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.

Injectors

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.


28 thoughts on “What Breaks When – LB7 Duramax

  1. you didnt mention the water pump issue that likes to hide behind the harmonic balance that is torqued to 250ft lbs ..

  2. 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.

  3. 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 ??

  4. 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.

  5. 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.

  6. 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.

  7. 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?

    thanks
    Mark

    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

  8. Valuable information guys, great job! I have a 2003 2500HD with 575,000 miles with original injectors, in fact I have only changed the alternator twice. No smoke and great compression as of 550,000 miles.
    Of course owning an Allison transmission repair shop I have done everything possible to the LD 1000. It has never failed or even showed signs of mechanical problems but I have rebuilt it four times over the years as new technology and HD parts have become available.
    I’m retiring the truck this spring as a parts truck in LA and sending it to our ranch near Kalispell MT, with a new paint job and a few other treats. We have always purchased new trucks every two years or so as well but this truck has given us great faithful service, second only to our 2003 Dodge Cummins with 865,000 miles (many in-frame engine repairs.
    Dave

  9. I got a 2001 went threw flood motor want turn over going to take injectors out replace see if she runs then put in 51 Chevy truck 4 x4 dually I want 5 speed manual shift tranny plus after market wireing what u think

    1. Now that is an awesome project, would love to see its progress. So, on the flooded Duramax engine, are you saying like it was submerged, or the cylinders were flooded with fuel, or other? If it was indeed submerged in water, the first step is to make sure the engine will at least turn over and isn’t rusted at the crank. If it is turning over but won’t fire, it most likely isn’t the injectors causing it to not start, typically it’ll just run really rough and lots of white smoke, but will still fire. Obviously, based on the condition of everything, there’s a bit of a laundry list of items to check if the truck won’t start, but if you’re needing any guidance on where to start, feel free to give us a ring or shoot an e-mail to [email protected] and we’d be happy to help you along the way.

  10. Recently purchased a low mile, stock 2001 from original owner. Transmission was replaced by dealership within first 20k. Truck runs great and is in near perfect condition, except for failing injectors. Have been quoted $4500- $5000 by a number of diesel shops in my area. Willing to put the money in, just curious if that sounds like a national average on parts and labor? Thanks

    1. Congratulations on the new-to-you truck. Unfortunately, the injectors are a very common failure on the LB7 Duramax, at least with the original design of the injectors, which have been superseded, so at least rest assured you should only have to do this job once. Regarding the $4500-5000 bill, its definitely a big job. We sell genuine Bosch injectors for $254.74 per ($2037.92 for all four), which is the biggest chunk of the bill. Then, you’ll also need to replace the injector lines, which we typically recommend Alliant branded, which total $565.92. Finally, you’ll obviously need an installation kit, and for that, Merchant automotive has put together a great kit that runs $190. This puts our total so far at $2793.84, and most of the remainder being labor. Many shops will quote 16-20 hours for the job, so if you look at the low side of 16 hours at $100 per hour, that’s a total of $4393.84. Overall, yes, its a big job. At $5000, that’s on the high side, but still within the range of a “normal” estimate for this job. Otherwise, if you’re up to the job, you can literally save yourself thousands of dollars if you want to tackle the job yourself or have a buddy help. Below is a link with all of the products I’ve mentioned, which I hope helps. If you have any other questions, feel free to reach out.

      https://www.dieselpowerproducts.com/c-277-2001-04-66l-lb7-gm-duramax-fuel-related.aspx#/filter:cfm_manufacturer:Bosch/filter:cfm_manufacturer:Alliant$2520Power/filter:cfm_manufacturer:Merchant$2520Automotive

  11. I recently picked up an 02 gmc lb7. I get a strong smell of diesel while driving if my windows are down at all. I thought it was coming through the vents but I’m pretty sure it’s when my windows are cracked, or at least it’s stronger. I don’t really have any smoking at idle very very little. But I think there is some fuel getting in the oil. I keep checking the oil and and I really can’t tell if I smell the diesel or I’m over thinking it and it doesn’t seem to been over filling to the point where it’s noticeable or I just can’t see the diesel as dumb as that sounds, I need to drain the oil and I’d be able to tell then how much fuel is in there. But what’s the likelihood of a leaking return line bango gasket or a cracked injector. Is there a way to tell or diagnose without tearing the valve covers off?

