08 GM 6.6 Gas Problems With Easy Solutions

There is a wrong perception about the GM 6.6L gasoline engine is that many people think 6.6L Duramax and GM 6.6L gas engines are the same.

But they are entirely wrong.

They are not the same engines.

General Motors, in other words, GM introduced 6.6 gas engines in 2020, whereas 6.6 Duramax diesel engines were introduced in 2001.

Both engine variants have pros and cons of their own.

Today, we are going to discuss GM 6.6 gas problems.

We will explain 8 common GM 6.6 gas engine problems with solutions.

GM 6.6 Gas Engine Specs

GM 6.6 Gas Engine Specs

The 6.6L L8T gas engine is the updated replacement of the 6.0L V8 engines used in the Chevrolet Silverado and GMC Sierra 2500 and 3500 models. GM 6.6 gas engine has a direct injection system.

As you know, Silverado and Sierra are the best-selling vehicle models GM.

That means GM has updated their engines by adding extra features like more horsepower and torque capacity in the 6.6L gasoline engine.

GM 6.6 Gas Engine’s Horsepower and Torque-

  • Horsepower: 401 hp @ 5,200 RPM (SAE certified)
  • Torque: 464 lb-ft @ 4,000 RPM (SAE certified)
  • Max engine speed: 5,600 RPM

GM 6.6L Engine Dimensions Are-

  • Height: 31.30 inch
  • Length: 29.75 inch
  • Width: 26.10 inch

GM 6.6 Gas Engine Materials Are-

  • Engine block: Cast iron
  • Cylinder head: Aluminum alloy
  • Intake manifold: Composite
  • Exhaust manifold: Cast stainless steel

What Are the Common GM 6.6 Gas Engine Problems?

gm 6.6 gas problems

GM 6.6L L8T is a 5th-generation small block V8 engine.

It has a 6-speed automatic transmission system.

It’s specially designed for heavy-duty use only.

GM 6.6 gas engine is used in-

  • 2020+ Chevrolet Silverado 2500 HD/3500 HD (along with 3500 HD chassis cab)
  • 2020+ GMC Sierra 2500 HD/3500 HD (along with 3500 HD chassis cab)

But GM 6.6 gas engines have some common problems.

In this article, I have mentioned 08 common 6.6 gas engine problems, including lack of horsepower, acceleration issues, transmission issues, unusual noises, low fuel efficiency, faulty valves, alternator issues, and low towing capacity.

01. Lack of Horsepower

As mentioned above, GM 6.6 gas and Duramax 6.6 are not the same engines.

They are a different and ideal competitor.

2020 GM 6.6L gas has 401 horsepower @ 5200 RPM and 464 lb-ft torque.

Its competitor 2020 Duramax 6.6L diesel engine, has 445 horsepower @ 2800 RPM and 910 lb-ft torque.

GM 6.6 L gas engine has 44 less horsepower than its competitor 6.6L Duramax engine.

That’s why Duramax 6.6 has more towing capacity than GM 6.6 gas engine.

So, if you need more horsepower, then you should choose Duramax.

High-horsepower trucks are more comfortable to ride and drive.


The horsepower of a vehicle can be increased by medication and proper maintenance.

To increase the horsepower of the 6.6 gas engine, you can do several things, including-

  • Upgrade the air filter with a cold air intake
  • Install an aftermarket exhaust system
  • Install a performance engine tuner and perform a tune-up
  • Clean the fuel injector
  • Install a supercharger or turbo kit

You can do these things to increase the HP, or simple, you can buy a Duramax 6.6L engine instead of a 6.6L gas engine.

02. Slower Acceleration

Slower Acceleration

Acceleration is a significant factor that affects the overall performance of a vehicle.

The more hp a vehicle produces, the better its acceleration is.

GM 6.6 Gas Vs 6.6 Diesel 0-60 MPH Comparison-

In 2WD Mode

  • 6.6 gas: takes 9.06 seconds
  • 6.6 diesel: takes 7.80 seconds

In 4WD Mode

  • 6.6 gas: takes 9.23 seconds
  • 6.6 diesel: takes 8.00 seconds

GM 6.6 gas engine is a little bit slower than the 6.6 diesel.

GM 6.6 gas accelerator is not only weaker than its 6.6 diesel competitor but also other competitors too.

