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Six Problems Often Found in Duramax 6.6L Engines

Duramax Engines have an excellent reputation for reliability. Each model year is associated with problems, especially when the engines reach 100,000+ miles. However, while Cummins engines may continue to edge out Duramax in terms of overall reliability, Duramax fans love their higher horsepower.

We think Duramax still makes a great engine. However, there are some problems you may encounter with them. Knowing about them ahead of time will help you spot problems early. The earlier you can spot a problem with your engine, the more likely you will avoid serious trouble.

You want to keep an eye on:

  • Glow Plugs
  • Injectors
  • Fuel System
  • Water Pump
  • Fan Clutch Failure
  • Oil

Who Makes the Duramax Engine?

You may think of the Duramax as a General Motors engine. You are half-right. Well, 60% right. DMAX Ltd. Builds Duramax engines. DMAX is a joint venture. General Motors owns 60% of DMAX, while Isuzu Diesel Services of America owns 40%. DMAX produces the Duramax 6.6L V-8 turbo-diesel engine for the American market. You can find this engine in two GM pickup models, the GMC Sierra and the Chevrolet Silverado HD. 

Glow Plugs

Glow plugs are heating elements. They heat fuel and air to improve your engine’s combustion efficiency. When glow plugs start to fail, you can experience many problems. One big sign is you notice a drop in fuel efficiency. Your truck may also have engine misfiring, rough idling, or hard starting. Your exhaust can also give you a clue like other potential diesel problems. If your smoke is white or black instead of thin and gray, you could have a problem with your glow plug.

Not all Duramax 6.6L models have glow plug problems. Instead, you want to beware if you have a 2006 model. Both the 2006LLY and LBZ models are known for their glow plug issues. GM has recalled them because the plugs are faulty. If you have not handled the recall, take care of it as soon as possible. Over time, the defective plug may become misshapen, increasing the chance it will break. 


Injectors are what get the fuel into the combustion chamber. Before it does, it pressurizes that fuel. When your injectors stop working correctly, you can get a range of problems with your engine performance. You may not be able to start the vehicle. Even if the truck starts, you may have to crank it repeatedly before it starts. Idling speeds can be uneven. Misfiring is another sign of injector problems. Of course, fuel efficiency is another problem. 

There are some non-performance indicators of injector problems, too. If you smell fuel inside your cabin, it indicates a leak somewhere in the fuel system. Injectors are a potential source of that leak. If you notice white smoke, it means your emissions are dirty. Faulty injectors can cause dirty emissions.

Some 2001 to 2004 Duramax models have injector issues. If you have an LB7, you want to be aware of them. The problem was with the injector design. DMAX fixed the issue after the 2004 model year.    

Fuel System

While most known Duramax problems are associated with specific model years, fuel system issues plague all model years. How do you know if you have a fuel system problem? It starts with starting or actually with difficulties starting. Some things to watch out for include problem starting, excessive smoke, lower gear selection, poor fuel economy, a loss of engine power, lower RPMs, and excessive noise.

While several possible fuel system problems exist, the 6.6L Duramax consistently has two. The first is that the Duramax does not use a lift pump but an injection pump. That can crack the fuel filter housing. Second, the engine design means air can quickly get into the fuel lines, preventing enough fuel from getting to the engine.

Water Pump

Drive a vehicle long enough, and the water pump will fail. For the average diesel truck, you can drive 100,000 miles or more before having to worry about the water pump failing. Unfortunately, if you drive a Duramax, you may have to replace your water pump around 80,000 miles.

Why is that a big deal? A broken water pump means that your engine will not get enough coolant. It will overheat. An overheated engine can be a nuisance, meaning you have to stop driving and let your engine cool down immediately before proceeding. What if it is not safe to stop or you do not notice in time? That can mean serious problems. Overheating can lead to expensive problems like blown head gaskets, damaged cylinders or pistons, and a cracked engine block.

Since you know your Duramax is prone to water pump issues, you need to be on the lookout for potential problems. If you notice any of these issues, have a trusted mechanic check out your cooling system, including your water pump:

  • Overheating
  • Coolant leaks
  • Signs of corrosion on your water pump
  • Whining noises from the front of your engine

Fan Clutch Failure

Since we have already told you that water pump failures are a problem, you know to be on the lookout for overheating. However, there could be another culprit behind your overheating engine. Think of your fan clutch as a way to get airflow through your radiator. Your fan clutch operates as a fluid coupler and uses your water pump to power the fan blade. When it begins to fail, your radiator may stop cooling properly. When your fan clutch is working correctly, it partially disengages when the pump is at an average temperature. 

Unfortunately, in models from 2005 and earlier, Duramax had a problem with its fan clutches. That can lead to overheating in your engine.


When you worry about oil leaks, you may think of oil leaking out of your engine. In the 6.6L Duramax, there is a more severe problem. Sometimes the oil leaks into the turbocharger. The real problem is a faulty PCV design. When a PCV valve works properly, it lets waste gases out of the crankcase and prevents other gases from entering. Duramax’s faulty PCV design allows oil out of the engine into the turbocharger. That loose oil coats the components and can lead to more serious engine problems.

Is your Duramax experiencing problems?

If your Duramax 6.6L is starting to have problems, you want to fix them as soon as possible. Most of the common Duramax problems are nuisances but allow you to avoid severe damage unless you ignore them. Bring it in as soon as you notice any change in your vehicle’s performance. We can give it a thorough inspection, identify any problems, and get you back on the road. Even with these challenges, a Duramax is a great engine. Take care of it, and you should be able to drive it for hundreds of thousands of miles.