Duramax LLY Common Problems

The 6 Most Common Duramax LLY Engine Problems

Jake Mayock

Meet Jake

Jake is a founder of 8020 Media and has been creating automotive content online since 2017. He has been the lead writer for Chevy Trucks and has transformed it from the old and outdated site it was into what it is today. Jake creates a ton of GM related content for the 8020 Media YouTube channel and specializes in Duramax and Vortec information but has a wealth of knowledge across all GM cars and engines. Jake believes the L5P is the best diesel on the market today.

In order to meet stricter emissions regulations, the LLY replaced the LB7 engine and was the first Duramax to use EGR emissions technology. Unfortunately, the new EGR system was problematic leading to common valve failure. Aside from this addition the new engine received a variable vane turbo and eventually got some block and internal upgrades in 2006, further strengthening reliability.

The new upgrades to the LLY removed some of the common LB7 problems such as fuel injector and fuel pump failure. However, there are a number of new issues that have arisen due to the changes, and a number of shared problems with the LB7. Additionally, 2006 LLY models are very comparable to the LBZ engine, creating a number of overlapping common problems there as well.

The Duramax LLY engine suffers from problems with bent rods, blown head gaskets, EGR valve failure, glow plug failure, overheating, and fuel pressure relief valve failure.

Duramax LLY Reliability

Ultimately, despite the common problems we are going to discuss in-depth below, the LLY is still a very reliable engine and has proven capable of making it to the 300,000 mile mark with minimal issues.

Two of the common problems, bent rods and the FPRV, are really only common on tuned and modified LLY’s running high power levels. The EGR problems only become an issue at high mileage. Glow plug issues are more than likely fixed on any LLY by now. And overheating and head gasket failure can be prevented with an upgraded turbo inlet manifold.

Overall, the common problems here are all avoidable and preventable. The block and internals on the LLY Duramax have stood the test of time. With common and consistent maintenance, these engines are superbly reliable, especially when left in stock form.

Duramax LLY Engine Problems

  • Bent rods
  • Head gasket failure
  • EGR valve failure
  • Glow plug failure (2006 only)
  • Overheating (pre-2006)
  • Fuel pressure relief valve

If you would rather consume this content via a video, check out our Chevrolet LLY Duramax Common Problems video below:

1. Bent Rods

While the LLY made competitive performance numbers back in 2004, the engine is certainly not competitive to the modern day diesels. For this reason, adding performance modifications to the Duramax is a common route to improve performance and hauling capacity. While light modifications such as intakes, exhausts, and modest tunes shouldn’t cause any problems, bigger or more aggressive modifications can.

The LLY uses the same forged-steel rods as the LB7 used. These rods were some of the lightest used in any Duramax (outside of the LML). In addition to a higher compression ratio, the VVT turbo which produces significantly more torque at lower RPM’s, the LLY rods are more susceptible to bending at increased power levels. Once you break the 500whp mark, bent rods becomes a likely reality. Additionally, be cautious of any modifications which create significant low-end torque as low-end torque is the primary killer of rods.

2. Head Gasket Failure

The LLY has the largest turbo in it out of any other 6.6L Duramax variants. Outside of it being the largest turbo, it had the largest compressor wheel and the tallest exhaust vanes. The end result is having an extremely efficient and high-flowing turbocharger. While this is great for those looking to add significant horsepower, this is bad for head gaskets.

Aside from the large turbocharger, the problem is amplified by the size of the turbo inlet manifold / mouthpiece. The inlet manifold is small and highly restrictive in its design. The restriction decreases turbocharger efficiency and makes the turbo have to spool faster to produce adequate boost which creates hotter engine temperatures. When you couple that with the biggest compressor wheel, the turbo has to work extremely hard which increases cylinder pressure and heat.

Over time head gaskets naturally deteriorate and fail due to the heat and pressure they are subject to. When you add the excess heat produced by the turbo with an undersized cooling system, the gaskets fail frequently.

Head Gasket Failure Symptoms

  • White smoke from the exhaust
  • Leaking water out of the tailpipe
  • Discolored oil and foamy residue in the oil cap
  • Engine overheating
  • Coolant leaks on the engine block and garage floor

While the replacement option is as easy as replacing the head gasket with a new $40 part, getting to the head gasket and properly cleaning and applying a new gasket is challenging. There are approx. 20 major parts that need to be removed to access the gasket. Book labor time for the job is 40 hours which can easily eclipse $4k of labor alone with today’s hourly shop rates.

