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Extended oil drains may cost you an engine.


Fuel getting into petrol and diesel engine sumps isn’t a new problem, but it’s now being seen in brand new, not worn, engines. Manufacturers keep capped price servicing as cheap as possible by extending oil drain periods, but fuel-diluted engine oil can destroy your engine.

On the face of it, capped price servicing allows new vehicle buyers to predict their regular maintenance costs and therefore should be viewed as a Good Thing. However, it’s promoted as an example of the lower cost of ownership of a particular new vehicle, so the service intervals are stretched.

Capped price servicing is also a cunning way of keeping buyers returning to the dealership for service, rather than in the after-market servicing arena.

Let’s see what’s happening
in too many buyers’ real world situations. The new petrol or diesel powered vehicle sits proudly in the driveway and all seems fine. The diligent owner checks the sump oil level every week and the level stays at the middle mark.

“What’s the upper mark for?” he or she may wonder, assuming there must be a good reason. There is a vitally important reason, but buyers are unlikely to hear about it from the dealer.

That upper mark on many modern engine dipsticks shows a high sump level and the cause of that increased sump volume is fuel getting into the oil.

When the sump level reaches the upper mark it indicates a dangerous percentage of fuel in the engine oil and it’s urgent time for an oil change. In theory, that contamination may be below the critical five-percent point, but if the engine is using oil – and many turbocharged engines do – the contamination level may be much higher.

The SAE viscosity range is broad enough that fuel dilution up to five percent can be tolerated and synthetic motor oils maintain their grade better than mineral oils.

Fuel dilution in oil is a condition caused by excess, unburned fuel mixing with engine oil in an engine crankcase.

Hydrocarbon-based fuel, usually with a lower vapour pressure than the lubricant, has a thinning effect, lowering the oil viscosity. Oil film strength is reduced, increasing cylinder liner and bearing wear.

Dilution is likely in many modern diesel engines that have diesel particulate filter (DPF) designs with passive regeneration. Engine makers have tried several approaches to raise exhaust gas temperatures to burn off fuel soot and the most widely used approach is late in-cylinder fuel injection. Fuel is injected very late in the cycle, typically near the time of exhaust valve opening.

The fuel vaporises but does not oxidise in the cylinder, thus generating unburned hydrocarbons. The hydrocarbons should oxidise in the diesel oxidation catalyst (DOC) and raise the exhaust gas temperature. This hot exhaust gas then flows through the DPF and, hopefully, oxidises the trapped soot.

This late injection approach is a low cost approach by engine OEMs. However, an increase in fuel dilution has been observed all around the world. Toyota’s latest 2.8-litre diesel has a fifth injector and this has been promoted as a way of avoiding over-fuelling the cylinders, but the system has been plagued with problems.

Oil contamination and other issues with modern diesel engines are explained in this Tech Torque piece.

As for petrol engines, some of the latest small-capacity turbocharged and direct injection engines produce much more unburnt fuel blow-by past the piston rings than older, larger-capacity engines.

So, what’s all that have to do with capped price servicing, you may ask.

Capped price servicing intervals are typically at least 10,000km and many are double that figure. If you’re periodically checking your oil and the level is normal you assume that the engine isn’t using any, which is a tad unrealistic. The level is being ‘topped up’ by fuel leakage past the piston rings.

By the time the programmed service interval arrives the oil may be dangerously contaminated by fuel, greatly diminishing its lubricating quality.

The situation can be worse for owners who don’t check their oil levels, because the fuel build-up in the sump can raise the oil level to the warning point, but they never see it.

Budget priced, capped cost servicing doesn’t include oil condition analysis, so you’ll never know how much fuel was in your sump.

Older diesels that required oil changes every 5000-8000km were less affected by fuel dilution, because of lower combustion pressures, but even if some fuel did make its way into sumps, was drained out more frequently.




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