Question: My engine burned a piston. What does that mean and why did it happen?
Answer:It means the piston failed due to excessive heat in the combustionchamber. A burned piston will typically have a melted appearance, or ahole burned completely through the top of the piston. Aluminum can onlywithstand so much heat, and when it gets too hot, it melts. Theunderlying cause is usually detonation and/or pre-ignition.
Detonationoccurs when the temperature and pressure inside a cylinder exceed thefuel’s octane rating. Instead of igniting when the spark plug fires,the air/fuel ignites spontaneously much like a diesel engine. Thiscreates multiple flame fronts within the combustion chamber thatcollide and hammer the top of the piston, producing a metallic knockingnoise called “spark knock.”
Common causes of detonationinclude a buildup of carbon deposits in the combustion chamber and onthe top of the piston that increase compression, no EGR (exhaust gasrecirculation), overadvanced ignition timing, a bad knock sensor, alean fuel mixture, low quality gasoline that does not meet minimumoctane requirements, or any cooling problems that causes the engine torun hotter than normal (coolant leak, low coolant level, bad waterpump, stuck thermostat, restricted radiator, defective cooling fan,even exhaust restrictions that back up heat into the engine.
Pre-ignitionoccurs if a hot spot develops in the combustion chamber that ignitesthe air/fuel mixture before the spark plug fires. The hot spot may bethe spark plug itself, an overheated exhaust valve, carbon deposits inthe combustion chamber, or a sharp edge in the combustion chamber.
Commoncauses of pre-ignition include the wrong heat range spark plugs (toohot for the application), carbon deposits in the combustion chamber andon the tops of the pistons, a lean fuel mixture, detonation or anythingthat makes the engine run hotter than normal.
On engines thatare turbocharged or supercharged, too much boost pressure and/or notenough fuel can burn a piston very quickly. Check the operation of thewastegate and boost control system. If the turbo system has beentweaked to deliver higher than stock boost pressure for more power, theturbo may be pushing more air into the engine than the stock injectorscan handle, causing the fuel mixture to lean out and burn the piston.
Anoften overlooked cause of piston burning is a weak or dirty fuelinjector. If an injector is not spraying enough fuel into thecombustion chamber, the air/fuel mixture in that cylinder may becometoo lean increasing the risk of detonation, pre-ignition and pistondamage.
Installers may diagnos the problem using a scan tool to check for leancodes and to look at fuel trim values. If a P0171 or P0174 code isfound, or the long term fuel trim readings are 10 or higher (indicatinga lean mixture), the engine may have one or more weak or dirtyinjectors.
The only way to know for sure is to remove theinjectors, clean them on a fuel injector cleaning machine, then flowtest all the injectors and compare the results. Any injector that doesnot flow within 5 to 8 percent of the rest should be replaced.
Question: What is a ‘scuffed’ piston?
Answer:A scuffed piston is one that has been damaged by rubbing against thecylinder wall. The metal-to-metal contact smears the metal on the skirton the piston and damages the piston.
Scuffed pistons can becaused by too much heat in the combustion chamber, engine overheatingor inadequate lubrication. The piston-to-cylinder clearances in mostlate model engines is much less than it used to be to reduce pistonrock and noise. Consequently, if the piston or cylinder gets too hot,the clearance goes away and you get metal-to-metal contact.
Ininstances where piston scuffing occurred due to a loss of lubrication,the underlying cause may be a low oil level in the crankcase (due to alack of maintenance or an oil leak), low oil pressure (a worn oilpump), poor oil quality, or oil breakdown (not changing the oil oftenenough).
When diagnosing a scuffed piston, note where the pistonis scuffed. If the cause is overheating, the scuffing will mostly be onthe upper ring lands and on the sides near the wrist pins.
Theremay also be oil carbon and lacquer burned onto the underside of thepiston indicating it got too hot. Scuff marks on the lower skirt areaoften indicates a lack of lubrication (check the oil pump and pickupscreen). Scuff marks on the edges or corners of the thrust sides of thepiston may be the result of bore distortion. Scuffing on both thrustsides would indicate binding in the wrist pin.
Some originalequipment pistons and many aftermarket performance pistons now havespecial anti-scuff skirt coatings to reduce the risk of scuffing.
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