3. EGR Valve Failure

Unfortunately, exhaust gas circulation (EGR) systems were standard on every Duramax LLY produced. The EGR system included a EGR valve and cooler. At a high level, the EGR system recirculates exhaust air back through the engine so that it can be re-burned in the combustion cycle, reducing the amount of nitrogen oxide released into the atmosphere.

The EGR valve is responsible for determining when and how much exhaust gas is recirculated into the intake manifold. Because you don’t want to shoot hot exhaust air into the engine, you also have an EGR cooler which is responsible for cooling the air that is recirculated. To cap off the emissions features, you also have a catalytic converter in the turbo downpipe to further filter exhaust air.

EGR system failure is not common on low-mileage LLY’s, but the valve, cooler, and catalytic converter are all common failure points for high-mileage engines.

EGR Failure Symptoms

  • Misfires
  • Rough idling
  • Poor performance and sluggish acceleration
  • Engine stalling
  • CEL codes for EGR issues or AFR related codes

4. Glow Plug Failue – 2006 Only

In 2006, the glow plugs were paired with a problematic glow plug module. The glow plug module was known to overload the plugs with current, causing the ends of the plugs to actually break off which would cause catastrophic engine damage.

The good news is that fixing this issue is as simple as re-programming the control module, which the dealership would do for free. Nowadays, this problem has probably been fixed on 98% of these engines out there, but if you are buying a low-mileage 2006 LLY it makes sense to confirm that this reprogramming has happened.

Common Glow Plug Failure Signs

While this issue won’t have any warning signs, I wanted to provide some symptoms of failure since these things are like spark plugs and do need to be replaced from time to time. Fortunately, they are inexpensive and pretty easy to DIY replace on these engines.

  • Starting issues – slow or hard starting
  • Poor idling and misfires
  • Engine light for glow plugs

5. Overheating in Pre-2006 Models

In the head gasket failure section above we discussed how the LLY diesel is subject to high heat levels due to the turbocharger and design of the turbo inlet. Additionally, the pre-2006 models had smaller radiators and cooling fans. The combination of added heat and a small cooling system can lead to frequent overheating on pre-2006 LLY models. However, overheating tends to really only be a problem when are towing heavy loads in really hot temperatures.

In 2006 year models, the turbocharger was improved to increase efficiency and reduce heat generation. Additionally, the size of the inlet manifold increased and was redesigned to prevent the heat issue from the previous year models. The engine was also outfitted with a larger radiator and cooling fan to improve cooling capacity and prevent overheating.

One of the best reliability mods for the LLY is upgrading to a larger and less restrictive turbo inlet. 

6. Fuel Pressure Relief Valve

The fuel rail on the 6.6L Duramax injects fuel at nearly 23,000-26,000psi of pressure. Included on the fuel rail is a pressure relief valve which is a spring-loaded valve on the back side of the fuel rail. The valve’s job is to release pressure in the event that it becomes too high. When pressures surpass the allotted threshold, the spring compresses, releasing rail pressure.

Over time, from constantly being subjected to extreme pressure, the spring can weaken, causing it to being releasing rail pressure earlier than it should. The end result is rail pressure continually dropping slowly over time which will make your Duramax create less and less power. So you will end up with continually decreasing power.

While this problem is only common on tuned and modified LLY’s, replacing the spring valve with an aftermarket valve or simply a new OEM valve should solve any potential problems here.

Common FPRV Symptoms

  • P0087 engine code (low fuel rail pressure)
  • Decreased performance
  • Noticeable decline in power
  • Engine misfires
  • AFR codes running lean

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  1. I replaced the LLY turbo inlet with the LBZ inlet, and the power increase was very noticeable. The Temp gauge is running cooler on the hottest days now. I additionally replaced the flexible ducting between the air filter with 4 inch hard ducting and clamps, and the truck just overall has more power and better fuel economy I used a PPE tuner to boost power by a very modest 90 hp and 120 torque, and that boosted power to a very respectable 405 hp and 740 torque. The truck now has 358 k miles, and it runs better than the day I bought it. I occasionally use short bursts of power to pass or merge, but typically keep my foot out of it which is being rewarded with very solid reliability and decent mpg. I also use the Michelin tire pressure guide based on axle weight and tire size, giving me amazing ly comfortable ride and tire wear. I am running Cooper Discoverer AT3’s 265-75/16E on stock rims. My unloaded payload psi (7,000 lbs., 4,000 front and 3,000 rear) is set to 42 psi front and 34 psi rear. This allows the tires to absorb many bumps and the ride is never harsh, just smooth and controlled. But if I put the pressure back to “factory specs”, of 50 psi front and rear, the truck bounces everywhere and it is a constant battle to drive down the highway in a straight line.

