Piston failures in late-model engines are relatively uncommon thanks to computerized engine controls that keep a close watch on the air/fuel mixture, and knock sensors that back off spark timing advance if detonation is detected. Pistons are also lasting longer because many engines now come factory-equipped with stronger hypereutectic alloy pistons that can withstand higher operating temperatures, or even forged pistons. Some pistons also have a factory anti-scuff coating on the skirt to prevent damage if the engine overheats or runs low on oil.
Yet in spite of these things, pistons do sometimes fail. The symptoms of a piston failure can include engine noise (rattling or knocking noises while the engine is idling), oil burning, misfiring and loss of power.
If the Check Engine light is on and you find a misfire code for a specific cylinder, and the engine is suffering any or all of the above symptoms, it may be a clue that you’re dealing with a mechanical problem rather than a fuel or ignition problem. Misfires can be caused by a variety of problems, including a dirty or dead fuel injector, a bad spark plug or plug wire, or a bad ignition coil on a coil-on-plug or multi-coil distributorless ignition system. But a fuel or ignition problem won’t cause engine noise or blue smoke in the exhaust.
Loss of compression also can cause a misfire and a loss of power. A compression loss can occur if the engine has a burned exhaust valve, a bent valve, weak or broken valve springs, a blown head gasket, a rounded cam lobe — or a bad piston. If a compression test shows little or no compression in a cylinder, and a leakdown test reveals compression is being lost into the crankcase and not out the intake or exhaust ports, chances are the piston has a hole in it or is badly cracked. Unless you have a bore scope (a fiber-optic tool that lets you peer inside the cylinder with a small probe inserted through the spark plug hole), the cylinder head will have to come off the engine so you can inspect the piston.
If the engine is burning oil and there is blue smoke in the exhaust, the problem could be worn valve guides and valve guide seals, worn or broken piston rings, or even a cracked piston.
Diagnosing a Burned Piston
If the top of the piston has a melted appearance, or it has a hole burned through the top, the piston has been running way too hot. Preignition and/or detonation have destroyed the piston. Aluminum melts when combustion temperatures get too high, so don’t blame the piston. The fault is whatever created too much heat in the combustion chamber.
One of the most likely causes of a burned piston is a dirty fuel injector that is running way too lean. If you found any trouble codes such as a P0171 or P0174 that indicate a lean fuel condition on the same cylinder bank as the bad piston, bingo, the engine likely has a dirty injector on that cylinder, and possibly dirty injectors on the other cylinders, too. The only way to know for sure would be to pull all the injectors, clean them on a fuel injector cleaning machine, then flow-test all the injectors and compare the results. Replace any injector that doesn’t flow within 5 to 8% of the rest.
If you don’t have a fuel injector cleaning machine with a flow tester, the next best thing to do would be to replace the injector on the cylinder with the burned piston, then clean the rest of the injectors once the engine is back together and running again.
Other conditions that can cause a burned piston include the wrong heat range spark plug (too hot for the application), over-advanced ignition timing (unlikely with today’s electronic spark controls), possibly a bad knock sensor that failed to detect detonation, low octane gasoline (bad gas that doesn’t meet a minimum octane rating of 87, or someone using 87 octane gas in a high compression engine that requires premium fuel), or anything that would cause the engine to run hotter than normal (low coolant level, bad thermostat, weak water pump, cooling fan that isn’t working, or a clogged catalytic converter that is creating a restriction and backing up heat in the engine).
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.
Just remember, if an engine burned a piston, it did so for a reason. So until that reason has been diagnosed and repaired, it’s pointless to replace the piston.
Detonation is the most likely cause of a cracked or broken piston. The hammer-like blows of detonation can literally beat a piston to death. The causes are similar to those that can burn a piston: a lean fuel mixture, over-advanced spark timing, a bad knock sensor, low octane fuel or anything that causes the engine to run hotter than normal.
Loss of exhaust gas recirculation (EGR) is a common cause of detonation (spark knock) because EGR has a cooling effect on combustion temperatures. Consequently, if the EGR system isn’t working, combustion temperatures may exceed the octane rating of the fuel causing the air/fuel mixture to ignite spontaneously before the spark plug fires. The knock sensor should detect the rattling noises produced by detonation and signal the PCM to back off spark timing. But if the knock sensor isn’t working, or the PCM fails to retard timing, detonation may continue unchecked and eventually damage the engine. Possible causes for loss of EGR include a bad EGR valve, loss of vacuum to the EGR valve (due to a leaky hose or EGR vacuum solenoid) or carbon buildup under the EGR that restricts exhaust flow back into the intake manifold. If you suspect an EGR problem, check the operation of the system and remove the EGR valve to inspect the intake manifold passageways for carbon buildup (clean as needed).
Scuffed pistons can be caused by too much heat in the combustion chamber, the engine overheating or inadequate lubrication. The piston-to-cylinder clearances in most late-model engines are much less than they used to be to reduce piston rock and noise. So if the piston or cylinder gets too hot, the clearance goes away and you get metal-to-metal contact. That’s where the anti-scuff skirt coatings on some late-model pistons come into play. The coating provides a layer of protection that can reduce scuffing under adverse operating conditions. It can’t protect the piston indefinitely, but it can provide a degree of protection that is nonexistent with a standard uncoated piston.
