List of Problems with GM 6.6L Gas Engine

The GM gas 6.6L engine (L8T) has some problems reported by owners like:

  • Knocking noise
  • Random tick sound
  • Bad lifters and valve springs
  • Valve noise
  • High gas usage
  • Low power

The engine Knocking happens when idling with transmission in park or neutral, and can be heard coming from bell housing area, or engine area.

Users have reported fixing this by changing to a synthetic oil. Others have noted it is just the normal noises and not to worry. Not everyone notices.

It could be caused by bad lifters of valve springs.


Low power and slow acceleration is due to not enough engine horsepower.
The gas version makes less power at lower rpm compared to diesel.  Note the diesel is more than $10k extra.

The GM Chevy 6.6 gas engine does not seem to have a good tranny. There is a 6 speed option, but some users think it would be better with a 8 or 10.

Note that owners have reported that GM has fixed most of the problems after taking it in.


Engine Specs:

Generation 5 small block engine, with 300k mile lifespan. The previous engine was the L96 with: 360 hp, 380 lb-ft torque.

HP    401@5,200
Torque    464@4,000

Compression is increased to 10.8-to-1bore and stroke, 4.06 x 3.86 / 103.25 x 98
Cast iron block with nodular iron main caps
Direct fuel injection
Overhead-valve, two valves per cylinder, variable valve timing

Is used in 2020 Chevy Silverado and GMC Sierra HD


Here is what people like about it:

  • Large towing capacity
  • Solid
  • Smooth
  • Good MPG
  • Direct injected

Here is a video showing the its towing power:



If you need more power and highly reliable engine, then I recommend spending the extra $10k and getting the diesel.

The engine is used in Chevrolet Silverado HD and 2020 GMC Sierra HD, here are some photos: