Diagnosing Diesel Piston Damage
Content provided to AA1Car.com by MAHLE Clevite
Got a diesel engine with one or more bad pistons? The following can help you figure out what may have caused your diesel engine's pistons to fail:
Melting at the top piston ring land area of a diesel piston
Signs of melted piston or ring land damage include:
• Erosion at the piston crown is visible.
• Melted areas can be seen at the piston crown – right up to a completely melted off top of the piston
• In extreme cases, there are seizure marks all along and around the piston. There is a hole in the piston.
This damage is attributed to the thermal overload of the piston. There are two causes for this.
Abnormal combustion can be diagnosed via the following features:
• The bowl edge has been "gnawed off."
• The injection nozzles display a poor spray pattern.
• The injection pressure and the delivery rate of the injection nozzles are set incorrectly.
• The top land shows seizure marks in the piston pin axis.
Melted piston crown on top of a diesel piston.
An abnormal combustion can be caused by a number of factors such as if the air-fuel mixture in the combustion chamber is too rich. This can be the result of the following:
• The air supply is reduced, e.g. the air filter is clogged.
• The fuel delivery is set incorrectly.
• The start of fuel delivery is set incorrectly.
• The nozzle needle is either wedged or stiff.
• The exhaust gas system is clogged.
There is ignition delay and misfiring, which may be caused by:
• The incorrect fuel or fuel with an insufficient cetane rating is being used, or there is gasoline in the diesel.
• The valves are leaky, resulting in compression loss.
• The air pre-warming is defective (especially for very low ambient temperatures).
Overheating of the piston crown can be identified via the following features:
• The combustion bowl is not damaged.
• An excellent spray pattern can be observed at the top of the piston.
The excessive temperature level of the piston crown can be caused by:
• The cooling oil nozzle is either bent, has become detached or has not been assembled (assembly error).
• The time between oil changes is too long. In this case, there is a risk of polymerization of the engine oil, especially when using biofuels, such as rapeseed and soybean oil, which can result in the cooling oil nozzles being clogged.
• Contamination, such as gasket residue, etc., prevents the required circulation in the oil circuit.
• Set the injection amount and timing according to the manufacturer's specifications.
• Check the injection nozzles for any leaks, the injection pressure and the spray pattern.
• Pay attention to correct alignment when assembling the cooling oil nozzles.
• Thoroughly clean the oil channels in the engine block, the crankshaft and the cylinder head.
• Make sure the pressure-regulating valve is functioning correctly.
• Ensure that the time between oil changes is much shorter when running the engine on biofuels.