In cases where piston scuffing is lubrication related, the cause may be a low oil level (due to a lack of maintenance or oil leaks), low oil pressure (the pistons and rings rely on splash lubrication, so reduced oil flow due to a worn oil pump is a possibility) or poor oil quality.
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 also may 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 indicate 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.
As before, it’s a waste of time to replace a scuffed piston until the cause has been determined and corrected.
Grooves or scratches down the side of a piston and/or dents, dings and marks on the top of a piston are the result of dirt or debris getting into the cylinder. Chances are more than one piston will be affected. The cause could be a missing or ill-fitting air filter. On a turbocharged engine, check the turbo compressor wheel for damage that may have thrown shrapnel into the engine.
Vertical scratches on the piston rings would also tell you that dirt and debris have been getting into the engine. If the damage is mostly on the top ring, check the engine’s air filter and intake system. If the scratches are on the oil ring, the contamination is in the crankcase. Check the PCV system and crankcase breather for leaks.
Damaged Piston Ring Lands or Grooves
Too much heat is the most likely cause of ring land or groove damage. This problem most often occurs in the top ring groove because it is exposed to the most heat from the combustion chamber. Anything that causes the fuel mixture to run lean or the engine to run hot can be a contributing factor.
On many late-model pistons, the top ring groove is very close to the piston crown to reduce emissions. The crevice between the top ring and the piston crown can trap unburned fuel, increasing hydrocarbon emissions. Also, many pistons are shorter to reduce weight, so the ring pack is positioned further up on the piston body.
The top ring on many engines today runs at close to 600° F, while the second ring is seeing temperatures of 300° F or less. Ordinary cast iron compression rings that work great in a stock 350 Chevy V8 can’t take this kind of heat. That’s why many late-model engines have steel or ductile iron top rings. Steel is more durable than plain cast iron or even ductile iron, and is required for high output, high load applications including turbocharged and supercharged engines as well as diesels and performance engines. Some piston manufacturers also anodize or coat the ring grooves to improve piston and ring durability.
If a piston has shattered and self-destructed, it likely hit a valve. On interference engines, there’s not enough clearance between the piston and valves to avoid direct contact if the timing belt breaks. In most instances, the pistons will survive a timing belt failure but bend the valves. But, in some instances, the sudden impact of a valve against a piston is more than the casting can handle, causing the piston to shatter like a hand grenade. The debris goes into the crankcase and may cause additional damage to the bearings or other pistons.
This kind of piston failure is really bad news because the engine must be torn down and thoroughly cleaned to flush out all the debris before it can be rebuilt. A piston can also shatter if an engine sucks a valve or the head breaks off of a valve. A dropped valve can occur if the valve spring keeper fails or pulls through (sometimes as a result of over-revving the engine), or the valve spring fails and allows the keeper to pop loose.
If the head of a valve has broken off, the cause is often a lack of concentricity between the valve guide and seat. This causes the valve to flex slightly as it closes, and eventually forms fatigue cracks that lead to valve breakage and failure.
Forged pistons are much more ductile than cast pistons, and are much less likely to shatter if they hit something in the cylinder. But no piston can survive a valve or valve head dropping into a cylinder.
Piston Wrist Pin Damage
Wrist pins can wear out prematurely if the pistons are not receiving enough splash lubrication, but they can also fail as a result of over-revving the engine or too much rod flex. A worn crankshaft thrust bearing that allows too much back-and-forth movement of the crankshaft can flex and twist the rods as the engine runs. Over time this can damage the wrist pins, and cause fatigue cracks to form in the rods, which may lead to rod breakage and failure.
Sometimes a wrist pin will work loose and chew into the cylinder with each stroke of the piston. The underlying cause here may have been improper installation of the retaining lock rings on a full floating wrist pin, improper fit or installation of a pressed-in wrist pin, a twisted or bent connecting rod, excessive thrust end play in the crankshaft or taper wear or misalignment in the crankshaft rod journal.
Piston slap is a classic symptom of too much clearance between the pistons and cylinder bores. Piston slap is most audible when a cold engine is first started because clearances are greatest then. This doesn’t necessarily mean the pistons are worn, because some new engines will slap a bit when first started. But if the slap doesn’t go away as the engine warms up, it usually means the pistons and/or cylinders are worn. A compression test and/or leakdown test can be used to confirm the diagnosis.
The wrist pins on many pistons are offset slightly so as to load the piston slightly to one side. This reduces piston noise as the piston approaches top dead center (TDC) on its compression stroke, then passes over TDC and starts down on its power stroke.
Latest posts by Larry Carley (see all)
- Battery Service and Diagnosis - Oct 23, 2014
- Improving the Head Gaskets, Fasteners Relationship - Oct 16, 2014
- Emissions Codes: Mercedes-Benz Case Study - Jul 17, 2014