Let's Get Technical: Cooling Water Pumps

By Tom Dobush                                                              July 1st, 2014                            


Well summer is here and in full swing, now is a good time to go over keeping your car cool. AC is usually on the list of service items this time of year, and that is all fine and dandy for you, but what about your engine’s cooling system? It is vital that the cooling system be checked and serviced regularly to ensure optimal and reliable performance.

Regardless of the age and level of complexity of your vehicle, a properly functioning cooling system is relatively simple to maintain. The concept is that internal combustion engines produce a lot of heat and therefore need to be cooled by air and water or another type of coolant. Coolant is pumped through channels inside of the engine and then through a series of pipes and hoses to a radiator. The radiator is a metal box designed with lots of surface area to remove the heat energy from the coolant as it flows through. It is this flow that is most important, if the coolant does not circulate properly through the engine, hot spots can develop and systems can fail commonly known as “overheating”.

A basic diagram of a generic cooling system layout.

Coolant is usually a water based liquid that also contains chemicals to provide lubrication and keep it from freezing in the winter months, which is where the term antifreeze comes from. To keep the system flowing properly, the vehicle is usually equipped with a thermostat and water pump in some shape or form. Everything used to be very mechanical up until the last ten years or so, where now we see many of these devices being controlled and monitored electronically by the cars on board computer system. Before, as on older vehicles, one can sometimes hear a faulty water pump, as the bearing that keeps it spinning tends to make noise when worn out, kind of a low hum. With the newer computer controlled devices they tend to not give much warning and simply stop working. This usually results in some manner of warning light on the dash. In order to avoid a warning light, especially where it may already be too late to prevent overheating damage by the time the light comes on, regular maintenance intervals are recommended. These intervals are typically something like 30-50k miles depending on your model.

A well seasoned water pump on the bench after removal from a Mercedes C230. Proof that even metal impellers can fail.

This maintenance involves replacing the water pump and thermostat devices along with any seals or hoses that may also be required and then flushing the cooling system with fresh coolant. It is during this maintenance service that sometimes an option can be considered. All water pumps are not created equal. Some have plastic impellers while others have metal. I was recently asked to discuss the idea and benefits associated with each. Typically, these days water pumps come with plastic impellers to fit design parameters and keep costs down, however we often recommend upgrading to a new water pump with a metal impeller for most applications when available.

Upon closer inspection, it is clear that the impeller has completely separated from the shaft. Not good for coolant circulation. This car did over heat and was well over the recommended maintenance interval. Unfortunately, she was so far gone, the car could not be saved.

Plastic tends to do the job fine, and is usually cheaper at first but after several years of use, and thousands of hot and cold cycles, they run the risk of becoming brittle and slowly disintegrating. The little plastic pieces of the impeller can then be spread throughout the cooling system and clog up vital cooling passageways, potentially causing big problems. So going for the cheapest solution can have very expensive consequences. With that said however, some designs require plastic, as metal could do damage as well. In some engines the water pump is located very close to the engine block and if a metal impeller was to fail and score the block from rubbing against it, major damage can occur.

Two BMW style water pumps. On the left is the new part, with metal impeller, on the right is a used, worn out water pump with a plastic impeller, removed before excessive deterioration.

There are obvious risks to each side of the argument, my point of view being that the water pump should be replaced as a maintenance item, however I feel plastic debris in the cooling system comes with potentially higher risk as a "silent killer" not being easily detectable, at least potentially not until damage of hot spots and/or oil/coolant “sludging” has already been done. With the metal impeller design you do run a slight risk of possibly gouging the case, however there are auditory warnings from the bearing in most cases well before enough "wobble" causes damage.

Regardless of the material used for the impeller, it should be replaced at 30-50k intervals depending on usage and risk avoidance preferences. It comes down to the question of which is more damaging, carries a higher risk, and is more likely to occur? From my experience with these types of water cooled engines, we know that plastic impellers will deteriorate, sometimes faster than anticipated, and that metal impellers do not nearly at the same frequency. If the water pump is going to be replaced at 30-50k mile intervals anyways, why not choose the impeller that is not going to deteriorate, and that will show signs (noise) of bearing wear before any issue occurs?

Of course, neither is a perfect answer and there are risks associated with each side of the argument. Ultimately, it comes down to personal preferences in the matter. My philosophy has always been the more often a specialist can inspect/maintain a vehicle, the greater likelihood they will find an issue before it becomes a damaging problem. This is why we encourage our clients to bring their cars in for regular check ups and not only when they are fearful of an issue that may be occurring.

Think of it like insurance and protecting your investment, much like regular doctor or dentist visits. We would rather see a client and their car once a month and only have preventative maintenance to discuss rather than less often and have only bad news and expensive repairs to talk about.

What would you like to read about in my next article? I greatly appreciate your questions, input and feedback. I can be easily reached at BavarianRocketScience@gmail.com. Thanks for reading and happy motoring!