2007.5-2010 GMC Duramax LMM Cooling Parts

What GM built into the LMM Sierra HD

The LMM arrived with the new Sierra HD body in mid-2007 and brought the first diesel particulate filter to the Duramax, so GM gave it more cooling capacity than the LBZ along with cooled EGR. The driver-side radiator tank holds the heavy-duty air-to-oil engine oil cooler, and every HD came with an auxiliary external transmission cooler. Behind the grille sit the intercooler, A/C condenser and radiator, and a mechanical fan clutch does the pulling. WT, SLE and SLT trucks share that hardware; the factory cold-weather extras were the block heater and, on 3500HD diesels, grille and bumper-cover openings.

Where these trucks lose coolant and heat

Start at the water pump. As it ages and heat-cycles it seeps at the weep hole, leaving crusty residue, coolant that disappears slowly and a needle that sits higher than it used to. The fan clutch is the next suspect, and owners report it engaging later than they expect because it reacts to the temperature of air coming through the stack instead of coolant temperature. Pack that stack with road debris between the intercooler and radiator and you blind it, which is how a Sierra passes a normal week and then overheats with a trailer. Coolant that vanishes with no leak on the ground points at an EGR cooler leaking internally, with white smoke and a rough cold start usually along for the ride, or at a head gasket that gave up after an earlier overheat, which pressurizes the system and forces coolant out.

OE-grade service parts vs. upgrades

Fan clutch, thermostats and water pump get replaced together on most towing overheats, and the coolant goes in fresh at the same time. Use GM-spec coolant, and pressure test cold and hot if the loss has no obvious source. Gaskets, O-rings and seals cost little next to a second teardown, so order the sealing parts with the hardware. Where Sierra owners upgrade, it is for durability under heat rather than horsepower: silicone hose kits in place of aging rubber, billet tank caps, stainless steel coolant tubes and metal hose connectors. None of that substitutes for clean airflow through the cooling stack.

What we carry

This collection is the service-and-upgrade mix for the whole coolant loop. Dayco covers the molded heater hose and the curved upper, lower and heater-outlet radiator hoses. Dorman brings upper thermostat housings, heater hose assemblies, metal heater hose connectors and inlet hose tee connectors. Merchant Automotive is deep on the parts that seal: water pump seals and gasket kits, thermostat housing gaskets, lower radiator coolant pipe O-rings and rear cover coolant seals, along with engine coolant thermostat kits, coolant tank float switches and complete water pumps with housings. PPE lists silicone coolant hoses, stainless steel heater hose connectors, T304 coolant tubes, drain tubes, engine coolant return pipes and radiator hose kits. XDP adds Xtra Cool water pumps and housings, radiator coolant drain pipes and engine coolant temperature sensors with pigtail. Wehrli Custom Fabrication has billet coolant tank caps and low coolant level sensors, Fleece Performance Engineering and PPE both list billet coolant block-off plates, and Mishimoto covers silicone radiator hose kits with high- and low-temperature thermostat sets. ACDelco, Mahle and CSF parts for the 6.6L Duramax are here as well.

Fitment and ordering notes

Two dates matter. A 2007 Sierra HD can be a Classic with the LBZ in the old body or a new-body truck with the LMM, so verify the engine RPO rather than the model year. And 2011 brought the LML in a revised chassis, which means a part listed for 2011 and newer is not a safe guess on a 2010. Inside the range, catalogs often list one water pump or thermostat for 2006-2010 because the LBZ and LMM share them, and that is expected. Hoses, tubes and sensors are tighter. Dually, chassis cab and box-delete trucks can differ from the pickups. Check the VIN with the fitment checker on the product page.

Frequently asked questions

Which cooling parts should I replace as a set?

On a Sierra that overheats towing, the fan clutch, thermostats and water pump are the trio that get done together, with fresh GM-spec coolant. Add the gaskets, seals and O-rings for whatever you open, because most of the cost is access. If the loss has no visible source, pressure test cold and hot before you spend anything.

How do I tell an LMM Sierra from a 2007 Classic?

The LMM only came in the new HD body that arrived mid-year in 2007, while the Classic is the previous body sold alongside it with the LBZ. Body style is the quick tell and the engine RPO is the certain one. It matters less for pumps and thermostats, which are usually shared across 2006-2010, and more for hoses and sensors.

Air comes out when I crack the surge tank cap after shutdown. Is the cap bad?

Probably not. Air released at the surge tank cap after shutdown is a head gasket red flag on these engines rather than a tank or cap fault, and a system that stays pressurized and keeps pushing coolant out says the same thing. Overheating is the usual cause, so have it tested before you chase parts.

Not sure what fits? Call the shop at the number in the header, or use the fitment checker on any product page. We install everything we sell.