Connecting Rod Breakage Explained (B09080B)

CONNECTING ROD BREAKAGE EXPLAINED

TECHNICAL SERVICE BULLETIN

Service Information

EDITOR'S NOTE:  This article replaces "Why Do Connecting Rods Break?" in the November '05 issue of ServiceNews  .

Connecting rods are specifically made to withstand brutal forces and temperatures. Yet, every once in a while, they bend or break and wind up as cool conversation pieces for a coffee table or desk. Why?

Fig 1: Bent Connecting Rod

For a connecting rod to bend or break, either the engine must hydrolock or the connecting rod bearings must fail. When troubleshooting a bent or broken connecting rod, here's some food for thought:

• Hydrolocking is the number one  culprit for connecting rod breakage. It happens when the volume of liquid that's entering the combustion chamber (water or fuel) exceeds the chamber's volume. Since liquids don't compress, the piston stops before it reaches top dead center (TDC). But the crankshaft keeps on turning from inertia, so as a result, the connecting rod bends and gets shorter. With each compression and power stroke, that rod flexes. You won't always notice anything wrong, until it finally gives out and breaks from metal fatigue.
• Hydrolocking can happen if you're driving through deep, standing water that's above the level of the air intake, or you're driving fast enough through it to splash water over the hood. It can also happen if you're driving through deep, standing water and someone driving the other way splashes water over your hood. Vehicles with cold air intake systems are especially vulnerable to hydrolocking since the air filter is mounted down low to draw air from the bottom of the engine compartment or the front of the radiator. Water sucked into the air filter gets sucked right into the engine.
• If you suspect hydrolocking, check to see if the air cleaner element and air cleaner housing are wet. Often times, there'll be dirt or mud from the road deposited on the inlet side of the air cleaner element or even in the intake air duct. Keep in mind, hydrolocking isn't  a manufacturing defect so it's not  covered by warranty. If hydrolocking is the problem, your service client needs to contact his or her insurance provider; it's considered accidental damage.
• Although it's rare, sometimes a bad ECM/PCM or fuel injector can cause too much fuel to enter a cylinder, causing hydrolocking and resulting in a bent rod. Before that happens, though, there are usually some sort of driveability problems that crop up and remain after the engine is fixed.
• A bent connecting rod might not show any symptoms until it actually breaks. But a sure-fire indicator is the carbon line it leaves on the cylinder wall. A normal connecting rod leaves a carbon line that's about 6 mm  deep. A bent connecting rod, since it's shorter, leaves a second  carbon line that's even deeper.
Fig 2: Carbon Line

Fig 3: Second Carbon Line

• Connecting rod bearings fail for lots of reasons. To find out what really caused a rod bearing failure, you've got to take apart all of the rod bearings and main bearings and compare them side by side. If all or most of the bearings and journals show similar damage (scoring, exposed copper, or heat discoloration), the likely culprit is oil starvation, poor maintenance, customer abuse, or running the engine with an aftermarket power booster (a nitrous oxide setup, supercharger, or turbocharger).
• If just the bearing or journal for the failed rod looks bad, but all the other ones look good (no scoring, exposed copper, or heat discoloration) and the crankshaft journals are smooth with no heat discoloration, then that particular bearing/clearance would be suspect.