AfterMarketNews AfterMarketNews Auto Care Pro AutoCareCareerHub Brake&Frontend BodyShopBusiness Counterman EngineBuilder Fleet Equipment ImportCar Motorcycle & Powersports News Servicio Automotriz Shop Owner Tire Review Tech Shop Tomorrow's Tech Underhood Service Speedville

BMW Recognized for Engine-Building Excellence

BMW Group’s engine-building prowess was recognized with four wins at the latest International Engine of the Year Awards. The drive unit in the BMW i8 earned two class wins as well as being declared overall winner, with a further class win being garnered...

Read more...

Diagnostic Dilemma: The Case of the Missing Code

When doing mobile diagnostic work, no-code stalling complaints are a major part of your agenda. In most cases, the client shop is simply too busy to duplicate the failure or, in some cases, a long test drive will yield nothing in the way of useful...

Read more...

Secondary Ignition: The Art of Spark

What is a coil? From the beginning of the internal combustion engine, several different ignition systems have been used to create a high-energy spark. The most popular system, and the one that’s in use today, is a step-up coil. A coil is nothing...

Read more...

Regulators Launch Investigation Into Jeep Grand Cherokee Brake Defect

Federal regulators are investigating whether the automatic braking systems in some 2014 Jeep Grand Cherokees may be defective after receiving a number of complaints from motorists about a potentially dangerous glitch that caused their vehicles to come...

Read more...

Electronic Proportioning Valve: Doing More With Less Hardware

Anti-lock brake systems (ABS) and the HCU are replacing proportioning, combination and other valves to change the braking forces in the front and rear. This is called Electronic Brake Distribution (EBD) and it can dynamically change the proportioning...

Read more...

NHTSA’s GM Brake Line Corrosion Investigation: Reading Between the Brake Lines

There will be no recalls on some GM vehicles for brake line corrosion. Instead, we received an advisory from the National Highway and Traffic Safety Administration (NHTSA) about brake line inspection and car washes. What was not discussed was the corrosion...

Read more...

ZF 8-Speed Transmission Replacement

The ZF 8HP transmission made its debut in 2009, and since its introduction, has been one of the top choices for international car manufacturers. BMW, one of ZF’s largest customers, uses the 8HP across its entire product portfolio. BMWs featuring...

Read more...

Shop Folds After Shoddy Brake Work Endangers Woman

A Florida woman was thankful to walk away without injury after her vehicle’s brakes went out for the second time in as many months due to negligence on the part of her local repair shop, ABC Action News reported. Shelly Darling of Zephyr Hills, FL,...

Read more...

Air Ride Diagnostics: Reservoirs and Compressors

Just about every Asian and European luxury import nameplate manufacturer has a 7- to 10-year-old vehicle on the road with an air-ride suspension at all four corners. Chances are one of these vehicles will be coming to your shop sooner rather than later. Regardless...

Read more...

Can Cordless Power Tools Replace Pneumatic Tools?

I recently attended Milwaukee Tool’s annual media tool preview event where they unveiled over 80 new products that will be available later this year and early next year. Milwaukee Tool is known primarily as a leading supplier of professional-grade power...

Read more...

OTC's Technician Challenge Tour Will Award Ultimate Diagnostics for Life

The OTC Technician Challenge is going on tour with dates in six states, showcasing the Encore with new Bravo 2.0 software. Technicians nationwide will compete to win one of three Ultimate Diagnostics for Life packages. Ultimate Diagnostics for Life winners...

Read more...

New Productivity Features Added To Coverage In Snap-on Software Upgrade 15.2

Snap-on has increased its vehicle coverage in the new Software Upgrade 15.2 for the Auto-ID, One-Touch Full Vehicle Code Scan and the One-Touch Code Clear features to provide effective, accurate and fast diagnostics out of the box and miles down the road. The...

Read more...

Print Print Email Email

By Larry Carley
Technical Editor

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 combustion chamber. A burned piston will typically have a melted appearance, or a hole burned completely through the top of the piston. Aluminum can only withstand so much heat, and when it gets too hot, it melts. The underlying cause is usually detonation and/or pre-ignition.

Detonation occurs when the temperature and pressure inside a cylinder exceed the fuel’s octane rating. Instead of igniting when the spark plug fires, the air/fuel ignites spontaneously much like a diesel engine. This creates multiple flame fronts within the combustion chamber that collide and hammer the top of the piston, producing a metallic knocking noise called “spark knock.”

Common causes of detonation include a buildup of carbon deposits in the combustion chamber and on the top of the piston that increase compression, no EGR (exhaust gas recirculation), overadvanced ignition timing, a bad knock sensor, a lean fuel mixture, low quality gasoline that does not meet minimum octane requirements, or any cooling problems that causes the engine to run hotter than normal (coolant leak, low coolant level, bad water pump, stuck thermostat, restricted radiator, defective cooling fan, even exhaust restrictions that back up heat into the engine.

