Common 6.6L Duramax Diesel Engine Problems

Every engine's got its quirks, and the 6.6L Duramax diesel is no exception. While some common problems with the Duramax diesel are limited to certain model years, others are frequently experienced across several engine generations. Many of these issues can be corrected by aftermarket upgrades or adopting specific practices, but the best way to avoid engine reliability concerns is to inspect and maintain your engine religiously.

Fuel Injector Failure

Affected engines: 6.6L Duramax LB7 (primarily)

Premature injector failure was a critical problem for the 2001 to 2004 model year LB7 Duramax. A recall was issued by GM and injectors were replaced with a revised design that also came with an extended 7 year, 200,000 mile warranty. The design flaw of the original LB7 fuel injectors was largely due to their placement beneath the valve covers, where fuel leaks would ultimately cause fuel dilution of the lube oil supply. When the LLY engine generation was rolled out during the 2004 model year, the valve cover was revised so that the fuel injectors were mounted externally in the cylinder head. Not only did this increase serviceability and make it simpler to identify potential fuel leaks, it eliminated the previous concerns.

Fuel Injector Harness Chafing

Affected engines: 6.6L Duramax LB7, LLY (primarily)

Fuel injector harness chafing is a common problem on LB7 and LLY engine generations, but also affects later models. This can cause a number of issues that include no-start and rough running conditions, and it is often misdiagnosed as a faulty fuel injector. DTCs will usually reveal which injectors are affected and will relate to an "open circuit". Correcting this problem requires locating the chafe point and repairing any broken wires or any wires that have had the insulation rubbed off. Often times, the impacted area of the harness can be found by wiggling the wires around the injector; if the problem goes away (even momentarily), you've identified the affected zone.

Fuel Starvation, Air in Fuel Lines

Affected engines: 6.6L Duramax LB7, LLY, LBZ, LMM, LML

The 2001 to 2016 Duramax diesel is largely susceptible to fuel starvation, air in the fuel lines, and difficulty bleeding the fuel system. This is largely due to the fact that these model year engines do not use an electric low pressure lift pump to move fuel from the tank to the injection pump. In lieu of a lift pump, these engines rely on a mechanical fuel pump mounted to the rear of the injection pump to draw fuel from the tank. It is also quite common for the fuel filter housing to develop micro cracks and allow air to slowly seep into the fuel supply. While the LB7 seems to be the most prone to such problems, all models are affected. A small plunger built into the fuel filter housing is used to prime the fuel system, but it does not always do an adequate job of removing air trapped in the fuel lines.

Water Pump Failure

Affected engines: 6.6L Duramax LB7, 2004-2005 LLY

Water pump failures are fairly common on LB7 and LLY engines, and a redesigned, larger water pump was fitted to the 2006 model year LLY to address this concern. It is not unlikely for a water pump on 2001 to 2005 model year engines to require replacement in as little as 75,000 miles; this seems to be inherent of the early water pump design.

Engine Overheating, High Engine Coolant Temperature, Blown Head Gasket(s)

Affected engines: 6.6L Duramax LB7, LLY (primarily)

Overheating issues typically occur while towing during the Summer months and they are most prevalent in 2005 and earlier models as a larger radiator, fan, and water pump were introduced beginning with the 2006 model LLY. In many instances overheating conditions are the result of a failed fan clutch, preventing the engine fan from providing supplemental airflow across the radiator when necessary. There's some speculation that a dirty/clogged radiator also contributes to the problem, as the grime that builds up on the radiator over time reduces its effectiveness in removing heat. Water pump failures will also commonly result in engine overheating. Head gasket failures are commonly the result of engine overheating.

Glow Plug Failure, Glow Plug Tips Breaking

Affected engines: 6.6L Duramax LLY (2006), LBZ

The 2006 model year LLY and later LBZ both experienced a high rate of glow plug failure due to faulty glow plug modules. The glow plug module would cycle the glow plugs for excessive times, causing the glow plug tips to become brittle and, in some instances, break off in a running engine. In such instances, catastrophic engine damage is likely. The problem was identified by GM and affected trucks were recalled so that technicians could reprogram or replace the glow plug module to eliminate the risk. While there shouldn't be many vehicles on the road still at risk, you should err on the side of caution if you decide to purchase an affected model year and have your local GM dealer verify that this problem was addressed by a previous owner.

Turbocharger Oil Ingestion (PCV Design Flaw)

Affected engines: 6.6L Duramax LB7, LLY, LBZ, LMM, LML

The Duramax PCV design vents excess crankcase pressure into the intake. As a byproduct of the process, engine oil is introduced into the turbocharger. Over time, this engine oil coats the inside of the intercooler and intercooler tubing. The primary concern is that large quantities of oil settle in the intercooler boots, causing them to rapidly degrade. There are several aftermarket products on the market that reroute the PCV line so that it vents into the atmosphere rather than into the intake, and a number of "DIY" methods of remedying the problem.

NOx Sensor Failure

Affected engines: 6.6L Duramax LML

The Duramax LML was introduced for the 2011 model year featuring the most sophisticated emissions system yet. The latest additions included the selective catalytic reduction (SCR) system, which requires a constant supply of diesel exhaust fluid (DEF) to allow the reaction that converts nitrous oxides into more favorable byproducts. The system relies on various sensors plumbed into the exhaust system in order to accurately control the injection of DEF into the catalyst.

