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Half-Dayin’ It: Tackling the Dreaded Water Pump Replacement on an LBZ Duramax…In Four Hours

Posted in How To: Engine on April 11, 2018
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Thanks to its reliability, fuel efficiency, and performance potential, GM’s 6.6L Duramax diesel enjoys a very loyal following in the heavy-duty pickup segment. One of the most sought after engines of the lot is the LBZ version. Available from ’06-’07, the LBZ represents a period in Duramax lineage where the LB7’s (’01-’04) injector problems had been addressed, the LLY’s (’04.5-’05) overheating issues were solved, and the most stringent emissions controls had yet to be implemented (LMM, ’07.5-’10). But even though the LBZ’s rock-solid reputation precedes it, minor repairs are inevitable from time to time. After the ’06 Chevy Silverado 2500 HD in this story displayed a low-coolant message on the dash, we detected a Dex-Cool leak at the front of the engine, and it was time for a new water pump.

Due to the water pump’s location on the Duramax, it’s often a dreaded process to replace it. Some horror stories even claim the job can take as long as 8-12 hours to complete. Hell-bent on disproving this myth, we set out to perform the job on the aforementioned ’06 model year, 230,000-mile LBZ. What we found was that—even though the clutch fan and cage can be a pain to remove, the harmonic balancer has to be pulled, and the coolant bypass tube can be finicky to reinstall—it’s not as bad as you think. With the right tools, replacement parts, and mechanical skill, the job can be pulled off in four hours’ time. Follow along as we highlight the ins and outs of the job, as well as offer a few time-saving tips to help you tackle it in your own driveway.

Water pump replacement on a Duramax has a reputation for being a highly involved and extremely time consuming job. Make no mistake though, with the right tools and parts this job can be knocked out in half a workday—two hours out, two hours in.
With no drain plug present in the factory radiator used on the Duramax, the passenger-side wheelwell liner can be pulled to access the lower radiator hose. From there, the coolant can be drained and the lower radiator hose (which can become brittle with age) and its O-ring can be thoroughly inspected. For the wheelwell liner removal, make sure you take your time removing the liner clips; pick up a tack remover–style screwdriver to make removal of the clips easier, or have a handful of additional clips on hand before you get started.
Once the lower radiator hose is separated from the radiator via its quick connect, things can get messy in a hurry. We improvised by squeezing a piece of cardboard between the radiator and framerail to divert coolant into a clean, 5-gallon bucket. Not a single drop of Dex-Cool hit the ground.
Next up, we pulled the serpentine belt, unbolted and temporarily relocated the transmission control module (TCM), and set to work removing the fan clutch. The LBZ utilizes a two-piece fan shroud: an engine-mounted cage for the fan and a shroud positioned around the radiator. Once the cage is out of the way, the fan clutch can be loosened and removed, followed by the fan hub, and then the radiator shroud. Last but not least, the camshaft position sensor has to be unplugged.
In order to access all the bolts that secure the water pump, the engine’s harmonic balancer must first be removed. This is accomplished by holding the crankshaft in place with a flywheel lock, installed through the inspection plate in the Allison transmission’s bellhousing. Luckily for us, a Lisle flywheel lock was included with the water pump kit we purchased from Merchant Automotive.
Breaking the harmonic balancer’s mounting bolt loose calls for a 12-point, 36mm socket; a 3/4-inch drive; and a breaker bar. It’s worth mentioning that you can buy a complete Lisle tool kit (PN 22100), which comes with two flywheel locks and a 36mm socket, for roughly $30.
With the harmonic balancer out of the way, all water pump mounting bolts are now accessible and can easily be removed. The two rear 12mm water pump nuts are best accessed from underneath the truck. Notice the cardboard across the radiator—this is done to rule out the possibility of damaging any fins during the job.
Believe it or not, the process of separating the water pump from the engine’s front cover should be delicately performed. This is because you don’t want to upset the O-ring seal that exists where the coolant-to-oil cooler tubes meet. With 230,000 miles on that particular O-ring (and us not having a replacement on hand), we definitely didn’t want it to spring a leak.
While a worn-out pump seal had been the culprit in our particular water pump failure, cavitation is a common occurrence. As you can see here, cavitation was beginning to take place in the OEM unit. Eventually, the superheated steam present in the cavitation process would’ve broken down the internal surface of the pump even more, along with causing irreparable damage to the pump’s impeller.
One major advantage the Merchant Automotive water pump has over the OEM unit is its impeller. For utmost reliability, the Merchant pump utilizes a cast-iron impeller (versus the plastic impeller employed on the original). While cast-iron impeller water pumps can be found in early Duramax mills, GM phased the plastic impeller units into its engines in 2006 with LBZs, and even some late-production LLYs received them.
Merchant Automotive’s water pump kit retails for $289.95 (at time of publication) and includes a new cover, the aforementioned flywheel lock, and all necessary gaskets and O-rings required to tackle the job. In our particular pump’s case, its impeller and drive gear had both been TIG-welded to the shaft. “Welding the shaft” is an upgrade that provides insurance for engines that see 4,000 rpm or more. The factory water pump is notorious for its drive gear slipping and the impeller being able to walk around on the shaft in performance applications. This eliminates that possibility.
After the supplied seal that sits between the back of the water pump and the engine’s front cover was installed on the new water pump, the outlet gasket was placed over its respective mounting studs. To keep the outlet gasket in place during the installation (and also to guarantee a good seal), a fair amount of Ultra Grey RTV was used.
Reinstalling the coolant bypass tube can be a tedious task. Once the tube has been fitted with fresh O-rings (which are supplied in Merchant Automotive’s water pump kit), it pays to coat them with engine oil. The engine oil provides enough lubrication for you to work the top of the tube into the thermostat housing bore. Once the bypass tube starts into the bore, apply upward pressure and rotate the tube counterclockwise until the mounting flange on the bottom of the bypass tube lines up with the water pump cover’s provision to accept it.
With the lower coolant bypass tube O-ring positioned in the machined groove of the water pump cover, added sealing insurance was once again employed in the form of a layer of Ultra Grey RTV. Then the two bolts that anchor the bypass tube flange to the water pump cover were torqued to 18 lb-ft.
Next, we set the harmonic balancer back into place on the crankshaft snout and torqued its mounting bolt to 74 lb-ft. After that, the bolt and its washer were marked and the bolt was tightened another 105 degrees. Also notice the points of reference we made prior to removing the balancer. This was done to safeguard against installing the balancer backward, which believe it or not would be easy to do.
With our main objective accomplished, the race was on to reinstall everything removed during teardown. The fan hub, clutch, and cage were installed, along with the camshaft position sensor, radiator shroud, and TCM. Removal of the flywheel lock and the replacement of the inspection cover in the transmission’s bellhousing followed. Then, finally, the lower radiator hose was reattached to the radiator, the passenger-side wheelwell liner was reinstalled, and the cooling system was refilled.

Quick Tips & Time-Saving Measures

1. You don’t have to pull the starter like the GM book says.
2. Contrary to what the GM book states, you don’t have to remove the thermostat crossover.
3. Mark the harmonic balancer before you remove it. (This way you won’t install it backward when it’s time to put it back on the crankshaft.)
4. Carefully break the seal between the pump and the engine’s front cover when removing the original water pump. There is an O-ring present in the nearby coolant-to-oil cooler tube that you don’t want to disturb.
5. Water pump replacement is an opportune time to install new thermostats.
6. Inspect and replace any questionable radiator hoses (or O-rings).


Merchant Automotive
Zeeland, MI 49464
Lisle Corporation

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