Pre-ignition occurs if a hot spot develops in the combustion chamber that ignites the air/fuel mixture before the spark plug fires. The hot spot may be the spark plug itself, an overheated exhaust valve, carbon deposits in the combustion chamber, or a sharp edge in the combustion chamber.

Common causes of pre-ignition include the wrong heat range spark plugs (too hot for the application), carbon deposits in the combustion chamber and on the tops of the pistons, a lean fuel mixture, detonation or anything that makes the engine run hotter than normal.

On engines that are turbocharged or supercharged, too much boost pressure and/or not enough fuel can burn a piston very quickly. Check the operation of the wastegate and boost control system. If the turbo system has been tweaked to deliver higher than stock boost pressure for more power, the turbo may be pushing more air into the engine than the stock injectors can handle, causing the fuel mixture to lean out and burn the piston.

An often overlooked cause of piston burning is a weak or dirty fuel injector. If an injector is not spraying enough fuel into the combustion chamber, the air/fuel mixture in that cylinder may become too lean increasing the risk of detonation, pre-ignition and piston damage.

Diagnosis requires using a scan tool to check for lean codes and to look at fuel trim values. If a P0171 or P0174 code is found, or the long term fuel trim readings are 10 or higher (indicating a lean mixture), the engine may have one or more weak or dirty injectors.

The only way to know for sure is to remove the injectors, clean them on a fuel injector cleaning machine, then flow test all the injectors and compare the results. Any injector that does not 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 the cylinder wall. The metal-to-metal contact smears the metal on the skirt on the piston and damages the piston.

Scuffed pistons can be caused by too much heat in the combustion chamber, engine overheating or inadequate lubrication. The piston-to-cylinder clearances in most late model engines is much less than it used to be to reduce piston rock and noise. Consequently, if the piston or cylinder gets too hot, the clearance goes away and you get metal-to-metal contact.

In instances where piston scuffing occurred due to a loss of lubrication, the underlying cause may be a low oil level in the crankcase (due to a lack of maintenance or an oil leak), low oil pressure (a worn oil pump), poor oil quality, or oil breakdown (not changing the oil often enough).

When diagnosing a scuffed piston, note where the piston is scuffed. If the cause is overheating, the scuffing will mostly be on the upper ring lands and on the sides near the wrist pins.

There may also be oil carbon and lacquer burned onto the underside of the piston indicating it got too hot. Scuff marks on the lower skirt area often indicates a lack of lubrication (check the oil pump and pickup screen). Scuff marks on the edges or corners of the thrust sides of the piston may be the result of bore distortion. Scuffing on both thrust sides would indicate binding in the wrist pin.

Some original equipment pistons and many aftermarket performance pistons now have special anti-scuff skirt coatings to reduce the risk of scuffing.

Latest articles from our other sites:

Chicago Auto Show Display Takes 1.0L EcoBoost Apart

One of many highlights in the Ford booth at the 107th Chicago Auto Show was a large layout of the pieces that make up the first-ever Ford three-cylinder EcoBoost engine. This 1.0 liter turbocharged, direct-injection...More

Performance V6 and Four Bangers

Any engine builder worth his or her torque wrench keeps their eyes open. They’re looking for anything and everything to make work easier, better and, of course, more profitable. And just as important...More

When is Road Force Balance Overkill?

Can road force balancing really make a difference? To answer the question of whether road force balancing is overkill, we should probably start by reviewing some basics. We balance tire/wheel assemblies...More

Telematics & Tires

Just as vehicle telematics are affecting maintenance and repair, tire telematics are becoming reality, as well. Though several tiremakers may be working on “smart” tires or TPMS developments, Continental...More

ZF 8-Speed Transmission Replacement

The ZF 8HP transmission made its debut in 2009, and since its introduction, has been one of the top choices for international car manufacturers. BMW, one of ZF’s largest customers, uses the 8HP across...More

Shop Folds After Shoddy Brake Work Endangers Woman

A Florida woman was thankful to walk away without injury after her vehicle’s brakes went out for the second time in as many months due to negligence on the part of her local repair shop, ABC Action News...More

Homak Has Serious Tool Storage Covered

Homak has introduced its new 35" seven-drawer Service Cart. It has plenty of storage for all your tools and is offered in three unique, high-gloss finishes. Featuring full-extension gas struts in...More

Alert Stamping Introduces Versatile Rechargeable Hood Light

Alert Stamping’s new rechargeable SMD LED Hood Light offers dual brightness levels: 1,200 lumens on high and 700 lumens on low. Its bracket extends up to 88" and can stand on flat surfaces. The bottoms...More