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Previously, we determined which was the best Duramax engine. Now, we’re taking a different approach and identifying each engine’s worst problems.
While every Duramax has its strengths and weaknesses, some exhibit downsides that outweigh their benefits.
LB7: 2001 – 2004
The original Duramax, the LB7, remains highly popular among diesel enthusiasts because it was produced before emissions standards were implemented.
It holds up as a durable, reliable engine. Some issues will crop up when this engine hits the 100,000-mile mark, the most common of which is injector failure.
The LB7 model’s injector failure is so notorious that GM addressed it by eventually correcting the design and offering a 200,000-mile warranty on the updated Duramax parts.
If you’re considering buying a used LB7, check whether it still has its original factory injectors. Any LB7 with its original injectors will need to be replaced soon.
Other significant problems with the LB7 include:
Lack of a factory lift pump
Fuel filter housing O-ring leaks
Water pump leaks
Overheating
Head gasket failure
LLY: 2004.5 – 2006
The LLY Duramax engine was only in production for a short time. It’s almost identical to the LB7 engine, but like the LB7, it lacks emissions control parts. By the time the LLY came out, GM had fixed its injector problems.
Instead, the LLY tended to overheat when towing heavy loads or driven in high temperatures. The engine’s stock air intake system was often to blame.
The engine also ran hotter because the original turbo mouthpiece was too restrictive. The best way to fix this is to use Duramaxaftermarket parts, such as a cold air intake or a turbo inlet that’s either aftermarket or from a stock LBZ engine.
Other Duramax LLY problems included the following:
Head gasket failure
Injector harness chafing
Water pump failure
EGR valve issues
Glow plug failure
Fuel pressure relief valve
Lack of factory lift pump
LBZ: 2006 – 2007
As mentioned in our look at the best Duramax engine, the LBZ is generally accepted as the greatest Duramax engine ever built. There are several reasons this engine is a favorite:
Lack of emissions control parts
GM worked out the bugs of previous engines
The first Duramax to come with a six-speed Allison transmission
For the most part, the LBZ didn’t have any major issues. It’s only real issue is that GM didn’t install a factory lift pump. Fortunately, it’s one of many Duramax diesel parts that can be added easily and inexpensively.
The only other minor issue is one found by expert tuners. Provided you spend $20,000 in mods to exceed 600 horsepower, you might encounter an issue with pistons cracking.
LMM: 2007.5 – 2010
The debut of the LMM engine coincided with new requirements for emissions control parts. Mechanically, the LMM Duramax and LBZ are almost identical. The only major difference is that the LMM included emissions control parts that the LBZ didn’t. These parts, as well as the pistons, sometimes resulted in mechanical issues.
Excessive modifications to achieve over 600 horsepower can lead to cracking in LBZ pistons, and due to their injector configuration, LMM pistons are even more susceptible to breakage.
The LBZ injector had seven fuel-spray holes, but the LMM had six. This meant that two streams of fuel came out in opposing directions over the piston wrist pin. This caused hot spots to form on the weakest part of the piston, which could lead to cracking.
LML: 2011 – 2016
The LML was completely redesigned for its 2011 model-year debut. This year, its power was increased to almost 400 horsepower and 765 pound-feet of torque.
GM had to increase the fuel system pressure to increase its power without compromising emissions standards. Unfortunately, this made the injectionpump a ticking time bomb.
Sudden fuel pump failure on this engine causes metal shavings to spread throughout the fuel system, contaminating everything. Repairing this requires an entirely new fuel system kit: a new pump, high and low-pressure fuel lines, a new fuel filter, new injectors, and a pair of new fuel rails. You’ll also have to drop the tank to clean out the metal debris. Read more about the “Glitter Maker” here.
It’s also possible to prevent this issue by installing a lift pump with better air and debris filtration. Many LML owners actually install the older Bosch CP3 that was first used in the LB7 engine. This lift pump has slightly lower pressure but is actually capable of more horsepower.
L5P: 2017 To Present
The L5P is often considered to be the best post-emissions Duramax engine. All the previous injector pump and emissions system problems were largely fixed. Mechanically, this engine is sturdy, and owners rarely report problems.
The only common issue is the MAP sensor becoming clogged with soot and triggering a check engine light.
Fortunately, this problem is easy to fix. Add a MAP sensor spacer and clean the soot with an electrical cleaner.
Conclusion
When selecting the best Duramax engine, one model clearly stands out: the LBZ has proven its longevity. None of the Duramax engines are failures. Most of their issues can be relatively easily resolved, if not completely avoided, with some aftermarket modifications. If you’re deciding between options, it largely depends on your specific needs.
For example, the LMM is almost exactly the same as the LBZ except for the emissions control parts. If you want an engine without those, you’re still likely to choose the LBZ. When it comes to issues with the stock engine, the LLY and LML are largely considered to be the most problematic.
Count on ProSource Diesel for L5P upgrades, LML Duramax parts, LMM Duramax upgrades, LBZ Duramax upgrades, LLY Duramax upgrades, LB7 Duramax upgrades, and a wide selection of hard-to-find kits, Duramax performance parts as well as thousands of other diesel parts.
Duramax parts at ProSource Diesel
2017-2019 L5P parts
2011-2016 LML parts
5-2010 LMM parts
2006-2007 LBZ parts
5-2005 LLY parts
2001-2004 LB7 parts
1982-2000 IDI parts
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