the oil becoming very thick at cold start up so that it does not flow through the engine correctly and causes increased wear rates. The excessive moisture levels combine with the soot and carbon in the oil producing sludge deposits especially in the valve cover.
OTHER FACTORS TO CHECK
Other factors that contribute to oil thickening are:
- Starting with a high viscosity oil - i.e. Sae m20w/50 should not be used.
- Using an oil of lower performance level - i.e. Api sf/cd is totally unsuitable.
- Engine running too cold due to faulty thermostat or even worse, no thermostat at all.
- Faulty injectors or fuel pump set incorrectly
The way to head off these problems is to ensure a good quality brand name sae 15w/40 of at least api cf4/sg specification is used eg Castrol RX Super.
And the oil and filters are changed every 5,000km. 1/9/99
(A future fact sheet will explain oil specifications / classifications).