What Is High Mileage for a 6.6 Duramax V8 Diesel? We Explain

If you are into General Motors technologies and need a good heavy-duty vehicle, you may be thinking about getting a truck powered with the Duramax 6.6 Diesel engine. This is the V8 engine that consumes diesel fuel and is made by General Motors and Isuzu in the Ohio plant. The engine has been manufactured since 2001 and is extremely popular even today.

In short, the Duramax 6.6 Diesel engine is capable of going about 300,000 miles, on average. But what should be considered high mileage when you buy a truck or van? For this engine, anything higher than 250,000 should be seen as high mileage and a risky purchase. Although some of these engines are capable of driving over 500,000 miles with only minor repairs.

Key features and my opinion about the engine

• Production years:2001-now
• Average lifespan of Duramax V8:240,000-280,000 miles
• Fuel supply type:Common Rail
• Power range:250-550 hp
• Fuel efficiency:average
• Engine block material:cast-iron
• Engine reliability score:medium
• The most common problems:DPF and EGR problems, weak sensors, injectors contamination, fuel pump problems.

Key facts about the 6.6 Duramax V8 engine

So, this engine was introduced for heavy-duty trucks made by GM in 2001. Since then, it has been used as one of the options for trucks, vans, and some commercial vehicles made by General Motors. The first version was LB7, then it had several modifications and now the company offers L5P and L5D versions.

Actually, the technologies haven't changed dramatically since the engine was first introduced. It's still a reliable and long-lasting engine that can be called bulletproof. But still, with every new modification, GM introduced new eco-friendly technologies that affected the overall durability of the engine.

Here are some key facts about the 6.6 Duramax Diesel:

• the latest L5P version of the Duramax Diesel engine is the most powerful diesel engine GM has ever manufactured;
• the L5D version is the downrated 6.6 Duramax for commercial use, they reduced the power to improve durability;
• of course, now these engines are fitted with EGR and DPF systems to make them more eco-friendly, but these systems affect their longevity;
• current versions of the 6.6 Duramax Diesel engines offer 13-15 MPG which is not bad at all if you consider their displacement and power;
• the most important feature is their torque - the current V8 Duramax can deliver over 1000 lb-ft of torque;
• the engine is turbocharged, also the turbocharger has variable geometry to help the engine make more power and torque;
• these engines are mated to the legendary 10-speed Allison automatic transmission which is extremely durable.

The engine seems to be a great choice for heavy-duty use. But of course, this is an awful daily driver. I can't imagine a person who would want to own such a car for driving it to a supermarket and to the office. Poor gas mileage, excessive vibrations, and focus on the commercial benefits of the engine make it only a good workhorse.

But you should also understand that this is one of the several diesel engines available in the US. If you want a relatively economical heavy-duty truck, you better start ordering your 6.6 Duramax Diesel now because no one knows how many years or months it will be manufactured.

Which vehicles use the 6.6 Duramax Diesel?

GM installs the 6.6 Duramax V8 engine in two models: GMC Sierra HD and also Chevy Silverado HD.

HD means "heavy duty", so these cars are made for commercial use and for towing and hauling something heavy. They are optimal for companies, farmers, etc. Of course, you may buy one of these trucks for your personal use but it's unlikely that you will get any pleasure driving it for your everyday purposes.

Previously, the 6.6 Duramax engine was also used in other vehicles such as Chevy Kodiak, GMC Topkick, Chevy Express, GMC Savana, and even Hummer H1 Alpha.

What's high mileage for the 6.6 Duramax engine?

There are two different categories of people who will ask this question: those who want to buy a used truck powered by the V8 diesel engine and those who already own this truck and want to sell it before it starts pumping money out of their budget.

And the answer for those two categories will be different.

So, if you want to buy a used Sierra HD and are looking at a vehicle with 250,000 miles on it, you should know, this is a very high mileage for this car. Actually, I would try to find something with less than 150,000 miles, in this case. After 200K miles, this engine may start showing many minor problems just because it wasn't serviced properly by the previous owner.

But if you own such a vehicle and you see that the engine is completely OK, you service it every 15K miles and don't get any problems, then you may drive it over 300,000 miles and even up to 400,000 miles. This will be considered high mileage but even after the engine hits this milestone, it will still be able to do some work for you.

Anyway, after the 6.6 Duramax Diesel hits 400,000 miles, it should be considered a high-mileage engine with barely some life left in it. It will most likely throw many problems with ecology equipment and will start hinting that it's time for it to retire.

What are the common problems with the 6.6 Duramax Diesel engine?

Unfortunately, this engine will still throw some minor issues and you will have to invest in repairing the powerplant. It doesn't mean that owning the Duramax V8 is a bad idea. It may start showing problems after it reaches 200,000 miles. And these problems are easy to solve. Also, some issues may come earlier than that if you don't maintain the engine when needed.

1. Timing belt and water pump don't match their mileage

The timing belt is designed to live 150,000 miles which is ridiculously much. It actually will live that long but I still recommend replacing it at least at 100,000 miles to avoid stretching and jumping a tooth or two.

But there is a problem. The water pump will not live even 100K miles. It may fail at 80,000 miles or even earlier and make you replace the whole timing belt kit. Replacing it every 80,000 miles even if everything still seems to be fine doesn't seem to be a bad idea.

2. Fuel pump problems

Engineers decided that this engine doesn't need the fuel lift pump or low-pressure fuel pump. It only has an injection pump that builds up high pressure to inject diesel fuel directly into combustion chambers.

But the problem is that the HPFP needs to work extremely hard to get the fuel from the tank of the vehicle and then pressurize it optimally. This leads to the loss of fuel pressure once you press the gas pedal hard. I know that some owners add lift pump and this problem disappears.

3. Bad injectors

You may not notice that one of the cylinders misfires because of the poor fuel injection system. First of all, this happens because the injectors in this engine are not as durable as other parts. Also, injectors in the 6.6 Duramax are not really cheap and they are pretty hard to replace. So, this problem will cost you some money.

Usually, diesel injectors start going crazy at about 150,000 miles. If you don't change the fuel filter often enough, they may go bad much earlier than that.

4. Low gas mileage

This is not the most economical engine in the world and even in the manual GM writes that the Sierra HD or Silverado HD will only allow you to go about 15 miles per gallon of diesel fuel. Also, in the city traffic, the mileage will be much worse.

But in reality, some owners say that they only get about 10 MPG and are happy with that because it can be even worse if you tow or haul something. So, these engines are obviously not for saving your money on fuel. But they are made to do some work for you, so this is quite fair.

Can you prolong the life of the 6.6 Duramax V8 Diesel?

There are several things you may do to prolong the life of this engine:

• follow the offered maintenance schedule - GM knows well how to maintain this heavy-duty engine;
• try buying only OEM parts and fluids for maintenance and repair;
• avoid aggressive driving and stepping on the gas pedal to the floor;
• avoid overloading the vehicle - this will affect the engine and the tranny;
• if driving in dusty areas, change the air filter in the engine twice more often than recommended;
• prefer dealership for maintenance and repairs;
• don't ignore problems with the turbocharger and other important parts of the engine.

Ignoring some symptoms and driving this engine with minor problems can quickly lead to fatal issues. So, better go to the repair shop or to the dealership once something is wrong with your vehicle.