Common Duramax Diesel Problems

Common Duramax Diesel Problems, Troubleshooting, and Solutions

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

Whether you currently own a 6.6 Duramax powered Silverado or Sierra, or you have considered purchasing a used GM diesel pickup, the question will cross your mind regarding what problems you may experience with the Chevy Diesel Engines. All told, there are six different iterations of the Duramax diesel engine: LB7, LLY, LBZ, LMM, LML, and L5P spanning from 2001 to the present day. Each version of the engine has its own issues and quirks, and some are less problematic than others. Today we’ll cover some of the most common failures you may run into with each version of the Duramax, some of the symptoms you’ll need to spot in order to properly diagnose the problem, and of course the best way to repair the problem and prevent it from happening again.

This is by no means a comprehensive list of all the problems you may run into, so if you are stumped by your Duramax or just need a little more information to help make the best decision on which parts will help get you back on the road, give us a call at Diesel Power Products, and our team of experts will be able to get you sorted out in no time.

Duramax Engine LB7 2001 to 2004

Duramax Engine LB7 Problems

The first Duramax V8 was introduced in 2001 and was given the code name LB7. The introduction of common rail injection powered by the Bosch CP3 injection pump made the LB7 Duramax stand apart from other diesels of the day with excellent power and efficiency. As the first batch of trucks started reaching the 100,000 mile mark, it became apparent there were some issues with the new engine, and one of the first symptoms exhibited would be a rough idle from the LB7, excessive smoke from the exhaust, long cranking times, and even fuel diluting the oil in the crank case.

We know today LB7 injector failure is a common problem, and because the operating tolerances inside the fuel system are so tight, proper lubrication from the diesel fuel is critical. If there is even the slightest bit of air in the fuel or worse yet contaminants like dirt or rust, you can bet the internal parts of the injectors will wear prematurely and you’ll be doing an LB7 injector change. If you suspect yours are on the way out, the best way to identify which one(s) are at fault, you need to hook up a scanner and read the balance rates. The trucks computer keeps track of how much fuel needs to be added or subtracted from each individual cylinder to make the engine run smoothly, and this value is called the balance rate. If one cylinder is contributing too much or not as much as the rest, the cause is usually a bad injector and the cure is a replacement.

Duramax Engine LLY 2004.5 to 2005

Duramax Engine LLY Problems

In terms of LLY injector problems, GM had sorted out the internal failures with the 2004 ½ redesign, but the LLY Duramax would occasionally exhibit a trouble code for the injectors, but in most cases it turned out to be an issue with the injector harness rubbing through and causing a short or open in the wiring. One of the biggest complaints on the LLY is it has a tendency to overheat, especially when the truck is being worked hard with a heavier trailer or on a hot summer day.

While it may not be the first place you think to look, the stock air intake system can actually contribute to a hotter running LLY Duramax. Many people add a cold air intake with hopes of increasing horsepower and efficiency, but on the LLY you need to take it one step further and add a less restrictive turbo mouthpiece, a part which doesn’t come with most cold air intake kits. When the engine is working hard, the turbocharger is drawing in a ton of air, and the restrictive stock turbo inlet tube can effectively choke the VGT turbocharger causing the compressor outlet temperature to skyrocket, heating up the intercooler, and ultimately raising the temperature of combustion and the coolant temp. However, if you install a stock LBZ or aftermarket turbo inlet, you can let the engine breath freely which will lower air temps and keep the coolant temps under control.  

Duramax Engine LBZ 2006 to 2007

Duramax Engine LBZ Problems

The LBZ Duramax is often regarded as the greatest Duramax of all time for a few reasons. It was the first to come bolted to the six-speed Allison transmission which provides a lower cruising RPM and greater fuel efficiency when compared to the five-speed on the LB7 and LLY. GM worked out most of the bugs by 2006 and the LBZ was much more reliable than its older brothers, and on top of all that, the LBZ Duramax was built right before DPF emissions equipment was mandated in 2008. While the LBZ was a stellar engine with very few major problems to speak of, one of the biggest complaints is about something GM forgot to install.

