6.6 Duramax, learn me

I have a friend considering a gmt800 with the Duramax and I don't do diesels.

What should he look out for and what does the hive recommend?

What year? I own one (and am selling it). If it is equipped with the CP4 fuel pump, a lift pump is strongly recommended (or retrofit to the CP3). It is also recommended to use a lubriciry additive with the fuel since US low-sulphur fuel doesn't always play well with the pump. 

The Allison transmissions are robust but trans filters should be changed along with oil filters; they're super easy to access and are just spin-on filters. Fuel filters are recommended at 10 or 15k, I think, but they're pretty easy to do. 

DEF heaters and sensors can be a sore spot. I had to do both on mine. Use the exhaust brake when driving around town to keep things hot. 

Duramax diesels are generally good to go for many hundreds of thousands of miles. 

In reply to brandonsmash :

It's in the planning stages but 05 ish is what he's finding and likes

I had an 04 LB7 since new. The thing had about 600,000 miles on it before a tree fell on it and took it out. That truck was all over the continent. Mexico. Canada. Both coasts numerous times tugging a variety of trailers behind it. 

Biggest thing is stay on top of maintenence. An 05 will be an LLY. When the injectors fail they tend to fail as a high return rate offhand. Easy to swap. I did I want to say 3 maybe 4 sets on my LB7 through the life of the truck.  Rebuilt the rear axle once, replaced the converter. Replaced the transfer case due to pump rub, countless sets of brakes and suspension (at one point was lifted 6" all suspension on a 37, then eventually went back down to a 35").  Suspension stuff likes to wear out religiously every 50-60k.  I did a ton of ball joints and tie rod ends - but that was also due to running oversized tires. There's solutions out there. 

Water pumps are a common failure point as well. Especially the LLY as offhand I want to say they had a plastic impeller.  Again, aftermarket has solutions for that as well. Sinister diesel among others makes nice aftermarket pumps. 

Watch for injector harness issues. They like to rub through with the factory harness. Can't rememeber whom, but pretty sure a few places have harness routing solutions and harness wraps / casings to solve the issue.  Fuel pressure relief valves also like to go, you'll notice fuel pressure starts bleeding off. Again, tons of options for aftermarket replacement / upgrades. 

And on and on... theyre solid trucks. I'd buy another without hesitation. It was a great truck. Had it about 15 years before a tornado wiped it out. 

Here's what you need to know

LB7 - up to early 2004.  Fixed nozzle turbo.  These are the ones that GM did an injector recall, so they've all been replaced by now, either because GM did it, or customers did it out of warranty.  Otherwise darn near bulletproof.  These are often sought for performance builds.  Dad has one that is a DD with 30-over injectors, intake, exhaust, and 5-position chip.  It puts 938 lb-ft to the wheels and gets 22mpg on the highway empty.  It's wicked fun.

LLY - mid 04 - 06.  Moved the injectors to outside the valve covers (they learned from the LB7) and added a Garret VVT.  Bulletproof.

LLY/LBZ - 06 - 07.  Identical in mechanical construction, but the LLY got an upgraded tune.  Bulleproof.

LMM - 08 - 11.  Many upgrades, including the 6 speed transmission which (having owned both a 5 and 6 speed) is super nice.  Big bummer is the addition of a DPF.  Do not recommend.  Dad has an 08 LMM and it's a nightmare.  DPFs stay healthy only if you use a diesel as it's intended... 500 miles a day on the highway.  Not really, but short trips don't generate the heat necessary to complete the burnoff cycle.  

LGL and LML beyond 2011 I have little experience with, but they not only had DPF, they also had DEF.

The pre-08 versions are just bulletproof, easy to mod, and I wouldn't hesitate to have any of them.  I have a slight preference for the LLY/LBZ since you get the VVT.  If I were in the market for another one I would look for the cleanest LLY or LBZ you can find and open the big wallet.  Early ones can be a direct replacement for a gas vehicle.  Easy starting (it was 6 degrees here this morning), lovely to drive, sips fuel (relatively), and can be a super-versatile, do-it-all vehicle.  Later ones after 07 should be reserved for working trucks.

I would hard pass on the CP4 pump in the '11-'16 trucks. My cousin's '14 ate the pump. The total cost of repair was right at $12k. These pumps are lubed by the fuel. When they eat themselves they spread metal shavings throughout the entire fuel system. You have to replace pretty much the entire fuel system for the repair to be warrantied, including the fuel tank, injectors, lift pump, filters, and lines. 

The pumps are super sensitive to heat and trash. The only good fix seems to be never getting below 1/4 tank and running something like the FAAS Fuel System so you never lose a lift pump. These pumps are a piss poor design. There is nothing that keeps the roller in line with the lobe. They are known for spinning in the bore and destroying the pump.

That leaves you with a fuel system full of this. 

I had an LB7 and an LBZ.  In the 50K miles that I owned it the LB7 needed injectors, glow plugs (twice), a FICM, and a water pump, as well as the usual GMT800 front end stuff.  The LBZ was better, but I only owned it 30K and it was throwing codes about the VNT turbo when I sold it.  Might have just needed cleaning (I had all the emissions stuff on there, including the EGR), or it might have needed a new turbo.  It also needed a bunch of front end bits.  I didn't put a lot of miles on them (this was over about 12 years), but I don't daily my trucks so the majority of those miles were towing.

The most frustrating part about those trucks to me was the steering box.  They wear out after 80-100K miles, but there are no new ones, all the replacements are reman.  The remanufacturing process kinda sucks, often doesn't really fix them, so the pipeline is full of junk.  The LBZ had 3 or 4 replacements  before I got one that was usable.

Awesome guys, ill pass the info on!