Due to its weak acceleration, you will feel a lack of power while driving on the highway.


There will be a significant increase in the acceleration of 6.6 gas by-

  • ECU tuning
  • Installing a cold air intake
  • Installing aftermarket throttle response controller
  • Re-gearing
  • Reducing vehicle’s weight
  • Improving aerodynamics

Don’t try to apply these changes if you are not an expert.

Instead, take help from an experienced automobile mechanic who has past experience.

03. Less Transmission Speed

Less Transmission Speed

GM 6.6 gas engine has 6 speeds automatic transmission system while its other competitor has 8-speed or 10-speed transmission system.

So, the transmission speed of GM 6.6 gas is much less than its competitors.


There is only one solution: to change the transmission system to an 8 or 10-speed one.

Practically it’s expensive and complicated to change a transmission system.

So, we advise against it.

04. Less Towing Capacity

Less Towing Capacity

GM 6.6 gas engine’s max towing capacity is 14,500 pounds, while the 6.6 diesel engine’s towing capacity is 18,500 pounds.

It means a 6.6 gas engine has 4,000 pounds towing capacity than a 6.6 diesel engine.

6.6L gas engine is far behind in towing capacity compared to its competitors.


If you increase the horsepower by following our advice mentioned above (check problem 01) in this article, then there will be a slight increase of around 1,000 pounds in towing capacity.

You can engine the whole engine with higher horsepower to increase the towing capacity.

Otherwise, there is no noteworthy solution to this problem.

05. Knocking Noise from the Engine

Many GM 6.6 gas owners reported a knocking noise from the engine in the brand-new GM vehicles.

After turning the engine off, this sound can be heard.

Many users suspect the noise is caused by bad lifters of the valve spring, but they are entirely wrong.

Every brand-new vehicle has more or less noise after turning off the engine.

Actually, this is not a mentionable problem.

GM 6.6 gas engine has 8 quarts of oil capacity.

If you put less than 8 quarts of engine oil in your vehicle, the knocking noise will be more than usual.

Using mineral and semi-synthetic engine oil also triggers the knocking noise.


There is no way to stop the noise.

The noise can be reduced if you-

  • Use full-synthetic engine oil
  • Put not less than 8 quarts of engine oil
  • Use SAE 5W-30 grade engine oil

After driving 12k to 15k miles, your engine will have no knocking sound.

So, don’t worry about the knocking noise if you own a 6.6 gas engine.

06. Low Gas Mileage

Low Gas Mileage

GM 6.6 gas has less horsepower, torque, and towing capacity than the 6.6 Duramax diesel engine.

As GM 6.6 gas has less horsepower, torque, and towing capacity, you may think it will provide more gas mileage than the 6.6 Duramax diesel engine.

All the GM 6.6 gas engine has the same issue of low gas mileage.

Gas Mileage of GM 6.6 Gas in 2WD Mode

  • 12 mpg in the city
  • 14 mpg on the highway

Gas Mileage of 6.6 Duramax Diesel in 2WD Mode

  • 15 mpg in the city
  • 18 mpg on the highway

In a word, GM 6.6 gas engine consumes more gas than its competitors but provides less hp, torque and towing capacity, which is very disappointing.


To save gas, always drive your vehicle in 2WD mode.

Don’t use 4WD mode unless it’s necessary.

Cause it consumes less gas in 2-wheel driving mode.

Change the oil filter and air filter timely.

It will help you to gain extra mileage.

07. Lifter and Valve

Lifter and Valve

GM 6.6 gas has 8 cylinders.

There are 2 valves in each cylinder, 16 valves in total.

Duramax 6.6 diesel engine has 16 lifters and 32 valves in total.

GM 6.6 gas engine’s lifter and the valve don’t function properly.

On the other hand, Its competitor engine’s lifter and valve function more accurately than the 6.6 gas engine’s valves and lifers.

The lifespan of the GM 6.6 gas engine’s lifters and valves is also less than average.


There is no solution to this problem.

You can change the valve and lifters by spending a lot of bucks, but the function of lifters and valves won’t improve much.

08. Low Alternator Power

The alternator charges the vehicle’s battery and supplies electrical power to the whole system of the vehicle while driving.

