In reply to TIGMOTORSPORTS :
Duramax-powered Express vans never came with Allison transmissions. Up until 2009 they had the 4L80E and after that you could get the 6L90 transmission. They're detuned to 260 hp and 525 lb. feet torque due to this. The Allison transmission is too big to fit. I work as a fleet mechanic for an ambulance company in a large urban area and I'm surprised the abuse that the Express van based rigs can take. We got 56 2011 Express duramax ambulances and 1 2015 Savana duramax. Minus 4 units that have 100-150k miles, the rest of the fleet has 300k and hours inbetween 28000 to 31500.
Some of the big problems we see:
Bosch CP4 injection pump fails and puts metal through the fuel system, requiring high pressure fuel system replacement and cleaning out the fuel tank/fuel lines. Requires tearing the engine down to the long block basically to service. 2010 was the 1st year for the CP4 pump I believe. Every automaker using this pump has this problem, not just a GM thing.
Head gaskets failing, pushing coolant out of the surge tank. We now replace head gaskets when replacing a fuel system if the engine has never been opened up before.
The band clamp holding the turbocharger together breaks and you can't build boost.
2010-2012 model year GMs have a reductant system that is touchy to say the least. Sometimes our rigs will flash a "EXHAUST FLUID LOW/EMPTY" and sometimes with "SPEED LIMITED TO 55 (or) 4 MPH" message and scare the crews, to only find there is nothing wrong with the unit. It got bad enough that GM came out with a recall to disable the speed limiter when there is a failure in the def system. Def freezes in cold temperatures, and frozen tanks can lead to a multitude of problems. We're in an area that doesn't get that cold so we don't see that often.
Plugged diesel particulate filters, mainly due to idling 90% of the time. People ignoring the "CLEANING EXHAUST FILTER" message and driving the rig until it goes into reduced engine power mode, requiring a service regeneration to get it back running again. This is mainly a personnel problem.
Engines snap crankshafts in half, normally at the #3 main bearing. We've replaced 8 engines out of the whole fleet.
Transmissions go 75-120k miles while being treated like rental cars. Most common failure is losing 4th-5th-6th gears. We just replace when there's a failure. Our rigs have a 12,300 lb curb weight and I believe max GVW is 18,000 lbs.
We've got a few new rigs, they're all Fords with V10 gas engines. We've done nothing to them but change oil and replace tires. They don't have enough miles on them to have any major problems so far.