People often ask what is the best lift pump for an LBZ, and truthfully ANY lift pump is better than none at all. The CP3 injection pump sits in the valley of the engine between the cylinder heads and is driven by the camshaft gear, and it has two main jobs: get fuel from the tank up to the engine, and pressurize the fuel to 26,000psi and ultimately inject it into the engine. Because the CP3 is doing two jobs at once, it sometimes has a hard time keeping up, but you can help it out with a lift pump. By installing a lift pump with extra filtration from FASS or Airdog you will extend the life of the injection pump and injectors by allowing only the purest fuel into the system, and by feeding a steady supply of low-pressure fuel into the injection pump, it can better focus on its main task of generating high-pressure fuel.

FASS Titanium Signature Series for Duramax Diesel Engine
The FASS Titanium Signature Series Lift Pump is an excellent upgrade for any Duramax.

Duramax Engine LMM 2008 to 2010

Duramax Engine LMM Problems

When GM changed body styles with the 2008 Duramax, the redesign coincided with some new regulations from the EPA regarding emissions equipment on light duty trucks. Mechanically, the LMM Duramax is almost identical to the LBZ, and if you do a quick search of the web for LMM Duramax problems, you’ll find much like the earlier LBZ it had very few problems related to the engine itself.

One problem can rear its head when you are pushing the power of the LMM to a level GM never imagined, and that’s with the pistons. Simply put, they can crack under pressure. There are a lot of factors which dictate the power level where a piston bites the dust, but it’s usually north of 600 horsepower. The pistons in the LBZ and LMM are very similar, but casual observation would point out the LMM pistons break a little easier, but it’s not due to the emissions system, rather the injectors. The LBZ injector nozzle has seven holes for fuel to spray out of, but the LMM has six which means there are two opposed streams of fuel directly above the wrist pin area of the piston, which causes hot spots directly on top of the weakest area which can lead to cracking. They say an ounce of prevention is worth a pound of cure, and if you want to avoid a costly and time consuming rebuild, it’s advisable to keep your driving habits and tune level at a slightly more conservative setting than “Full Send”.

Duramax Engine LML 2011 to 2016

Duramax Engine LML Problems

In an effort to keep up with the ever-tightening emission regulations, GM redesigned its Duramax for the 2011 model year, and this time the code name was LML. Power was at an all-time high coming in at 397hp and 765 pounds of torque, and in order to provide this much power while still keeping pollutants low, GM needed to be able to increase the pressure in the fuel system to levels above what the tried-and-true CP3 could safely deliver.

On paper, higher fuel pressure sounds good, but you can’t talk about common LML problems without mentioning the ticking time bomb of an injection pump. A class action lawsuit was even filed against GM for its use of the CP4, alleging the injection pump was designed around European fuels which have more lubricating properties than the ultra-low sulfur fuel which we’re accustom to stateside. As a result (allegedly), the lack of lubrication in the fuel can cause a premature failure of the injection pump, to a point where it won’t allow the LML Duramax to run. You can be driving down the road and everything seems normal, and when the pump fails, the truck just shuts off. At first you might think you can just swap out the pump for another and hit the road, but sadly this is not the case. Because of the nature of the CP4 failure, large amounts of metal shavings are produced inside the pump, and they get sent straight into the rest of the fuel system which contaminates pretty much everything. In order to properly repair an LML after a CP4 failure, you’ll need a kit which comes with a new injection pump, high and low pressure fuel lines (all the way to the tank and back) a fuel filter, a set of eight injectors, a pair of new fuel rails with bypass valves and you’ll even have to drop the tank to clean out any debris.

CP4 injection pump on Duramax
The CP4 injection pump was first debuted on the LML Duramax engine, beginning in 2011 model years

Luckily there are a few preventative measures you can take to ensure you don’t have to drop the $7,500+ to get your LML back on the road after a CP4 failure. Just like an LB7, you can install a lift pump with better filtration which helps to clean air and debris out of your fuel which will minimize wear and extend pump life. Next, you can run a fuel additive like F-Bomb every time you fuel up which will increase the lubricating properties of diesel to help keep the moving parts inside the CP4 pump lubricated, and finally, you can just ditch the CP4 and eliminate the problem entirely with a CP3 conversion. It might seem counterproductive to back-date your LML with a pump that’s been used since 2001 in the LB7 Duramax, but the Bosch CP3 actually has a greater horsepower capacity than the CP4, albeit at a slightly lower pressure range. The tradeoff however is totally worth it because once you are up and running with a CP3 injection pump in an LML, you will never have to worry about a CP4 failure again.