Not particularly surprising, the new system proved littered with problems, most of which would be ironed out before the production of 2012 model year engines. By far and large, the most common problem was the reoccurring failure of one of the systems two NOx sensors. Additional problems included DEF level sensors, DEF pumps, and DEF tank heaters (which often allowed DEF to freeze in extremely cold environments). Owners of 2011 model year trucks may or may not be covered by an extended warranty on NOx sensors, as the widespread frequency of problems prompted GM to increase the warranty period for what they considered "affected trucks".

DPF Clogging

Affected Engines: 6.6L Duramax LMM, LML

Diesel particulate filter clogging was a significant issue with the Duramax LMM, but became less common when the selective catalytic reduction system was introduced for the LML. DPF clogging occurs when soot and ash build up in the particulate filter, restricting exhaust flow and causing a number of conditions that include running rough, excessive smoke, limited performance, and poor fuel economy. Soot is intended to be captured by the DPF and is easily removed through the regeneration process, but ash deposits will not readily burn under standard regeneration conditions. If you drive in heavy city traffic and make frequent short trips, you will be more susceptible to DPF clogging since passive and active regeneration cycles will be interrupted. The risk of DPF clogging can be drastically reduced by the following practices:
• When an active regeneration cycle is initiated, drive at highway speeds until the process has completed entirely.
• Avoid frequent start-and-stop conditions, especially when they prohibit the engine from reaching full operating temperature.
• Use a full synthetic low ash engine oil that meets the appropriate API category specifications.

Excessive Fuel Dilution

Affected engines: 6.6L Duramax LB7, LMM

Fuel dilution is a serious concern in the LB7 and the LMM, but for entirely different reasons. On the LB7, fuel dilution occurs because the injectors are located beneath the valve covers and are continuously doused with engine lubricating oil. Even a small fuel leak will perpetually add diesel fuel to the lube oil supply, and although the rate at which fuel leaks may be extremely low it adds up quickly between oil changes. Furthermore, fuel leaks tend to get worse and they typically go unnoticed for long periods of time because they are relatively undetectable. On later models, fuel leaks are easily spotted because the injectors are not hidden beneath the valve covers. Fuel dilution is typically caught by periodically taking an oil sample and submitting it for an analysis, or by noting that the engine oil level is actually increasing between oil changes (commonly referred to as an engine that is "making oil").

On the 6.6L Duramax LMM, excessive fuel dilution is caused by the regeneration process or, more specifically, the manner in which raw fuel is introduced into the exhaust stream. When active regeneration is in progress, the engine injects fuel into the combustion chamber during the exhaust stroke so that the fuel will not immediately combust and exits into the outgoing exhaust stream. High temperatures in the diesel oxidation catalyst ignite the fuel, creating the extreme exhaust gas temperatures necessary to burn off soot captured in the DPF. Fuel injected into the combustion chamber has a tendency to stick to the cylinder walls, and that tendency is even greater when there is no combustion. A portion of the fuel that sticks to the cylinder wall is captured by the piston ring(s) upon completion of the following stroke and ultimately ends up in the crankcase. This problem was considerable enough that the LML received a "9th" or downstream fuel injector that introduces fuel directly into the exhaust stream.

Fuel dilution is generally accepted as "excessive" when it exceeds 5% by volume. Oil analysis services are relatively inexpensive, non-invasive, and will report fuel dilution rates. Combating excessive fuel dilution may include altering your driving habits, performing any necessary repairs (fuel leaks, in particular), switching to a high quality synthetic engine oil (see 6.6L Duramax oil change guide for recommendations), and determining an appropriate service interval for oil changes that occurs before fuel dilution exceeds 5%.

CP4 Injection Pump Failures

Affected engines: 6.6L Duramax LML

The Bosch CP4 is renowned for its failure rates, and its worth noting that this is not limited to the Duramax diesel; the Ford Power Stroke and Ram Cummins trucks that utilize this injection pump have similar issues with premature failures and total fuel system annihilation when the dreaded "failure event" occurs. There are many theories as to why these injection pumps fail prematurely.

A very common argument is that the pump is manufactured by Bosch, a company with roots in Germany, and that it is designed for use with diesel fuels that have higher lubricity properties than the ultra-low sulfur diesel available in the United States.

Another argument is that the roller followers used in the CP4 are easily impacted by air intrusion, poor fuel quality, and ultimately wear prematurely. There is strong evidence to suggest that a CP4 failure event starts with metal filings being produced by a worn follower. Furthermore, the camshaft lobe on the CP4, which lifts the roller followers as it rotates, is somewhat aggressive with a steep ramp-up profile.

When the Bosch CP4 fails, it typically takes the entire fuel system with it. Fine metal debris is produced in the crankcase of the CP4 and is then drawn into the pumping mechanisms where it spreads to the common rails, individual injector lines, the fuel injectors themselves, into the fuel tank through the return line, and finally in each of the fuel filter elements. A typical repair requires replacing every inch of the fuel system (fuel lines, injector lines, injection pump, fuel injectors) and removing the fuel tank to perform a thorough cleaning. The repair bill for such procedures can run well above $10,000 USD. For additional information on the Bosch CP4, see Bosch CP4 failure overview.

Problems with the CP4 were sufficient enough that GM ditched this injection pump in favor of a Denso high pressure injection pump for the 2017 model year L5P.

Bosch CP4 injection pump

Bosch CP4.2 injection pump

Page Information

Last update: 12/14/2021

Copyright: Diesel Hub (, all rights reserved