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What Breaks When – LB7 Duramax

Diesel Truck Maintenance, GM Duramax, Products, Tips and TricksTags

Written By: Lawrence “LT” Tolman

Back around the turn of the century, General Motors teamed up with Isuzu and designed a revolutionary new powerplant specifically for use in pickup trucks. The joint venture (and resulting engine) was known as Duramax, and the LB7 was the first of many models to come. It’s a 6.6-liter, common-rail injected, 32-valve, V8 turbodiesel which put out 235hp and 500 pounds of torque in its first year, and for the final year of ‘04 the LB7’s output had grown to 300hp and 520 pounds of torque. When the Duramax powered HD trucks were brand new, critics were impressed with the power and response as well as the fuel mileage, but more than anything they were blown away with how quiet the common rail injection system was compared to the mechanical diesels of the time. Today if you want to get your hands on a Chevy or GMC powered by the LB7, they can be found for as little as a couple thousand bucks if you want a beater, but even clean, medium mileage trucks can be had for around twelve thousand bucks. At first when the trucks were new, the LB7 was found to be very reliable, but as the years wore on and miles accumulated, there were a few common issues that popped up.

It’s a fact of life any 18-year-old trucks will have problems that need to be addressed here and there, but rest assured its nothing that can’t be solved when you own a Chevy. With a little careful maintenance and repair, your aging pickup will give you many hundreds of thousands of miles of service. Today we’re taking a closer look at the LB7, some of its shortcomings, how to spot problems in advance, and of course, how to fix them.


The one issue the LB7 is best (or worst) known for are the fuel injectors. There are several causes of the failures, and the issues were so common, GM covered the injectors with a special extended warranty for 200,000 miles or 7 years after the vehicle was placed in service. For most buyers the first set of injectors will likely have already been taken care of since the failure usually occurs between 100,000 and 150,000 miles. The bad news is, some of the replacements from GM are known to fail again, so the truck might need a second or even third set depending on how many miles it’s racked up.

The first question a potential buyer will ask when looking at an LB7 truck is how do I tell if the injectors are bad? While a service record or invoice is a good start, usually you won’t get that lucky. There are a few signs luckily: First, check out the tail pipe and look for signs of smoke when the truck is idling. There shouldn’t be any. Second, check the engine oil level on the dipstick: if it’s over the full mark and smells like raw diesel fuel, there’s a good chance the oil has been diluted with fuel from a sticking injector. But the third and most accurate method of checking an injectors health comes from reading the cylinder balance rates with a scan tool. A balance rate is the amount of fuel that has to be added or subtracted from each individual cylinder to make the engine run smoothly, so if an injector is failing, its balance rate will be much lower or higher than its siblings. With the truck up to operating temperature, the acceptable range is ±4 mm3 in park and ±6 mm3 when in gear at idle with your foot on the brake. If you have one or more injectors with a total fuel rate that falls outside of the accepted range, chances are it’s going to need a replacement.

It sounds simple enough to just swap out a failed injector for a new one and be on your way, but it’s not exactly a repair you’ll make on the side of the road in 20 minutes. The injectors are buried underneath the rocker covers, and there are about a dozen other parts in the way. Based on the labor time it takes to replace the injectors, (book time is around 16 hours) many owners decide to replace the entire set of eight at once since labor costs would be duplicated if you had to come back later to replace the ones you skipped. If your final goal is anything less than 550hp, then there’s no reason to upgrade to a larger injector, so a genuine Bosch replacement will be your best bet get you back on the road. They are an updated design which eliminates the failures the original versions are known for, so you won’t have to worry about another failure. But…. If you plan on stepping up your power gains in the near future, it might be a worthwhile investment to install a slightly larger set instead.

If you’re lucky enough to have a set of eight injectors that are working just perfectly, or you just had them replaced, how do you keep ‘em running like new? In one (or two) words, it comes down to fuel quality. Modern ultra-low sulfur diesel doesn’t have the same lubrication properties as older blends of fuel, plus when air becomes entrapped in the fuel, the tiny bubbles can cause accelerated wear under the extreme conditions inside the injector. By running a quality fuel additive like F-Bomb each time you fill up, the lubricity of the fuel is increased, and the injectors are kept clean and healthy, which leads to a longer service life. Plus, you’ll see an increase in power and fuel economy. Additionally, running a lift pump with a good filtration system like the FASS Titanium Signature Series will remove water and air from the fuel along with any small particles of dirt, so nothing but clean pure fuel makes its way to your engine, keeping your injection pump and injectors protected. Once you have the injectors crossed off the to-do list, there aren’t a ton of major issues that plague the 2001 to 2004 GM’s, but there are several smaller ones which you should be aware of if you own or plan to own an LB7, and the next most common issue lies with the transmission.