Duramax Engine L5P 2017 to Present

Duramax Engine L5P Problems

Just like the LBZ was the best pre-emissions 6.6 Duramax engine, the L5P is the best post-emissions Duramax. All the problems associated with emissions system and injection pump have been taken care of, and for 2017 GM switched suppliers of the injection pump from Bosch to Denso. Mechanically, the L5P Duramax is stout and very few major problems have been reported. One common problem is with the MAP sensor however, as it tends to clog with soot and throw a check engine light, but a simple MAP sensor spacer and a can of electrical cleaner can get you back on the road worry free and prevent the soot from building up again.  

Transmission Problems

Nearly all Duramax powered pickup trucks were equipped with the Allison 1000 automatic transmission, with the five-speed version running from 2001 to 2005, and the six-speed starting in 2006 and running until 2019. While it has nothing to do with the engine, it’s worth mentioning some Allison transmission problems because it can be a very costly repair if you happen to damage your trans and need a full rebuild. At a stock power level, the stock Allison usually does an OK job at transferring the torque from the flywheel to the rear differential, but even a bone stock LB7 can put a hurting on the Allison, so you can only imagine what happens when you turn up the wick or hook up an oversized as many truck owners often do.

The five speed Allison is by far the weakest of the bunch, and if you are pushing over 400 horsepower, you’ll probably experience reduced engine power, or limp mode which engages when the TCM detects slippage. The six-speeds have a lot of mechanical similarities with the five-speed, but the few slight tweaks allow the trans to hold a bit more torque. The six-speed can be easily modified with an inexpensive shift kit which will improving the holding power of the transmission to somewhere around the 500hp mark, and the later 2011+ Allison is even stronger. Regardless of which version of the Duramax or Allison you are driving, if you have a heavy right foot and a bunch of modifications, chances are you’ll need to invest in an upgraded Allison so you can put your foot down with no fear. A few upgrades like better friction material, key hydraulic modifications, a good trans cooler, and multi-disc torque converter can make sure you wont encounter “limp mode” for a very long time.

Want to find out more details of known issues and fixes? Check out other resources here:

25 thoughts on “Common Duramax Diesel Problems, Troubleshooting, and Solutions

  1. I have a 2009 Duramax, I sometimes get a message ” Engine power reduced” when stopped at a stop light or coasting to a stop. Power is reduced for about 100 feet or so then it takes off and runs fine. When it is hooked up to the computer, it says it is the mass air flow sensor. I have changed it twice and replaced the air cleaner. Did not fix the issue. Any suggestions?

    1. Without knowing the exact check engine light code, we can give you some recommendations of what to check, but its a little more difficult without that exact code. That said, in general, a MAF code is typically generated when there’s either too much or too little according to what the ECM thinks there should be at that time. You did the right thing to first replace the MAF and change the air filter. Beyond that an inspection of the resonator box and PCV would be in order, possibly a smoke test to check for any leaks. Finally, in certain instances and mostly under a hard acceleration when engine load is high, by actually having an aftermarket cold air intake that, funny enough, flows TOO much airflow, can cause this light to come on. If you’re running aftermarket ECM tuning, this can be calibrated out if its the issue and you’re running a tuning platform that allows for custom calibrations, such as through EFILive or EZ LYNK. Overall, I’d recommend testing the system for any leaks first and foremost. Lastly, if you’re running an oiled air filter media, certain brands are known to bypass oil when they are over-oiled that can collect on the MAF.

  2. Curious if you know much about what’s going on with a constant transmission leak on a 2017 Duramax? Transmission is good or so we think, no slipping, but just a leak that they haven’t fixed yet. First it was the lines, then the o-rings, and now we are still having a leak!

    1. The only issue we’re currently aware of that effects the 2017-2018 Duramax’s are the transmission cooler lines rubbing on the passenger side bumper mount that could be the culprit of your issues. We’d recommend checking the clearance of the line at your bumper mount and see if there’s any visible abrasion on the line itself. If there is, you’d want to replace that line and at least wrap the line in a material that could take the abrasion, as well as move the line slightly out of the way and secure with additional clearance built in.

