High RPM Insurance – Dmax Welded Water Pump

Merchant Automotive’s TIG-Welded Water Pump
With a 19-year track record under its belt, it’s safe to say the Duramax is one of the more reliable diesels to ever grace the ¾-ton and larger truck segment. Sure the LB7 had injector problems and the LLY’s ran hot, but by and large the 6.6L V8 holds up remarkably well—regardless which generation engine it is. We’ve even seen well-maintained Duramax mills accumulate more than 700,000 miles without needing an overhaul or any other major repair. But while the Duramax is no stranger to racking up hundreds of thousands of miles, it’s not immune from requiring a minor repair from time-to-time.

Following the purchase of a high-mile ’06 Chevy Silverado 2500 HD, the truck’s new owner noticed the LBZ was low on coolant. The cause? The 13-year-old, 251,000-mile factory water pump was on its way out. After getting a quarter-million miles out of the original water pump, it was an easy decision to stick with an OEM unit. And thanks to Merchant Automotive’s water pump kit, not only was the owner able to start over with a genuine AC Delco unit, but the new pump was modified to hold up to elevated rpm and higher horsepower. If you plan to keep the water pump alive in your modified Duramax, look no further than Merchant Automotive’s TIG-welded unit.

With water pump failure striking anywhere between 130,000 and 250,000 miles on a Duramax, we’d say the factory unit on this ’06 LBZ had a pretty good run. When water pumps fail due to age or miles, it’s typically the fault of a worn pump seal, which in turn allows coolant to leak from the weep hole located on the top, gear side of the pump. Maintenance-savvy owners will typically spot a coolant leak before the “Low Coolant” message appears on the dash.
Jake Bosie of Flynn’s Shop in Alexander, Illinois would handle the 5-hour water pump job. He got started by pulling the passenger side inner fender well in order to access the bottom radiator hose.
Due to the Duramax radiator not incorporating a drain plug, draining the engine’s coolant can be a messy job. Using a piece of cardboard, wedged between the frame and radiator hose, Bosie was able to minimize the mess by deflecting coolant into a 5-gallon bucket.
Next, Bosie turned his attention under the hood, removing the transmission control module (TCM), the two-piece fan shroud, and the fan clutch. Then the serpentine belt was pulled and the camshaft position sensor was unplugged (shown).
From there, the fan hub was unbolted and the ½-inch rubber hose that supplies coolant from the bypass tube to the turbocharger center cartridge was removed (arrow). Brittle and deteriorating, Bosie would end up replacing the ½-inch rubber hose later on.
The bypass tube is the steel pipe section that links the water pump to the thermostats. Once the bypass tube was loose, Bosie carefully twisted it out of its respective mounting points. Also notice the piece of cardboard blocking the radiator, which Bosie put in place to protect the radiator while working in the tight quarters at the front of the engine.
To gain access to all water pump fasteners, the harmonic balancer has to be pulled off of the engine. Because Bosie has seen his fair share of novice mechanics install the balancer backward (a big no-no), he makes it a habit to add alignment marks to both the front cover on the block and the balancer, before removing the balancer.
Prior to breaking the harmonic balancer’s mounting bolt loose, the crankshaft must be locked in place. To accomplish this, a flywheel locking tool is used. A Lisle flywheel lock (22100) is included when you order Merchant Automotive’s water pump kit with cover (PN 10511). Using the flywheel lock entailed Bosie removing the inspection plate on the Allison transmission and setting it in place on the flex plate
Loosening the harmonic balancer bolt calls for a 36mm socket, ¾-inch drive, breaker bar, and a little muscle. Before applying any pressure to the breaker bar, Bosie re-checked to make certain the aforementioned flywheel lock was firmly in place on the flex plate.
With the balancer out of the way, all water pump nuts can be accessed. Also notice that the factory dowel on the crankshaft is exposed with the balancer removed. This is the 5mm dowel that’s known to sheer off in big horsepower, high rpm applications (when that happens, the crank and cam get out of time—causing all hell to break loose).
After pulling the bottom radiator hose, Bosie got to work removing the factory water pump using a ½-inch ratchet and 12mm socket with extension. When it came time to pry the water pump off the front cover, Bosie took special care to secure the coolant-to-oil cooler tube. Reason being, there is an O-ring where the tube meets the oil cooler that can leak if disturbed.