    1. Hello, I was just doing some research on the LLY engines and stumbled on this forum. I am planning on buying a 2006 Gmc savana short bus with a LLY 6.6 Duramax engine. I keep seeing that upgrading the turbo inlet from a LBZ is a good thing to do so i’d like to do that first thing. Where did you get your upgrade parts from ? TIA- I am new to the Duramax community so any help would be greatly appreciated !!!

  2. My 2005 LLY has over 315,000 miles on it and runs like a top. I did have to replace the fuel pressure relief valve at roughly 290,000 miles but other than that I haven’t had to do anything to it as far as replacing failed parts. I did install an S&B cold air intake and intake elbow, a 3 in hot side turbo pipe, rerouted the egr and installed a 4 in exhaust. I was really impressed with my fuel mileage and power increase. I drove from Illinois to Chattanooga, Tn and my avg fuel mileage was just over 25 for the round trip. The main problem I can see is the factory fuel pressure doesn’t increase till about 2200 rpm which actually keeps fuel mileage and power down in the normal operating rpm range but there are aftermarket modules you can buy to fix that. I think that’s going to be my next upgrade along with increasing the hole size in the fuel rail orifices.

  3. I. Have a 2006 Silverado 2500hd 6.6 duramax. I changed the egr valve and cooler. Plus mass air flow sensor, map sensor air cleaner and also had computer flashed and upgraded at the dealer. Be running E-zoil carbon crusher in fuel. It still throwing the same code p0402 (egr valve) what else could it be.

  4. id like to share some info, that ive encountered, that might be across all 2500hd’s gas or and diesel, and gm’s stupidity, and seemingly complete disregard for certain fundamentals principles in some subjects or what ever.
    some of this might be unique, to my experience, due to x many factors. But thought id share them all the same

    a bit of a preface to to this because it seemingly wouldnt effect a diesel being its a gasoline powered 2500hd BUT THIS IS REGARDING THE WIRING FOR THE FUSE BLOCK UNDER THE HOOD,
    and while i should of checked my 2004.5 LLY to compare and maybe verify i have not yet. maybe someone else has or can. maybe even goes beyond 04 04.5 idk

    this first one im sharing
    was something i came across from afriend of mine who bought a 2004 6.0l gasoline 2500hd that kept having starting and ignition issues, like randomly a 1/4 to 1/2 of the fact0ry under hood fuseblock would completely loose all power DOA, like no readings from a dmm etc truck wouldnt start etc. well one of the times he was trying to diagnose the issue, a friend of his, just had the random idea to remove the underhood fuseblock to inspect the wiring and connections etc that all the relays and fuses connect to. and he observed something quite perplexing and seemingly strange which struck him myslef and my buddy and any one who knows basics of DC electricity and AC ekectricity would id say start sratchig their head..

    upon removing the fuseblock, they saw a breadboard type of layout, which i guess kinda made things easy like a paint by number exccept this would be wire by number, Easy to route and and new circuit replace circuit wiring.

    ALL that was well and good, the confusing thing was, the type of wire that was found to be used…… type of wire you ask yes

    now im no electrical engineer, or any type of professional that has extensive experience in designing or implementing circuits be they dc or ac but ine thing i do know, that iwas taught that was told to me as something that was as basic and fundamental and that was a rule becuase of the way current was delivered differrently or the way current travels differently between dc and ac is that dc requires use of stranded wire and AC uses solid core, that will never change and you never use stranded for AC nor Solid for DC as doing so will emminently result in dangerous premature failure of the electrical system its been wired with

    which i then realisex exactly why hed been plagied with intermittent doa starting issues, etc, and the fuseblock randomly reading nothing to me as i saw it, it was the instance of using solid core wire in a dc system that was the cause of the intermittent yet reoccuring issue,

    i dont know if the gas and duramax chassis’s were assembled in the same plants in canada and mexico, or if depending on powerplant designation it was built in either place etc etc

    NOW i didnt delve into the how the why and who signed off on it as to how the reality became to be that there was a truck made a huge auto mfg and that theyd been making cars for 50+ years and should know that solid core wiring in a dc system is a HUGE nono or is at least or as far as i know is the first rule you learn about the 2 different types of electrical systems/circuits/ currents etc. and ive not yet heard of a use case scenario that superceeds that rule. except maybe if it involves profit margins,…… not a valid reason in this case nor should it be a reason in any case outside the scope of this or anything else.

