The Duramax L5P was introduced in 2017 as a replacement for the LML. While the engine uses the same 6.6L design, many major engine components were completely refreshed in order to increase power as well as engine reliability.
The L5P has made great reliability improvements and is, in my opinion, the most reliable diesel engine being produced today. However, it does suffer a few common problems such as MAP sensor failure, a weak transmission, and fuel injector failure.
I’m going to discuss the important reliability changes that were made to the L5P and then jump into each of the common problems in detail.
Duramax LML vs. L5P Differences
Despite the same 6.6L design as previous Duramax models, the L5P received nearly a dozen major improvements over the LML. Outside of the major items covered below, the L5P also received: a strengthened cylinder block, improved glow plugs, a new camshaft design, a high capacity engine oil cooler, an upgraded EGR system, and a stronger transmission with a new torque converter.
Understanding the differences and the new equipment on the L5P is important for understanding the reliability improvements made over the previous Duramax version.
Borg Warner Turbocharger
Previous Duramax engines had used a Garrett turbocharger which has been replaced with a Borg Warner turbocharger. Previous turbos were variable vane turbos (VVT) which were controlled and actuated via oil pressure. The new Borg Warner turbo is electronically controller and actuated and uses a variable geometry design (VGT). The VGT design is a lot more efficient and reliable.
Denso HP4 in Favor of Bosch CP4.2
The Bosch CP4.2 was a wildly common problem in the LML. Bad fuel would deteriorate the internals of the fuel pump and then send small metal shavings throughout the whole fuel system. This would cause a chain reaction that would then essentially destroy the whole fuel system and lead to $10,000+ of repairs.
Fortunately, in the L5P the CP4.2 was ditched for a Denso HP4 injection pump. The HP4 is capable of handling higher fuel pressures which allows for smaller injectors without losing power potential. The stock HP4 has is capable of handling 800+ horsepower, although upgraded injectors will be required to hit these numbers.
Denso Fuel Injectors
Alongside ditching the Bosch injection pump, the old Bosch fuel injectors were also left behind in favor of new Denso injectors. The new Denso injectors flow approximately 20% more fuel than the previous Bosch injectors and also use a solenoid-style design, replacing the piezo design.
New Intake & Hood Scoop
A hood scoop was added to the exterior design of the L5P. With the new design, approximately 60% of air is provided via the hood scoop while the remaining 40% is sucked in via a traditional intake system that sits within the front fender.
The hood scoop draws in cooler air and provides a ram air effect which improves turbo efficiency and reduces lag, resulting in overall better performance.
Duramax L5P Engine Problems
- MAP Sensor Failure
- Allison 1000 Transmission
- Fuel Injector Failure (#4 Injector)
Finding 3 problems with the L5P was difficult. While these engines are still relatively new, they seem to be extremely reliable so far with very few “common” problems that haven’t been solved or addressed.
If you would rather consume this content via a video, check out our GM L5P Duramax Common Problems video below:
1. MAP Sensor Failure
The L5P uses a manifold absolute pressure (MAP) sensor inside the intake manifold. The MAP sensor is responsible for sending the ECU vacuum and positive air pressure readings which are used to determine how much fuel should be sent to the engine for combustion. Additionally, the MAP sensor is critical in determining when ignition should occur within turbocharged engines.
Like any sensor, the MAP sensor is prone to becoming clogged or gunked up which can affect the readings being sent to the ECU. On the Duramax L5P, the MAP sensor is known to become clogged with soot or grime and either fail or send incorrect readings to the engines computer.
When a MAP sensor fails, inaccurate vacuum and air pressure readings are sent to the computer which then affects the amount of fuel that is being sent into the engine for combustion. Overall, a bad MAP sensor will cause poor performance and can lead to catastrophic engine damage if it’s not fixed in a timely manner.
Symptoms of a Bad MAP Sensor
- Engine misfires
- Rich/lean air-to-fuel ratios (AFRs)
- Check engine light
- Lack of power and overall poor performance
- Rough idling
- Engine stalling, hesitation, etc.
Replacement Options
The MAP sensor on the L5P requires constant cleaning. If you have a bad MAP sensor, installing a new sensor will only reset the clock. Eventually the new sensor will become clogged and fail again.
There are generally two options to prevent the MAP sensor from clogging:
- Constantly clean the MAP sensor with MAF Sensor Cleaner
- Install a MAP Spacer
A MAP Spacer reduces the amount of soot buildup by spacing it away from the EGR stream which is what predominantly causes the buildup.
2. Allison 1000 Power Limitations
The 6-speed automatic Allison 1000 transmission on the L5P is the strongest Duramax transmission yet. With a new torque converter and several other upgrades, the transmission is stout and shouldn’t cause any issues on a stock L5P. With that being said, this isn’t really a “common problem” with the L5P. However, we did want to note that there are some limitations to the transmission for those looking to make big power.
With the stock fueling system, turbos, and internals capable of handling up to 600whp without any modifications, it can be tempting to throw some additional power at your L5P. Unfortunately, the biggest limitation to big power is the Allison 1000 transmission. The transmission is generally capable of handling ~500whp without many issues, but once you start pushing past those levels, you will run into some expensive transmission upgrades.
For those with heavy modifications and a heavy foot, some upgrades such as an upgraded trans cooler, a multi-disc torque converter, hydraulic upgrades, and improved friction materials are likely necessary to avoid expensive transmission rebuild costs.
3. Fuel Injector Failure
While not the most common problem, the #4 fuel injector specifically is known to be problematic. The injector connector for the #4 injector is known to go bad and send the L5P into limp mode with a P0204 engine code. The engine becomes very shaky and various additional engine lights will appear such as Stabilitrak and ABS.
While the #4 injector itself can fail as well, a lot of people are reporting issues with the connector itself and not the injector. It seems the trick so far is to either use an LBZ connector or get a gold plated connector, both of which make a better connection.
Additionally, there was a recall for fuel injector calibration (recall N182179090) in February, 2019. While this recall isn’t tied to the #4 injector, it is worthwhile making sure you have this completed to prevent any future injector issues.
Symptoms of Fuel Injector Failure
- Engine codes P0204, P020D, P0300
- Limp mode, ABS and Stabilitrak lights come on
- Engine misfires, rough idling
- Shaky engine, poor performance
Duramax L5P Reliability
The L5P is considered the most reliable post-emissions Duramax made to date. With the removal of the CP4.2 fuel injection pump and various improvements made to the block, pistons, and other internals, the engine itself is extremely capable and reliable. The turbo itself is capable of handling up to 600whp, and the stock fueling system can handle up to 800whp, making this a very tuner-friendly engine.
With the Allison 1000 upgrades the transmission won’t cause you any issues on a stock L5P, although it may become problematic under heavy modification. So far there don’t appear to be any major issues with the emissions components (EGR, DPF, SCR, etc.) although DEF fluid may require flushing a bit more frequently.
With proper maintenance, the L5P should be able to easily clear the 350,000 mile mark, with some owners claiming they have had virtually zero issues through the 400,000 and 500,000 mile mark.
Overall, these engines have proved to be extremely reliable.