  3. 2017 GMC 3500 with Dura Max. When slowing down and light braking the engine rpm increases for a short time which increases the speed of the truck. I seem to recall a notice from GM on this but can’t seem to find it again. Any ideas?

    1. Thanks for the question. Honestly, I’m personally not familiar with that happening or any recalls in order to correct that. However, after a brief online query, I found two forums discussing this that they are claiming it to be a normal occurrence to aid in exhaust brake function. Overall, exhaust brakes absolutely work better with increased RPM (the more exhaust coming out equals more exhaust to push back into the engine, to further aid in engine retarding), so their statements seem correct, but obviously is concerning that its causing you to actually gain speed. Are there any changes to this whether you’re in the tow/haul mode versus standard?

      Here’s the references I noted above:

  4. 2016 duramax lml exhaust got unplugged , started , ran for like 30seconds and died, hasnt started since , gm dealer dealer said it was immobilizer in dash,, sent out to fix,, and reprogramed emc,, and still no start…please help

  5. hello i got a 2016 gmc sierra duramax its was running fine into the garage was looking at exhaust muffler and was planning on takin out muffler but before i did any cutting i uplugged the exhaust wires to see if it would still run…i started it.. it ran for like 30 seconds and hasnt started since..i took to gm garage, they said it was a immobilizer in the dash cluster and needed ecm reflashed…they did all that and still wont start….can you help me please

    1. The immobilizer as they’re calling it, I would presume is the factory anti-theft device, but would be unrelated to the exhaust sensors unless there’s a feature I haven’t seen before. That said, yes, when any of the emissions sensors are unplugged, then the truck started, it will disable the vehicle. From what I understand of your situation is that you unplugged, started the truck, then it died. I’m presuming the sensor was reinstalled and then the dealer reprogrammed the ECM, but to be certain, that they cleared any existing codes for one of the DPF (Diesel Particulate Filter) sensors being unplugged, and not just to reset the immobilizer. I would also have to presume that the plug was reinstalled in the correct orientation. Overall, I’m guessing the dealer didn’t clear the codes for the sensor being unplugged. Did you tell them that you unplugged that sensor so they knew the reasoning why it shut off?

  6. 2018 2500HD Duramax 43,000 miles
    After warmed up, my engine feels like it is missing, when at a stop. The miss is subtle but sometimes is more noticeable. I am concerned that it might be a bad or clogged injector.

    1. That definitely could be an injector that is misfiring. Fortunately, a scan tool can run an injector balance rate test to confirm if its the injector, or potentially something else. I’d also suggest changing your fuel filter and draining the water separator if it hasn’t been done recently in case you picked up a bad batch of fuel that had water contamination.

  7. 2020 2500hd ltz…

    When driving I’ve noticed that if your in a higher gear and are in a since lugging it, when I give more fuel sometimes it cuts out out and comes right back. Almost like a hiccup. Truck has 6,500 miles on it. Just seems like it may be a fuel issue? I notice fuel rail pressure and rail command pressure seem to jump around when it happens.

    1. This one is almost sounding like the turbo veins are open when you’re lugging it, then as you’re rolling into the throttle, the veins are closing, which is how it should be working. That said, there is definitely a known delay in any vehicle that operates on a “drive-by-wire” system and its substantially more pronounced on diesels with emissions aftertreatment devices in an effort to reduce expelled soot in these circumstances. Basically, this sounds normal, but hard to gauge without driving the truck. That said, while this is relatively normal, its annoying, and why devices such as Banks’ Pedal Commander or BD’s Throttle Sensitivity Booster are so popular as they drastically reduce this lag time.

  8. 1. 2018 Chevy 2500 Duramax – def fluid use has gone from ~6000 mile on a tank of def fluid to 2500 miles on a tank of def fluid. Dealer says there is no mileage on def fluid it uses, what it needs. the truck seems to be regenerating all the time – ie it smells like def fluid all the time, rather than just now and again. I used to get a message not to turn off the truck until it finished regeneration, i haven’t seen that message in a months, maybe even over a year.

    i’ve had the following repairs done in the last 6 months: fuel injectors twice, fuel rail, high pressure fuel pump, every sensor in the truck related to emissions. might be a few other things – can’t remember.