    i told him to simply i say simple yet huge undertaking, that hed just need to re run /replace all that solid core with the appropriate gauage size dc stranded and unless some other gremlins were at play id solve his doa issue, that palgued him all the years since purching it,

    so if anyone is having weird unexplained power issues or if you are curious I kinda am, check out the wiring thats under the engine compartment fuseblock. this might be something random or could be a something that we as consumers should raise a huge stink about as its agregiously stupid and wreckless in regards to the vehicles being safe let alone reliable might be interesting to see if this using solid core wiring was used in any other gm vehicle patform one that might be worth checking would be the same year IMPALAS as from memory they seemed to be plagued with a slew of abnormal amount of problems…

    NOw ONTO DURAMAX specific issues,
    the duramax im referenciung is the 2004.5 LLY which currently has 300,000+ miles on it.
    1: this is one that is an odd one, so 2 years ago, the truck, started displaying the infamous “low coolant” indicator across the info display in the gauge cluster, (which has been replaced now 4 times at least)

    and along with this warning, the truck would at some point, after being parked for the evening, or ill say more than an hour or 2 idk exactly when it would happen, just that once parked for the night and then starting it in the morning, the system would act like it went out on a bender and it would expell coolant from the resivoir, at first it wasnt every night, over the 2 years it would become worse to a point of a constant drip.

    so the water pump was replaced 2 times, and the coolant resivior assembly replaced 3 -4 times, toward the end of 2 years i had tenetaviley suggested that the egr cooler may need replacing. from my researching, via numerous online resources, reading personal experieces of others across several different duramax owners forums, mechanic specific forums, etc etc.. and it seemed logical. to deduce that the egr cooler had reached the end of its service life. Which after taking it to a very duramax knowledgeable private mechanic, to have it looked at, he confirmed and replaced the egr cooler, yet the problem still rearred its prescence, that is until july took it to another local private mechanic whod done a lot of or most of all the servicing on it, over the years since new, as it had a random no start condition which was seemingly cured by having to prime the fuel system, so took it have that looked at if it was a random instance or something hinting at something else as well as the low coolant warning,
    during that time the mechanic randomly came across this obscure bit of insightful and ultimately led to resolving and fixing the at time ongoing mysterious and head beating against walll frustrating of “low coolant”

    the tidbit, im not certain i can verbatim quote and re-utter the exact wording but upon hearing the info, he went and tested the info he had come across and sure enough it resulted in fixing it.

    not sure where the ball fell on this one. as to whos fault, it is, but it seems that you can encounter this issue no matter where you are when going about purchasing a replacement coolant overflow tank assembly for the 2500hd silverado, (note i didnt specifically notate be it a diesel or gas engine , )

    the long and short of it is, that unless its specifically notated that you are wanting to purchase a coolant overflow tank assembly for a 04-06 or maybe even past the 06 and up, not for certain
    IF you simply state you need an overflow tank assembly for an 04 2500hd silverado, they will hand you a tank assembly that is for the gasoline engine, that DOESNT come with a coolant level sensor in it,….. it alsoshould be noted that i guess the system doesnt prompt the clerk behind the counter to ASK if its for the a gasoine engine or a diesel, which normally wouldnt be a problem However from what i recall the guy saying is that it doesnt come up in their system or it doesnt inform the parts counter clerk that there is a difference between the 2 overflow tank assemblies, because the gasaoline version doesnt require a coolant level sensor yet the diesel platform does,
    again im not certain on the exact wording or the exact factors that play a part except that which is this,
    If you need to replace the coolant level sensor, in the overflow tank YOU HAVE TO TELL THEM THAT ITS FOR THE DURAMAX 2500hd which has the coolant sensior in it,
    Be it a silly over sight by this mechanic, or what ever, the reason that th low coolant warning kept showing up, was due to the fact that there wasnt a coolant sensor in the overflow tank for the system to veriffy the coolant level, and when he went to get the another one, with this new learned knowledge, and testing it, with telling them it was for a duramax this time he ended up with the correct tank, and thus the low coolant warning hasnt reoccurred,

    so if you might be eexperiencing a similar type scenario it might be worthwhile to verify that there is a coolant sensor present in your overtank, and if not then you receiveid the 6.0L v8 gasoline engine coolant overflow tank.

    those 2 are the most notable that ive encounterd aside from the standard things, hope they help someone else out

  5. I have a 2007 Silverado 2500HD- Diesel. I tow a 30 ft 5th wheel. This truck gives me no problems until I am towing up hill and the oil psi drops to around 29 or so and then the temp will start to rise. My mechanic has tested everything he could and tells me it checks out fine. We have cleaned up the entire cooling system, replaced the water pump, etc… About a year ago we had an oil pressure low warning and I was told a vane in the turbo was sticking so that was fixed but we still have this persistent over heating issue at high work load- only going up hills. Once I crest the hill the temps go right back to normal and stay there the whole time otherwise. Any thoughts?

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