Slippery Slushbox

Another industry first GM was able to take credit for was installing a medium-duty transmission into their pickups. The Allison A1000 was designed for trucks and equipment, but GM adapted it to the LB7, and it has been used in every Duramax produced since (except of course the ZF-6 manual versions). The Allison found behind the LB7 was a 5-speed model, and it will live a long life when used behind a stock powerplant. A few minor upgrades that will keep the trans happy revolve around keeping the fluid clean and cool, since ATF is the lifeblood of any automatic transmission. First, regularly changing the external spin-on filter is a super easy way to keep the fluid clean, and you should do it every other oil change. They say transmission life expectancy is inversely proportional to the temperature of the fluid, and heavy use like towing (and racing) can generate a lot of heat and shorten the lifespan of your friend Allison. To combat this, a larger transmission cooler will remove more heat from the fluid, and a deeper extra-capacity transmission pan will hold more fluid to absorb the heat from the transmission, plus the aluminum construction radiates heat for additional cooling.

Regardless of how strong they are behind a stock engine, when you get power hungry the Allison 5-speed has a hard time transferring the power to the ground. In short, the holding power of the stock clutches just isn’t sufficient for a high-performance application, and once you get to about 125 horsepower over stock, you’ll begin to experience some clutch slipping. There are a few dead giveaways that will let you know if you’re about to experience trans problems. If you are on the throttle and the engine RPM suddenly rises but you don’t accelerate or the truck goes into reduced power, also known as limp mode, you have received your warning. And if you slip the trans once, the likelihood of it slipping again goes way up and things will continue to get worse, so you have two choices: turn down the power or install an upgraded transmission. The good news is whether you have a tune-only truck or a fully built monster, you have a solution. A great place to start is ATS, and their stage one transmission has what it takes for the stock or mildly tuned truck that tows heavily, or if you need a lot more holding power, you can upgrade to their stage six version, which is good for 800+hp, and of course they have an option for every power level and application in between.

Miscellaneous Parts

As we said earlier, the LB7 powered HD Sierras and Silverado’s are very reliable, and many you’ll find on the market have well over 300,000 miles on the original engine. However, there a few small things that will have to be replaced along the way, and for GM, their IFS front ends are at the top of that list. While they do ride smoothly, there are a lot of moving parts that wear out, like tie rods, pitman and idler arms, steering boxes, ball joints, and wheel bearings. A stock height/tire size truck might make it to 150k miles or more on the stock parts, or a lifted truck might last half as long (or less) depending on how the truck is used. Either way you can count on a full rebuild of the IFS at some point, and the warning signs are uneven wear on the front tires, play or wandering in the steering, or excessive vibration coming through the steering wheel. Since the stock steering parts are a bit too small to begin with, it’s a good idea to make a few upgrades as well. Simple tweaks like PPE Stage Three tie rods can strengthen up the steering greatly, and when you combine those with the center link brace, your steering will be much more precise and stronger than before, and it will last for many miles to come.

Along the way, you’ll encounter a variety of smaller maintenance items on your LB7 truck, from oil leaks in the transmission cooler lines to the transfer case oil pump rubbing a hole through the side of the case, but the best thing about an older GM truck is the parts to fix the issues are inexpensive, and in most cases, you can permanently solve the issue, rather than simply replacing failed items with more stock parts. Two such upgrades for an LB7 are the Fleece Performance transmission cooler lines which are a much stronger design than stock, and the Merchant Automotive transfer case pump upgrade kit.

High Performance Build

Once you’ve taken care of all the maintenance items, you could just leave well enough alone and ride in your bone-stock LB7 until the cows come home. However, one question always comes up when talking with your friends, and that is “how much power can the stock engine safely handle?” The answer is never a simple one, as there are a lot of variables, but each engine has a general range where it starts to fail. For the LB7, that number is somewhere around 550 to 600 horsepower at the wheels. Some can hold more, and some have failed with less, but that’s a good ballpark. Once you get much beyond the threshold, pistons can start to crack from the cylinder pressure, head gaskets can start to fail, and connecting rods can eventually bend. The crankshaft and block are (basically) bulletproof, and when it comes to crazy power numbers, you can reach for the stars. In fact, some of the power records for a Duramax have been made by an LB7.

