When GM rolled out the L5P Duramax for the 2017 model year, they didn’t just update the 6.6 diesel platform—they rebuilt it.
The L5P was one of the most dramatic redesigns in Duramax history, and it instantly set a new standard for factory horsepower, refinement, and reliability.
Today, nearly a decade later, the L5P is one of the most popular diesel engines on the road. Whether you daily drive one, tow with one, or modify one, the L5P has earned a reputation as a powerhouse that’s as durable as it is impressive.
The Evolution: A Fully Redesigned Duramax
The L5P replaced the LML in 2017—but calling it a “replacement” undersells it. GM didn’t tinker around the edges… they redesigned the Duramax from the inside out.
Major changes included:
- New, stronger engine block
- New forged-steel crankshaft and rods
- New cylinder heads designed for higher flow
- A new, larger Denso high-pressure fuel system
- A massive factory turbocharger (the biggest ever fitted to a Duramax)
- All-new electronics and ECM architecture
What does all that mean? Simple: The L5P was built to handle power—lots of it.
L5P Power Ratings
The L5P launched with absolutely class-leading numbers:
- 445 horsepower
- 910 lb-ft of torque
Even today, that’s serious power. And because of the stronger internals and improved fueling system, L5Ps handle additional power better than any previous-generation Duramax.
Why Diesel Owners LOVE the L5P Duramax
1. It’s the Most Powerful Duramax Ever Made (Stock)
The L5P pulls like a freight train, whether you’re empty or towing at max capacity. The throttle response, low-end torque, and mid-range punch all outperform earlier generations.
2. It’s Extremely Reliable
While the LML struggled with CP4 pump failures, the L5P switched to a Denso high-pressure pump that’s significantly more durable. The turbo, pistons, rods, and bearings are also built to handle high loads.
3. It Tows with Confidence
The L5P combined with the updated Allison transmission makes towing feel smooth, predictable, and stable—even with heavy goosenecks or equipment trailers.
4. The Factory Turbo Is a Beast
The huge stock turbo supports big power numbers, even on mild tuning and fueling upgrades.
5. It Responds Incredibly Well to Upgrades
Even simple upgrades like:
- FASS Fuel System
- Cold air intake
- Exhaust or intercooler piping upgrades
- Larger intercooler
…can make the L5P feel like a totally different truck.
6. The Interior & Chassis Improvements Matter
On 2017+ GM HD trucks, the ride quality, steering, brakes, front suspension geometry, and cabin comfort are a huge step up compared to older Duramax generations.
The Not-So-Good: L5P Common Problems
No engine is perfect. While the L5P is overall extremely reliable, here are the most common owner-reported issues:
Turbo Actuator Failures (Early Years)
Some 2017–2019 trucks saw turbo actuator problems. GM updated the design, and newer replacements tend to be much more reliable.
Glow Plug Failures
Pretty common on high-mileage L5Ps, but inexpensive to fix.
Transmission Issues
2020+ GMC/Chevrolet 2500 and 3500 with the 6.6 Duramax got the 10-speed "Allison"; which by all accounts is not an actual Allison transmission. While some owners praise the 10-speed; it has quickly become notorious for failing.
In fact, Allison and GM are set to officially part ways; eliminating the "Allison" branding from GM's troubled 10-speed altogether.
ECM Encryption (2017+)
The L5P uses a heavily encrypted ECM, which made tuning difficult (and expensive) early on. While tuners have found workarounds, it’s more complex than earlier Duramax engines.
Performance Potential: The L5P Can Make Huge Power
The L5P has the strongest bottom end of any Duramax to date. With the right combination of fueling and turbo upgrades, owners routinely push:
- 650–700+ hp on stock internals
- 1,000+ hp with built engines
At FASS Motorsports, the most popular L5P upgrades include:
- FASS Titanium Signature Series Fuel System
- Cold air intake kits
- Upgraded intercoolers & piping
- Lift kits & leveling kits
- Wheels and tires
- S&S tuning solutions (depending on model year)
What L5P Owners Think Today (2025)
If you browse forums, Facebook groups, or talk to customers in our showroom, you’ll hear the same thing over and over:
“The L5P is the best Duramax GM has ever built.”
Owners love the reliability, the smoothness, the power delivery, and how well the trucks hold up under heavy towing.
FASS Motorsports: Your L5P Duramax Experts
Whether you drive a 2017+ Silverado or Sierra 2500/3500, FASS Motorsports has the upgrades, expertise, and installation services to take your truck to the next level.
From fuel systems to suspension to wheels & tires, we serve L5P owners across Franklin County and the greater St. Louis area.
👉 Shop L5P Duramax Products
👉 Visit our showroom in Washington, MO to plan your next upgrade
Have Questions? We're here to help.
- Phone: (636) 429-7020
- Email: info@fassmotorsports.com
FASS Motorsports
25 Town and Country Drive
Washington, MO 63090