    Now, i have rough idle – intermittent. no check engine light, no codes. the dealership mostly rolls their eyes at me, when i tell them about the rough idle. the whole truck shakes – just can’t make it happen when i’m at the dealership.

    any suggestions appreciated.

  9. I have a new Chevy duramax 3500. It is tripping the check engine light for code OBD2. PO26c. Quality low fuel injection. Reads 60 psi fuel pressure but not a straight line. First dealership trip was told nothing was wrong. On for a second trip. Runs fine. Any ideas ? Is using an fuel additive on the new 2021 duramax a good idea ? Thanks.

  10. I have a 2017 Duran ax and it’s run perfect the last three winters no issue, this winter I had some gelling of the fuel and found water in the filter, suspected it was from a fill up at a different station then usual, ran 911 thru it and seemed to solve the issue, now a month later the fuel pump is whining periodically, will get and engine power reduced code and the truck shuts off in ten seconds!, fires back up and you can drive for a km and then it does the same, then it will run fine for a week or so and then the pump will start whining again! Any ideas?!

  11. I have a 2021 sierra diesel.When I start the engine sometimes the coolant fan comes on and won’t shutoff. The next day it’s fine. It has been to the dealer 3 times with no repair.

  12. I have a 2020 gmc with the 6.6. 34K mileage. Just pulled in driveway and idling it misses. Just a miss as idling. don’t see any smoke out tailpipe. Just got back from 2200 mile round trip. fuel fuel, def I filled this morning. Any ideas?
    Thank you

  13. I have recently heard some minor growling noise. Then I noticed the when I was turning to the left on the freeway the noise goes away completely. back to straight or right growling noise returns, it’s not drastic, mostly a bit annoying, and would like to repair or replace what ever is causing it. 2007 Silverado 6.6 Duramax 2500 HD 4×4 Allison transmission, mileage is at 226,500. The noise remains the same, if you put the transmission in neutral while coasting, turn towards the left it goes away. It does not change in intensity while accelerating or slowing down. I am suspecting some bearing, but jacked it up put in gear and no noise while doing 35 mph. would love to hear any suggestions or past experience stories. Thanks, Stephen

  14. This answer is for Stephen Gordon. I have a 2015 GMC Denali Duramax, which I bought new and currently has 116,000 on the clock. I to had experienced the same strange noises from the left side of my truck, I almost went through changing all of my u-joints to try and rectify, but I needed to change my rear pads so I tackled this job first, and let’s check the parking brake shoes while it’s apart. Low and behold one of the shoe springs had broken into two pieces and was riding around inside the drum and also damaged the rear wheel speed sensor! Check yours out!

  15. I have a GMC turbo diesel – 2005 – 6.6 litre engine.
    PRoblem is engine knock – loss of power on acceleration.

  16. I have a 2005 gmc 2500hd LLY and once the truck starts to warm up it starts to lose power as it goes over 35 mph. When it does it I look around for codes but the engine light doesn’t pop up. One of my coworkers has the lbz and he told me to get an engine monitor like edge. Do you have an idea what it could be or should I get the monitor first?

  17. 2008 duramax. Keep getting reduced power change fuel filter. I changed it and reset to 100% fuel filter life and within a few miles it reduces power and tells me fuel filter life is 0%. I reset disconnecting batteries each time but same problem. Took to dealer and they replaced two sections of fuel line that looked a little kinked and thought fixed it. 24 miles later same thing. I had already installed a booster fuel pump with filters years ago. So kinda thought the fuel line replacement wasn’t the issue. Any ideas?

  18. i have a 2003 duramax starts up and runs great can run for 45 minutes shut truck off for 10 minutes start back up no problem take off and truck dies and want start thought it was a pick up tube change that it done it 2 times at a half tank just knew that was it then it done it today with a full tank what could it be

  19. I have a 2013 duramax 2500. Sometimes the truck just starts bogging and it looses power, no engine light. When I turn it off and on again. It works fine. It is deleted and has a bully dog programmer. Any insight will be appreciated.

Your email address will not be published. Required fields are marked *