Last time we looked at the best Duramax engine, now we look at the different problems in each engine to find out which one might be tagged as the worst. As previously mentioned, every Duramax engine has strong points and weak points. However, some Duramax engines have significant weak points that outweigh the strong points.
As the original Duramax model, the LB7 is still extremely popular with diesel enthusiasts due to its lack of emissions control parts. In general, it’s a very durable and reliable engine. However, as the first models hit the 100,000-mile mark, some issues cropped up. Injector failure was a common problem on the LB7 engines.
GM even recognized the problem as they eventually corrected the original design and offered a 200,000-mile warranty on the updated Duramax parts. When you go to buy a used LB7, it’s important to find out if it still has the original factory injectors. If it does, you know a replacement job is in your future. Other significant problems with the LB7 included:
The LLY Duramax engine was only in production for a short time. It was nearly identical to the LB7 engine and emissions control parts continued to be absent. GM had fixed the injector problems by the time this engine came out. Instead, the LL7 had issues with overheating when towing heavy loads or driving in hot temperatures. The stock air intake system was one of the culprits.
The original turbo mouthpiece was too restrictive, which contributed to the engine running hotter. The ideal fix is to use Duramax aftermarket parts like a cold air intake and either an aftermarket turbo inlet or one from a stock LBZ engine. Other Duramax LLY problems included the following:
Did the LBZ have any problems at all? For the most part, it really didn’t. The only real issue is something that GM simply didn’t install: a factory lift pump. Fortunately, it’s one of the Duramax diesel parts that can be added easily without spending a ton of money.
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.
The debut of the LMM engine coincided with new requirements for emissions control parts. Mechanically, the LMM Duramax was almost exactly the same as the LBZ, but it included emissions control parts that the LBZ didn’t have. That was one area of problems, but the other area was with the pistons.
As mentioned above, excessive mods to get more than 600 horsepower can cause the LBZ pistons to crack. However, the LMM pistons were liable to break even easier. This was due to the injector setup. The LBZ had seven fuel-spray holes. The LMM had six, which meant that two streams of fuel came out in opposing directions over the piston wrist pin. This causes hot spots to form on the weakest part of the piston, which can lead to cracking.
The LML was completely redesigned for its debut in the 2011 model year. The power was increased for this year to almost 400 horsepower and 765 pound-feet of torque. To get this type of power without compromising emissions, GM had to increase the fuel system pressure. Unfortunately, this led to the injection pump becoming a ticking time bomb.
The worst part about the sudden fuel pump failure on this engine is that it caused a large number of metal shavings to spread throughout the fuel system and contaminate everything. To repair this failure, you need an entire fuel system kit, which includes 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 in order to fully clean out the metal debris.
It is possible to prevent this problem by installing a lift pump that provides better filtration for air and debris. Many LML owners actually install the older Bosch CP3 that was first used in the LB7 engine. It has slightly lower pressure but is actually capable of more horsepower.
The L5P is largely considered to be the best post-emissions Duramax engine. All the previous problems that plagued the injector pump and emissions system have largely been fixed. Mechanically, this engine is quite stout, and owners have reported very few problems. The only common issue is the MAP sensor becoming clogged with soot and triggering a check engine light. This problem has an easy fix though. Just add a MAP sensor spacer and clean the soot with an electrical cleaner.
It’s easier to pick the best Duramax engine as the LBZ stood the test of time. None of the Duramax engines are exactly duds. Most of the problems can be fairly easily fixed if not prevented entirely with some aftermarket mods. If you’re trying to choose one over the other, it largely depends on what you need. 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.
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