    1. Hey there, thanks for the questions, but sorry to hear of the troubles your LB7 is giving you. Okay, the first thing to do is try and determine if you are in fact getting fuel in the oil. Overall, if you’re not experiencing hard start, white smoke at start up, rough idle, those kinds of things, most likely your injectors at least aren’t bad enough to cause a fuel smell when you’re driving. That said, yes, drain the oil and see if you can physically see any fuel. Also, sample some of the fuel and you can actually take it to a lab for testing, its not very expensive. Otherwise, you would need to run a fuel return rate test to determine if your injectors are passing excessive fuel. And if you had access to a scanner or mechanic, they can run that test with a scan tool. Now, lets presume again that the truck is running great, but still have this smell of fuel. The return line banjos are notorious for leaking on these trucks and its very possible you could merely have an external leak. To determine this, you’ll need to spend some time with a flashlight peaking around the fitting at the return lines and see if you can see any moisture. The external return line bolts to the cylinder head right at the end of the intake plenum at the front of the passenger head and the rear of the drivers side head. Another place that commonly leaks is the fuel filter housing at the top bleeder screw. There’s not a great way to test this, but you could get a hose up to the return banjo and give it really low air pressure, then see if you can hear any air coming from the PCV vent. Hope this information helps in diagnosing your fuel smell, good luck, and let us know if you have any other questions.

  12. I have a 2003 lb7 ran great when I first got it 161000 miles except sometimes the starter grinds no teeth are missing on the flywheel and I put a new starter on still does it. Then it at times would start right up and sometimes it wouldn’t if I let it set for an hour or sometimes all night it was totally random but it would start right up without doing anything to it it runs great when it does never just died just won’t start sometimes so I have replaced the FPR the cam shaft position sensor put a solid race valve in the ignition switch and key lock tumbler put the cat conversation filter on and the airdog 150 on still does the same thing after I did all that it started right up ran for 5 min shut off it has started and shut down 3 times all at way different times sometimes it will run for 20 min or sometimes for 5 min and sometimes after 2 or 3 seconds of cranking it will stop and sometimes it will crank for 30 seconds everything is just so random please help thank you

    1. What has been done to diagnose this issue? I see that some of the usual suspects have been replaced. Does it have a check engine light? If so what is the code?. When it shuts down is it a hard shut down, such as someone turning off the key? Or will it stumble like it is running out of fuel? I would go through and check all your power and ground connections. Make sure the grounds for the BCM/ECM are good and clean( driver side under the door on the frame). Check those things out and reach back out to us with what you found!

    1. Thanks for taking the time to read it. Did you get your blowby situation figured out? Did you change or hydrosonic clean the return lines? What injectors did you install?

  13. I wana buy a used 04 gmc c4500 diesel, wana know what are the common problems with these trucks, and would u recommend it. Thanks

    1. Thanks so much for reading and the question. The first thing to confirm is that the truck is an early 2004 with the LB7 engine, as opposed to a 2004.5 with the LLY just for clarification on your part since there are numerous differences between the two, such as injectors and turbo. Overall, this article still sums up any of the issues relatively well that you’ll need to be on the lookout for with this TopKick. Depending on the mileage, fuel quality, duration between fuel filter changes, you’re going to want to have the injectors checked out unless they’ve already been replaced. If the truck does have appreciable miles, the stock IHI turbo may be getting ready for replacement. Depending on what you’re planning to use the truck for, it may be helpful also to upgrade the factory Allison 5 speed to a 6 speed to reduce your RPM when cruising down the highway. The five speed is a good transmission, but having that extra gear is nice and oddly enough, the five speed is setup for this double overdrive mechanically speaking, merely needs an updated TCM and valve body to TELL the transmission to operate in this gear. DuramaxTuner makes a conversion kit you can check out here:

      https://www.dieselpowerproducts.com/tuner-6-speed-conversion-01-05-duramax

      Overall, any of the recommendations in this article are still relevant on the 4500 you’re looking at. If you have any other questions, feel free to reach out.

  14. I’m not too sure if many other people have had this issue or not, but when you’ve had other “older” Duramax’s particularly an LB7, then you already know if you’re run the hell out of your truck “because it’s a dirty” then you’d know that the Allison will only withstand so much abuse… SO, you send it off to build it to the gills to make sure it NEVER has to be done again, that’s been the first thing I’ve done on any and ALL of my Duramax’s. With that being said… something to keep in mind, is that when your transmission has the capabilities of withstanding now 3 times the power your engine is capable of producing… there forms torque issues… such as, (what I’m dealing with at this very moment…) cracked adapter plate… you do a mad rush to find and repair ALL coolant related leak issues… and come to find out, the adapter plate located on the rear of the engine between the engine and transmission is ACTUALLY a coolant passage cover plate… something you would have never even given a FIRST thought to check/repair… it IS costly, and an absolute pain in the rear (no pun intended) to change… so THIS time I’ve opted to sending it to a very good friend of mine, and one of the best diesel mechanics I know to fix it… not sure how much of a good friend I am pawning my issue off on him, but… I’m paying full price on what it costs to be fixed, so… just want to give a heads up on this issue… I’ve just noticed almost NO ONE has brought this issue up… so… hope I’m not making myself look too much like a dummy, and hope it’s somewhat helpful… thanks and have a good one.

  15. Does the 2003 Chevy 3500 have an egr and if it’s malfunctioning cause white smoke like an injector problem

    1. Duramax’s did not receive an EGR until 2004.5 model years with the LLY, so your 2003 LB7 would not have an EGR. White smoke, especially on that truck, is very indicative of injectors due to an extremely high failure rate of the original design.

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