Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com
Duramax Water Pump Replacement
We're here to save you money and provide the information you need to do the best job possible.
By Jim Bigley
Part I

Own one of these trucks long enough and you'll eventually be faced with the prospect of replacing the original water pump. Normal cooling system pressure is in the range of 8-10 psi. The water pump's coolant-side seal is there to contain this pressure against a spinning shaft. Over time, this seal will wear until it begins to leak coolant. That's your cue for a need to install a new pump. What we'll do in this two part series is explain how to correctly identify a leaking water pump seal, give you a list of part numbers you'll need for the new parts, identify the special tools you'll need, and show you how to do the work using large clear photos and easy to follow instructions. In short, this professionally written water pump replacement guide should answer all of your questions.

What questions, you ask? The water pump on my 2001 GMC LB7 Duramax had been leaking for some time - we'll discuss the exact symptoms in a moment. I knew the pump had to be replaced, so I began accumulating the information I needed to do the job correctly. I wanted to know what parts were preferred and which ones should be avoided. I wanted to know what special tools were actually necessary, and I wanted to know how long the job took. And finally, to adequately prepare for the task at hand, I wanted to know what engine/truck components needed to be removed to access the water pump. What I discovered at the end of this process was that as big as the Internet is, TheDieselPage.com remains the best information resource by a wide margin.

Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.comDuramax Water Pump Replacement - Copyright 2016 TheDieselPage.com

As the lead-in photo and the above photos illustrate, the water pump actually consists of two primary components - the cartridge that includes the impeller and drive gear, and the outer housing that routes coolant both coming and going. The impeller cartridge and the water pump housing are available separately, but most of you shouldn't need to replace the housing every time the water pump needs to be replaced.

Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com

The housings can be affected by a process similar to cavitation erosion. By definition, Cavitation Erosion is the process of surface deterioration and material loss due to the generation of vapor or gas pockets inside the flow of a liquid. The above KennedyDiesel.com photo shows an extreme example of erosion. If your engine's water pump housing looks like this one, I'd replace it as part of a water pump replacement project.

Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com

This photo shows what the erosion looked like on my engine's water pump housing (127,000 miles on the odometer). I didn't consider this enough of a problem to justify replacement. If you're evaluating the erosion on your water pump housing, I'd not replace it unless you can answer "yes" to the following questions: 1- Is the housing in danger of being perforated, which would result a major coolant leak? 2- Would you replace the water pump housing if you hadn't already been involved in a water pump replacement project? If you can answer "no" to these questions, then replacing the housing becomes optional. The ACDelco 97228188 water pump housing is priced at just over $80 at www.amazon.com.

A word about Dex-Cool coolant... The above photo shows the inside of the water pump housing removed from my truck, and you can see a small amount of Dex-Cool that had pooled in a low spot. This water pump housing and Dex-Cool coolant (orange liquid visible in this photo) were 16 years old at the time this photo was taken. The cooling system in this truck has never been serviced - never. From a corrosion standpoint, all wetted surfaces involved with the water pump are as pristine as when they were first assembled more than 16 years ago. This means that Dex-Cool is vastly superior to any other coolant used by GM. While this example proves what is possible, I'm not suggesting that anyone should neglect servicing the cooling system in their Duramax powered truck. Running a cooling system low on coolant, for example, could potentially explain the extreme erosion shown earlier or otherwise allow for more corrosion to develop inside the engine.

Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com

After reading in our forums that the gears had worked loose from a couple of recently installed ACDelco #252-838 water pumps, I followed the advice offered by John Kennedy, and had the gear TIG welded to the shaft. Great care must be taken during the welding process because of the somewhat vulnerable oil seal and coolant seal. These seals can safely handle up to 280° degrees or a little more, but care must be used during the welding process. The local welder I employed was told about the seals and the need to go slowly to reduce the amount of heat put into the shaft. The welder set the impeller side of the water pump in a shallow pan of water and used a wet-saturated rag on the gear to keep heat soaking to a minimum. The welding process itself was intentionally slowed (weld a little - let it cool, weld a little - let it cool), and took approximately 30 minutes to complete. Also, the inherent nature of TIG welding helps to reduce the amount of heat being driven into the part being welded better than other types of welding (i.e. arc/MIG/etc.). This cost me $32 at a local welding shop (30 minutes shop time).

Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com

I wrote a news story about DMAX in mid 2010 that celebrated the cumulative production of 1.2 million engines over a ten year time frame. Not until 2012 did I hear of a single instance of a water pump gear coming loose from the shaft, which occurred very soon after one of our members had installed a brand new ACDelco replacement. What changed, why now, you might ask? Millions of production water pumps and not a single report of a spun gear, then... The country of origin stamped on the gear shown here might offer some insight. The new ACDelco water pump is a marvel of modern manufacturing - and is beautifully machined. However, there is no evidence of heat treatment or of heat being used to properly fit the gear to the shaft.

These gears are not threaded onto the shafts. Water pump manufacturers have always used the shrink/interference press-fit method to produce a secure bond between the gear/impeller and shaft. Heat is used to expand the hole diameter in the gear while simultaneously cold shrinking the shaft just before pressing the gear onto the shaft. Once the material temperatures normalize, the interference fit is so tight that it will not slip. That being said, I can find no evidence that heat was used on the current (purchased 9/2016) ACDelco gear shown above to facilitate a shrink/interference fit or that the gear teeth were heat-treated.

Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com

This photo shows the original 2001 model year water pump that was removed from my truck in October 2016. You can clearly see evidence of heat being used on the gear hub and heat-treatment having been applied to the gear teeth. In addition, the photo at the top of the page is a photo I took in March of 2001 at the DMAX engine plant in Moraine, Ohio. You can see the same sort of heat treatment was used on that 2001 original equipment water pump as well.

Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com

What are the symptoms of a leaking water pump seal? The above photo shows an LB7 Duramax engine that had been removed from a truck for a custom power build (not mine). I happened to notice that this water pump had been leaking at the weep hole - apparently for quite a while. A weep hole is used by these water pumps, so they can leak coolant to outside the engine if the seal begins to fail - you don't want it leaking to the inside of the engine. This lower front corner of the engine can be seen by looking through the gap between the front driver's side inner wheel well and the frame - looking over/around the driver's side front-top shock absorber mount.

My truck's water pump seal first began leaking two winters ago. I first noticed a leak after starting the truck one cold winter morning. A thin 3-foot line of orange coolant was left behind in the snow when I backed the truck up in my driveway. Once a loose hose clamp had been eliminated as a cause, I theorized that the near zero F overnight temperatures had shrunk the worn seal just enough to allow a leak. Using the engine's block heater a couple hours before starting the engine the next morning seemed to prevent a repeat of the leak. As long as the overnight temperatures were above freezing, I couldn't detect a leak. Over the next 12 months, I needed to add just a quart of coolant, so the leak wasn't that big a deal, even though it slowly became worse over time. After approximately 15,000 miles since the leak first appeared, I decided to replace the water pump.

Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com

This photo shows a buildup of residue at the weep hole, indicating that coolant had been leaking out the weep hole for some time (pointed to here). Any decent mechanic will verify a leak at this site before beginning the process of replacing the water pump. Notice how pristine the cast-iron impeller appears here. This is yet more evidence of just how superior Dex-Cool is compared to any other coolant.

A leaking water pump seal can be the result of a bad head gasket (or LB7 injector cup seal leak), should compression pressure find its way into the cooling system. As mentioned earlier, a typical cooling system operates at about 8-10 psi when the engine is at operating temperature. The coolant surge tank cap is designed to vent pressure in excess of 15-psi. Because cooling system pressure is constantly pressing against the water pump seal, more pressure will produce more wear on the seal - shortening its life. Generally, if there is a compression leak into the cooling system, there will be visible evidence of that in the plastic coolant surge tank in the form of soot. An overnight shutdown in cool weather should result in the upper radiator hose having little to no residual pressure. If the root cause of a water pump seal leak is due to overpressure, simply replacing the water pump will just buy you a little time. I recommend having your truck's cooling system pressure tested before replacing the water pump, if there's any question.

In Part II, we'll take you through the process of replacing a water pump using the best parts, tools, and methods. Our goal here is to help you produce a long-lasting water pump replacement, while saving you time, money and effort. Come back next month, and there'll be a subscriber link available to part II right here.

The Diesel Page, November 2016


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Duramax Water Pump Replacement - Copyright 2016 TheDieselPage.com