The 6.6L Duramax is one hell of an engine and when modded correctly, can’t be beat. Here are some of the flaws and how to fix them.

  1. Snapped Crankshaft


This is one of the most devastating failures. When a crank breaks, it can do more damage to everything around it. Not only causing a long downtime, but also quite a chunk coming from your wallet. The crankshaft usually breaks near the number one rod journal due to a combination of excessive rpm and the large external counterweight. A factory firing order that beats up the front area of the crankshaft has also been blamed for imposing undue stress on the crank.

2. CP4 Failure

This one is specific to the LML code Duramax produced between ’11-’16, which used the Bosch CP4.2 high-pressure fuel pump (vs. the predecessor CP3 found on LB7, LLY, LBZ and LMM engines). Unlike the ultra-durable CP3 that came before it, the CP4.2 doesn’t possess the same track-record for reliability. Any time water, rust or debris infiltrates the CP4.2 (usually due to bad fuel or lack of maintenance) it’s prone to either seize up or come apart internally. Unfortunately, the latter scenario plays out more often than seizures do. Typically, the roller lifter (or bucket) that rides on the CP4.2’s shaft (the part that’s driven up and down via the plunger) fails due to lack of lubrication or debris hampering its ability to operate in an unobstructed manner.

3. Bent Rod(s)—LB7/LLY

Seeing a connecting rod failure in an LB7 Duramax (’01-‘04) or LLY Duramax (’04.5-’05.5) is almost always for those making double the factory horsepower or more, the good news is that these rods will usually bend before they break. If they break, beating the cylinder to death is a sure way to making sure you order a new block.

4. Cracked Piston(s)—LBZ/LMM


When power levels reach more than the stock 650-700go for the street driven 2006-2010 trucks these pistons start to crack. It can also happen in just the stock trucks as well because the pistons are low quality castings and have less meat in the wrist pin area. In most cases, the cast-aluminum piston cracks along the center line of the wrist pin, as shown above.


5. Leaking Injectors—LB7


One thing the ’01-’04 LB7 Duramax is notorious for is injector failure. The two primary failure points of the factory injectors are internal cracking of the injector body itself and corrosion-induced failure of the internal ball seat. The solenoid style Bosch units found in the LB7 employed a non-hardened internal ball seat to seal the injector. By comparison, later versions of injectors would utilize a hardened, chrome-plated ball seat and—not surprisingly—benefitted from much improved durability. Over time, this ball seat erodes and no longer fully seals. When an LB7 injector begins to leak, you’ll notice the truck hazing at idle and you’ll also be able to observe abnormally high injector balance rates (the amount of fuel each injector contributes to maintain a smooth idle) with the appropriate diagnostic equipment. To add insult to injury, a leaking injector means a considerable amount of fuel is making it into the crankcase.

Stay tuned for the next article on how to fix these issues!