Using electronic gadgets or turning on the headlights without starting the engine will consume power from the battery.

GM 6.6 alternator is low-powered.

So, it takes longer than standard to charge the battery fully after starting the engine.


You can replace the alternator with a high-powered one.

It will cost you around $780 to $850 to buy a high-powered alternator.

6.6 Gas Vs 6.6 Diesel, Which is Better?

Many people are confused between GM 6.6 gas engine and the 6.6 diesel engine; which is better?

GM and Japanese Company Isuzu jointly began to manufacture Duramax diesel engines in 2001.

On the other hand, GM started manufacturing a 6.6L gas engine in 2020.

6.6 diesel is better than 6.6 gas engines in every aspect.

There is no single aspect where a 6.6 gas engine is ahead of a 6.6 diesel engine.

So, without any doubt, the 6.6 diesel engine is the clear winner.

Always choose the 6.6 diesel cause it’s far better than the 6.6 gas engine in terms of horsepower, acceleration, transmission system, towing capacity and gas mileage.

2021 GM 6.6 Gas Problems

GM began to manufacture 6.6 gas engines in 2020.

If you own a 2021 Chevrolet Silverado or GMC Sierra, then you may face some common issues like-

  • Slow acceleration
  • Low horsepower
  • Low gas mileage
  • Less towing capacity
  • Knocking/Ticking noise from the engine
  • Low alternator power
  • Lifter and valve issues

We have discussed 8 common problems of GM 6.6 gas engines above.

You can read them for an explanation of all these problems.

The answer is the same if you are looking for 2021 Chevy 6.6 gas problems.

2022 GM 6.6 Gas Problems

2022 GM 6.6 gas problems and 2021 6.6 gas engine problems are the same.

Both model year has the same identical problems.

We have highlighted and discussed 8 common problems with solutions in the above of this article.

GM 6.6 Gas Review

GM 6.6L gas engine is a powerful engine that offers a noticeable power, torque, and towing capacity.

Due to its higher power output, the engine has lower fuel economy.

Here is a quick review of GM 6.6L gas engine:



Potent powerhouse

Oil consumption issues

Smooth operation

Carbon buidup problems

Cast iron durability

High price

Modern features

Complex engine

GM 6.6L gas engine is a perfect option for truck owners who want strong performance.

But its high oil consumption and higher price also need to be considered.

Its pros and cons are crucial factors that clearly define specific requirements.

If you really need high performance this engine is right for you.


Is the 6.6 gas engine reliable?

The 6.6 gas engine is a reliable one.

The engine is manufactured with strong material such as forged crank and cast iron block.

Its strong materials make it a long lasting engine.

However, the rods and pistons are not forged.

To run for an extended period of time, the engine requires proper maintenance.

With proper maintenance and care, the 6.6L engine can last around 200,000 miles without any serious issues.

What gas mileage does a 6.6 gas engine get?

According to FuelEconomy.gov, the estimated gas mileage of a 6.6L engine is 14 mpg in the city and 17-18 mpg on the highway.

The 6.6L gas engine has comparable or sometimes better fuel mileage than its competitors.

Remember that fuel economy depends on many factors, like driving habits, driving conditions, weather, and many more.

So, a 6.6L gas engine’s fuel efficiency varies slightly depending on the conditions.

What are Chevy 6.6 gas problems?

Some common problems with Chevy 6.6 gas engine are:

  • Acceleration issue
  • Reducing horsepower
  • Transmission speed less than Duramax 6.6
  • Low gas mileage
  • Knocking noise from the engine.

What is the gas mileage on a 2024 Chevy Silverado 2500hd?

The 2024 Chevy Silverado 2500 HD comes with a 6.6L gas engine.

The combined mpg of 2024 Silverado 2500 HD is estimated at 13.50 with a 0.28 mpg margin of error.

It is a heavy duty truck.

Final Thoughts

GM 6.6L gas engine is a reliable engine.

All the GM 6.6 gas problems are minor, and none of them is a major problem.

There is no engine without problems.

But you should know the pros and cons of the engine of the vehicle you are willing to buy or you already own.

We suggest vehicles with a 6.6 diesel engine.

Now the decision is up to you.

Whether you choose GM 6.6 gas or 6.6 Duramax diesel.

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