Once the factory water pump was available for dissection, Bosie separated the impeller cartridge from the housing. Within the housing, we could see that cavitation was beginning to occur. In addition to causing surface deterioration, cavitation can eventually contribute to impeller damage. Luckily, Merchant Automotive’s water pump kit with cover comes with a new housing, effectively ruling this problem out.
In this comparison you can see that the Merchant Automotive water pump (left) utilizes a cast-iron impeller vs. the plastic one found on the factory unit. Even though the plastic impeller pumps are known to be reasonably durable, it’s good reassurance to upgrade to a cast-iron one. GM used cast-iron impeller water pumps from ’01 to ’06, but somewhere in the early to mid 2006 model year switched over to the plastic units (note: some late model LLY engines were equipped with plastic impeller pumps).
Merchant Automotive also supplies the three seals and outlet gasket needed to install its water pump. Here, Bosie installs the supplied back of water pump to front cover seal.
For the ultimate seal, Bosie combined the supplied water pump outlet gasket with a coat of RTV silicone. In addition to ensuring a leak-free seal, the use of silicone helps hold the gasket in place while the pump is being installed.
For further insurance, all of Merchant Automotive’s water pumps come with both the driver gear and impeller TIG-welded to the shaft. This keeps the drive gear from slipping and the impeller from ever walking on the shaft, which oftentimes occurs when engines see 4,000-plus rpm. Also notice that the weld runs all the way around the diameter of the shaft.
All told, Merchant Automotive’s complete water pump kit with cover will set you back less than $300. If you ask us, this is a small price to pay for the added security Merchant builds into its water pumps. Every water pump it sells comes with its impeller and drive gear TIG-welded to the shaft, and the new water pump housing offers a fresh, cavitation-free start.
Next, Bosie installed the new water pump. On the Duramax, the water pump is secured to the front cover by three bolts (and the water pump also fastens to the pump housing via three bolts), all of which are torqued to 18 ft-lbs.
Because reinstalling the bypass tube can be a bit tricky, Bosie walked us through how he does it. First, the thermostat housing end of the tube is fitted with a new O-ring (supplied by Merchant) and then the O-ring receives a coating of fresh oil to help it slide into the bore in the bottom of the thermostat housing. After that, and with the provision for the turbocharger coolant facing the block and the outer diameter of the bypass tube outlet lined up straight with the thermostat housing bore, he walks the bypass tube up into the bore while twisting it counter clockwise.
With the top of the bypass tube fitted snugly inside the thermostat housing bore, Bosie installed the supplied O-ring into the machined groove in the top of the water pump housing. To make sure the O-ring was fully seated in the groove and in order to hold it in place (the O-ring is slightly larger than the groove), Bosie employed a thin coat of RTV silicone. From there, the bypass tube was rotated on top of the water pump housing and tightened down.
To make sure the top of the bypass tube is correctly installed in the thermostat housing bore (and that the new O-ring hasn’t been compromised), Bosie checked his work using a telescoping mirror. Note that you can also make sure the bypass tube feeds correctly into the bore by looking down through the top of the thermostat housing when installing it.
Because the reinstallation of the factory harmonic balancer calls for 74 ft-lb of torque plus a 105-degree turn, Bosie marked the head of the bolt to be as accurate as possible. Then with a helping hand keeping an eye on the head of the bolt, he completed its tightening sequence.
After the radiator hoses, fan clutch, fan shroud, serpentine belt and TCM were reinstalled, Bosie put the passenger side inner fender back in place and began topping off the engine with coolant. It’s important to make sure you run Dex-Cool coolant in a Duramax. Not only were these engines specifically designed by GM to run Dex-Cool, but Dex-Cool was part of the reason why this particular engine’s original water pump lasted 251,000 miles.
Once everything was buttoned up, we couldn’t help but wonder if the LBZ’s new and improved water pump would last as long as the original. While it might take a while to find the answer to that, there is no question that the Merchant Automotive piece will handle more horsepower and higher rpm, which the owner planned to add courtesy of EFI Live.


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