Should I use flushing oil to clean my engine?

Hello Paul, my Mercedes C200 has been parked for four years and I have been advised to flush the engine. Apparently the oil has become thick and lumpy. Can I flush it at a petrol station? George

Hello George, I have my reservations about the traditional flushing of engines. Before I share my concerns, let us understand why it is done. Engine flushing is the cleaning of internal engine sludge or dirt using a mixture of kerosene or diesel (automotive gas oil) and oil run in the engine for a few minutes.

Sludge is when your engine oil forms a solid or gel at engine operating temperatures. Sludge is usually a result of intrusion of water or excess blow by gases in the engine oil, which can happen due to a failure of the crank case ventilation or engine cooling system.

Many times, sludge develops when you use cheap or counterfeit engine oils that are not designed or blended with sludge inhibitors. Kerosene and diesel are more corrosive and can separate some sludge from metallic surfaces and break it down for dispersing.

While this procedure may clean some sludge inside the engine, I have my doubts about its appropriateness and effectiveness.

Engines are designed to be lubricated by engine oils of specific viscosity (oil thickness) and performance grade. Altering that recommended viscosity and oil grade by introducing a blend of kerosene or diesel with oil poses the risk of running the engine, albeit for a short time, without adequate lubrication and protection of sensitive metallic engine components in the valve train and crankcase against metal surface sheer or damage. The kerosene or diesel blend with motor oil will damage rubber seals in the long run.

It may also not be easy to fully clean out the flushing oil after its use, remnants of which will contaminate the new engine oil you intend to use for a longer service interval.

Consider opening the crank case or sump to clean it and the oil pick up tube. Buy a good mineral engine oil product that is designed with good sludge dispersant additives, run it for a few kilometres, drain it and change the filter.

This should sufficiently and safely clean your engine while protecting against further sludge buildup. Over the next couple of drain intervals with a good oil product, your engine should be cleaner.