Block Damage at Water Pump is a Non-Issue

We have had some pretty irate shop owners contact us regarding damage present on the engine block, on a fresh GoWesty engine they received. The complaints range from being accused of cutting corners to going as far as blaming the damage on mysterious overheating issues. This article is intended to put this issue to rest, once and for all.

The typical damage looks like this:

This damage is caused when water pump fails, and the backside of the steel impeller is driving into the block by the force on the pully via the v-belt. This is very common and, in fact, we do not deduct any amount from the engine core deposit for it. Any engine core turned in with similar damage in this area is deemed as 100% okay, and the engine block is used to build another, fresh GoWesty engine. This assumes, of course, the rest of the engine block is okay and/or fixable.

The first thing to understand is that the water pump on an engine is not a positive-displacement pump. It is not designed to produce high pressure, but rather to circulate high volume at low pressure in a closed system. The whole system is under pressure, controlled by the coolant cap. However, that pressure is due to the increasing temperature of the coolant—it has nothing to do with the water pump.

The water pump design on a waterboxer (1.9 and 2.1) moves coolant from ONE intake port (“low side”), where the entire impeller system is located. Here is a 2.1 style pump for reference:

This is the intake port, inside of which you can see the business-side of the impeller:

Then there are two main output (“high side”) paths on all pumps (plus a small, 3rd path on the 2.1 pump that flows to the oil cooler, not present on a 1.9 engine):

1) Straight into the left side of the case where it is bolted, around which is a surface that is commonly damaged from a previous failed pump, and

2) Via a coolant pipe to the right side of the case to cool the other two cylinders.

The coolant then flows past all four cylinders, up and through the numerous coolant passageways in each of the heads, comes back out of each head via two separate fittings (one on either head), then either comes back around to the pump (thermostat closed) or is forced up to the radiator (thermostat open).

Even on undamaged blocks, there is always a gap between the backside of impeller and the block. This part of the impeller is not responsible for any fluid flow at all. The coolant is being pumped into a common cavity located in the pump housing itself, as seen here:

The gap between the flat side of the impeller in this photograph and the surface of the block (damaged or not) simply adds a relatively small volume to this common cavity, or the “high side” of the water pump. It does not affect the flow of coolant at all. Contrary to popular belief, damage to the block in this area does not create “a leak between the high side and low side of the pump impeller.” This is because of the unique design of the water pump on a water boxer.

The common cavity created by the volume of the cavity in the water pump and the space between the flat side of the impeller and engine block (damaged or not), all together, is the “high side.” The “low-to-high” section of the pump is located between the inlet port and the common cavity. The coolant simply flows out of this cavity in multiple directions.

There is no way around the physics that govern the fluid dynamics of this system and the conclusion that damage created by backside of the impeller of a previously failed water pump on the engine block in this area is a non-issue. Not only that, there is the empirical evidence generated by the thousands of engines we have built with damage in this area.

Regarding the criticism that it should be fixed anyway because it simply does not look right, we have good reason to leave it alone. The only way to “fix” it is to weld it all up, then grind it all flat again. While we in fact do save money by not doing this, the reasons for not doing it is not simply monetary. In fact, the heat required during welding can end up warping the case, creating even more work, driving the prices of our engines even higher, and for no good reason in the first place.

And, what about these mysterious overheating issues blamed on this damage? The common complaint is that this only happens at idle, providing some proof to the concept that the damage creates “a leak between the high side and low side of the pump impeller,” and therefore only shows up at idle (low water pump speed). However, for the reasons stated above, no such leak is created by this damage, at slow or high water pump speeds. Overheating issues at idle that coincided with a damaged block are just that: coincidental. The overheating was happening due to some other issue. If only at idle that is usually an issue with the electric cooling fan. But, it could be something else, as well—like a stuck thermostat or some sort of blockage on the inlet side of the pump, not any issue with the outlet, or any other number of cooling system related issues.

One thing is for sure: it has nothing to do with the damage to the block!

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