Go Ahead and Try it at Home

Overall, the LB7 is a very reliable and efficient engine with exception to the injectors faults, but beyond that, the truck will only need basic maintenance and last for several hundred thousand miles. Just about any issue that might come up has readily available and affordable solutions, and they’re not hard to work on. You do have to look out for rust on the trucks, as the GMT800 trucks were prone to rotting away at the rocker panels and above the rear wheels, especially if you live in the rust belt. From a high-performance aspect, the LB7 is also a great candidate for a build, and the best part is, you likely got such a good deal on the truck when you bought it, you’ll have some money left for fun parts to make the truck go faster. Whether it’s in pristine condition or just an old beater, if you’re still rocking an LB7 every day, you deserve a 21-gun salute, since there’s nothing more American than an old-school HD pickup racking up the miles.

12 thoughts on “What Breaks When – LB7 Duramax

  1. Dead on, I have a 2001 Chevy 2500HD LB7. Replaced the water pump, fuel injectors, transmission line. 99% of the time I haul a Lance 845 camper. Truck does fine, a few upgrades, air bags, wireless controller keeps the bags even for all temperatures. Sway bar, certainly helps tighten up the ride too. Still have stock wheels with BFG 265 75 load range E, so far quite and wearing well. Just changing the oil regularly around 5k, fuel filter yearly, tranny oil change about every 30k. Other than that, it’s pretty much dependable. Thank you for posting, seems accurate.

  2. Tengo una duramax LB7 se me daño el motor por causa de los injectores y le voy a comprar uno a una compañía que los venden reparados pero le voy a poner la cp3 k que compre nueva y tengo dos pre bombas en el tanque. La pregunta es cuántos caballos le puedo sacar a un motor de fábrica solo con cp3k u dos pre bombas ??

  3. I have a 2001 Duramax LB7 run it out of fuel and could never get it to start again one mechanic told me I used to much starting fluid and now the rings are bad could this be true it was running just fine when it run out of fuel wouldn’t there have been some signs of the rings going bad just wondering because I can’t get it to start for nothing almost wants to start but doesn’t any help or ideas will be greatly appreciated thanks

    1. Hi there, well the first question I’d have is whether you’ve bled the air from the fuel system. Especially due to a lack of a fuel pump, your LB7 can be tricky to get all of the air from the system once you’ve ran it out of fuel. At the least, you’d need to crack the injector lines when cranking to allow the air to seep out of the system, then closing each one as you see fuel start to bubble out of each. Technically, its possible to blow the rings out if you’re using too much starting fluid, but that would mean you’d be having one heck of an ignition each time you tried to fire it. Overall, sounds like you’ve still got air in the system.

  4. Yes you’ve still got air in the line!! Install a lift pump of some sorts. Even a small pump to get positive pressure at your fuel filter head! Loosen the air screw and let air out. If not pump the primer button until it gets hard and loosen bleed screw.

  5. Great points, all valid for my 2002 LB7 with 390,000km. I would also note that the break lines are a sore point as many years were made with steel lines, leading to failure in the rust belt. My transfer case has an issue I’m wondering if anyone has seen – its not the oil pump rubbing through, but a corrosion issue with the bolts that hold the cases together. To me, it appears the lock rings they used on the bolts had some sort of dis-similar metal reaction leaving the t-case housing pitted where every bolt is fastened. Now the bottom ones are so bad, oil is leaking out. Not sure it can be fixed.

    1. Thank you for the response, and definitely agree that especially for those in the rust belt and with accumulated miles (or KM’s), you’ll definitely see failures to even go as far as rust penetration on the frame.

  6. thank you so much for the information on the LB7. My 2002 has stock injectors (hazing at idle) so I am want to change them out. Looking hard at the S&S injectors, Looking hard for the right mechanic to do the long job. I am their worst customer, mechanical knowledge but don’t have the drive or tools to take on this big replacement. Anyway I deleted my engine fuel filter when I installed the lift pump and do aid Lubricity products.
    again, your write up will make me keep my 2002.

    Can the 5 speed Allison be swapped with the next gen 6 speed?


    1. You’re very welcome, happy to help! When you’re ready to pull the trigger on injectors, feel free to give us a call or shoot an e-mail and we can help with your decisions. Either way you end up going, make absolutely sure you either replace the injector lines or have them sonic-cleaned. In regards to your question about swapping out the transmissions. The good news is you technically have six years, its just that your truck isn’t currently capable of accessing the secondary overdrive. Duramax Tuner has built a kit, though, that includes the new valve body and TCM you’ll need for the upgrade. Check it out here: https://www.dieselpowerproducts.com/tuner-6-speed-conversion-01-05-duramax

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