There are a ton of bolt-ons that make driving a Duramax-powered ¾-ton Chevy or GMC much more fun than operating a 7,500-pound pickup should be. Already the quickest trucks out of the box among the Big Three, it doesn’t take much to keep them at the front of the pack. But what about making sure your 6.6L oil-burner outlasts the competition in addition to outrunning it? No different from Cummins-powered Rams and Power Stroke-equipped Fords, each rendition of the Duramax has its own unique weakness that requires addressing if you want to rack up hundreds of thousands of trouble-free miles.
For LB7 models, you’ll want to ditch the original fuel injectors in favor of Bosch’s updated units. Believe it or not, a freer flowing turbo inlet manifold will keep coolant temps in check if you’re fighting overheating issues with your LLY. Even on the LBZ, one of the most durable versions of the Duramax ever produced, there is still room for improvement in the form of addressing the transfer case pump-rub issue in 4x4-equipped HDs. Moving on to the LMM, leaking transmission cooler lines will have to be addressed sooner rather than later. Then there is the LML, the only Duramax that’s ever required a lift pump to (hopefully) keep its injection pump from self-destructing.
If you can only treat your specific Duramax to one upgrade, make sure it’s this one.
2001-2004 LB7: Updated Bosch Injectors
At this point, there probably aren’t a ton of LB7 trucks still sporting the original injectors they left the factory with, but you can bet there are some. That’s reason enough to make this priority numero uno for ’01-’04 Chevy and GMC HD owners. The biggest problem with the original LB7 injectors was traced back to the non-hardened internal ball seat, which, thanks to debris in the fuel and/or aggressive tuning, is prone to failure. Once the ball-seat is damaged irreparably, it no longer has a flush surface to seal against. The result is an injector that essentially never shuts off.
Hazing smoke at idle, excessive injector balance rates, knocking, missing, fuel in the crankcase and piston damage are all commonly experienced when an LB7 injector goes south. A word of advice: When one does decide to check out, replace them all. As laborious as it is to access the injectors on the LB7 (they’re under the valve covers), labor represents a major portion of the repair bill. Plus, it’s no longer possible to install the old-style, faulty injectors. Bosch’s new, updated injectors feature hardened, chrome-plated ball-seats, revised and stronger bodies and DLC coated nozzles for ultimate durability.
2004.5-2005 LLY: Free-Flowing Turbo Inlet Manifold
Although the LLY came with the largest turbocharger ever offered on a Duramax, it was fitted with the most restrictive turbo mouthpiece, the smallest engine fan and also debuted the engine’s 50-state EGR system. The combination of having a larger compressor wheel yet having the air flowing toward it bottleneck down in the turbo mouthpiece (i.e. turbo inlet manifold) means the variable geometry turbo has to work harder to bring in fresh air. This creates hotter intake air and exhaust gas temps, while the added duty of the cooling system (to cool down EGR) doesn’t help matters. Long story short, the LLY runs warm to begin with and is known to overheat when it’s asked to do any heavy lifting.
Opening up the intake path to the compressor side of the turbo is one of the most cost effective means of lowering coolant temps on an LLY. S&B Filters’ high-flowing aluminum turbo inlet manifold solves much of the LLY’s overheating issues, simply by allowing the turbo to breathe better. With the turbo laboring less to maintain adequate boost pressure, intake air and EGT drop, coolant temperature decreases and turbo responsiveness (spool up) improves.
2006-2007 LBZ: Pump Rub Fix
This one is common on all ’01-’07 trucks with four-wheel drive, but we’re calling it out here due to a lack of major issues that plagued the LBZ specifically. From the factory, both the NP261XHD (manual) and NP263XHD (electronic) transfer cases utilize indexing tabs to keep the gear pump housing in position within the back (rear) section of the transfer case. The design of the indexing tabs allows them to dig into the magnesium transfer case, eventually rubbing a hole in it, allowing fluid to escape. However, fluid is only lost when the truck is operational (i.e. in motion), so there are literally zero warning signs that “pump rub” is occurring until it’s too late.
The key to stopping the pump rub issue (as it’s known) is to eliminate it before your transfer case is trashed. While it’s a bit involved to pull and then split open your transfer case, pump rub can be solved for just $75 in parts. Thanks to Merchant Automotive’s all-inclusive pump upgrade kit(s), a replacement billet-aluminum gear pump housing employs indexing tabs with much more surface area and smoothed edges so as to not dig into the transfer case. Though it’s only a preventative upgrade, this mod is completely worth the time, effort, money and peace of mind.
2007.5-2010 LMM: Aftermarket Transmission Cooler Lines
Leaking transmission cooler lines are common on all Allison-equipped ’01-’10 GM HDs. Unlike the previously mentioned pump rub issue on ’01-’07 trucks, you will notice a pool of ATF under your truck’s front skid plate when these babies start to seep. Easiest to notice in cold weather when the rubber hoses constrict, transmission fluid leaks past the crimps on the factory lines. With most cooler lines springing a leak within the first 50,000 miles, many Duramax owners had their lines replaced under warranty—only to have them leak again later.
The sound solution for leaking transmission cooler lines is to scrap all the factory hoses in favor of high quality hydraulic hose or braided stainless steel lines, along with threaded fittings as opposed to crimps. Companies like Fleece Performance Engineering, Merchant Automotive, Dirty Hooker Diesel and Deviant Race Parts all offer aftermarket kits, which hold up fine to both the elements and the extreme pressure the Allison requires during operation.
2011-2016 LML: Aftermarket Lift Pump
In order to meet tighter emission standards, GM ditched the tried and true Bosch CP3 injection pump on the LML Duramax. In its place sits the Bosch CP4.2, a pump that makes use of fewer internal parts but that creates higher injection pressure (up to 30,000 psi in the LML application). While both the CP3 and CP4.2 Bosch common-rail pumps employ a gear pump on the backside, GM never equipped any of its ’01-’16 trucks with an electric lift pump from the factory. However, with tighter tolerances present within the CP4.2 providing lesser room for error (along with a different design), any debris, lack of lubricity and especially aeration can lead to catastrophic internal failure.
The best way to keep the CP4.2 happy (aside from reverting back to a CP3) is to ensure air doesn’t make it into the high-pressure fuel system. This means an electric lift pump system should be added, which maintains a constant supply of fuel pressure for the CP4.2 to use. A means of monitoring low-pressure fuel supply is also highly advisable. In comparing the LML Duramax with the 6.7L Power Stroke—an engine that also uses the CP4.2 but is accompanied by a factory electric lift pump that supplies it a steady 55-60 psi—the CP4.2 failure rate is much lower.
