Why do my car water pumps keep dying?

Why do the water pumps on my car keep dying?

I rear-ended somebody about 8 months ago, crumpling the hood and damaging other things like the grill, the radiator, et c. No apparent problem with the water pump at the time, and no fluid leaks at all. They fixed the car; it looked great. A few weeks later the water pump started leaking coolant out its front seal - shop said the accident must have damaged it a little and it just showed up now. They replaced it. And since then the pump has gone bad and been replaced every 4 to 8 weeks. Also leaked out the lower radiator hose shortly after the accident, which the shop says is because they didn’t tighten it enough.

My car is a '97 Subaru Legacy. The front of the engine, with 2 serpentine drive belts and pulleys and brackets and all in front, showed no sign of damage, and the belt and pulleys all still look fine (none replaced). All the pumps fail by the front seal leaking; their bearings are all fine. I just noticed the radiator cap was cocked a bit because the radiator mounting bracket wasn’t straight and was pushing on it, which they fixed last week - could that have done it? Maybe some unusual system pressure resulted?

In my car the water pump is driven by the serpentine belt and, I think, has nothing else on its shaft - certainly not the crankshaft or anything. The shop is a big place with good local reputation and they have taken care of everything each time it’s died - they say they’re stumped as to why pumps keep dying, unless maybe we hit a batch of pumps with bad seals - last time they tried Subaru brand parts instead of another maker.

Any ideas out there???

Maybe something unseen kinked (radiator replaced?) when you hit. Just enough to slow the flow of coolant so as to put undue pressure on the pump.Did you post on newsgroups? Because there’s one for soob’s and they can be very helpful…

I have a theory. My brother had a VW Rabbit that had the same problem. Water pumps were going out after on a few weeks. He tried rebuilt, used, new, all the same. He changed one at my house and after he was finished changing the pump, he filled the cooling system back up and made sure it was full. I asked him if he was going to bleed the air out of the system. He did not know what I was talking about, I then showed him a bleed valve on the top of the thermostat housing. He had to add over half a gallon of water to get the cooling system full and the system had no air in it. He never changed another water pump. And his problem started shortly after he changed his antifreeze.

I later learned that air cavitation around the water pump impeller was causing my brothers problem. Normally the water/antifreeze mix will keep the impeller seals cool and prevent wear. Add air and the seal wears much quicker, they get hot and the rubber fails. Make sure whoever is doing the work makes sure there is no air in the cooling system. Also add some water pump lubricant, I have always used it and I have never changed 2 water pumps on the same car. Also make sure the radiator cap is the right pressure for your car. It may have been replaced with the new radiator. If your car has a 15 pound cap but should only have an 11 pound cap, the cooling system can develop too much pressure. This happens shortly after you shut off a car, the water is still absorbing heat from the engine but the water is not circulating in the cooling system.

You should also have your fan temp switch checked out. Many Subarus are wired so the cooling fan runs even after the engine is turned off. My daughter Subaru wagon did this. This would also cause the cooling system to overheat and develop too much pressure.

Well, I have 3 theories. I think one of them will be the cure to your problem.

I doubt it’s the radiator cap.
It could be a rash of faulty pumps from one supplier, such things do happen.
It could be related to excess air in the system, however I would expect some overheating in that case, and a good shop would not do this.
Another possibily to explore is electrical charge buildup in the coolant. Perhaps a grounding strap is now missing or disconnected. This can cause electrolysis that eats stuff.

The air caviatation is a possibility, but having been in an accident, I’d check the pulleys and the alignment. If one or more of the brackets got bent, or the water pump pulley itself is tweaked, it’ll stress the shaft as it turns. The seal goes first, but if you kept 'em there for a while, I’d wager they’d develop bearing problems shortly thereafter.

Alignment of pulleys in a serpentine system is fairly critical, as is proper tension, as is pulley run-out. In fact, if say the power steering pump pulley were bent, the varying belt tension could damage the water pump bearings.

I have a question. If it was leaking water out it’s front seal, why did you replace the pump? Sounds like a blowen gasket to me. Seems cheaper to replace an 89 cent gasket then the whole pump. Anyways where are you taking this to be fixed? A trusted mechanic or Joe Schmoe down the block? Perhaps you could try taking it to a dealer?

…the water pump started leaking coolant out its front seal…

If it was leaking water out it’s front seal, why did you replace the pump? Sounds like a blowen gasket to me. Seems cheaper to replace an 89 cent gasket then the whole pump.

The water pump’s front seal, or shaft seal, is an integral part of the pump. It is not serviceable in the field. If it’s leaking, the only repair option is to replace the pump.

The water pump gasket, which seals the pump to the engine block, is behind the pump, not it front. In this case, it is not the problem area.

Oooooohhhhhhh. Okay, that makes sense now. Got it!!

A wag here but maybe the belt (and corrosponding pully system) is out of alignment, meaning the belt is putting uneven pressure on the pully (i.e pulling it outwards, inwards or ‘twisting’ the pully)

whoops it looks like this was said already.

really long shot
if the radiator cap is not lifting at the design pressure then it could be excessive pressure blowing the seal
caps should be replaced regularly if in doubt but usually for loss of pressure leading to boiling.
?? a tricky one

You should take it to a Subaru dealer for a proper diag.

Many moons ago I had the opportunity to attend a training seminar sponsored by Stant, the worlds largest manufacturer of radiator caps. They stated that the incorrect radiator cap was the third leading cause of water pump failure behind overtightened fan belts and lack of cooling system maintainance. The radiator cap is designed to be the weakest link in the automotive cooling system, strengthen that link and the next weakest link, generally the front seal of the water pump, will be the first part to fail if the cooling system exceeds it’s recommend pressure.

For those involved in auto racing it is common knowledge that increasing the pressure in your cooling system will help engine run better. You can see this in the cars of today compare to the cars of 30 years ago. Most vehicles built in 1970 used 160 or 170 degree thermostats. But in later years, discovered through auto racing no doubt, that the closer to the boiling point of water your engine runs, the better it runs and you get better fuel economy. Most cars sold today contain 192 or 195 degree thermostats, some cars even have 205 degree thermostats. To run these higher temperatures, the cooling system pressure is also increased, the easiest way to to slap on a radiator cap with a higher pressure rating. My 66 GTO has a 9 pound radiator cap, that is what is recommended by Pontiac. I just replaced the radiator cap on my Toyota pickup and it is rated at 12 pounds. My Toyota also runs at a much higher cooling temp than my GTO.

When I built and drove race cars, I always use 18 pound racing radiator caps. And the first thing me and most everyone that competed would do at the end of a race is release the pressure of the cooling system. Why you may ask? Because if we didn’t, we would blow out the front seal in the water pump. In modern auto racing such as F1, CART, IRL and NASCAR, one of the first things done to the car at the end of the race is relieve the pressure in the cooling system. They do this because they know the damage that excessive pressure can cause after engine shut down.

As I stated in my first post in this thread, it is just a theory and one of the things that should be checked out.

Thanks racer72. I learned something new today.

That’s always good!


I thought of that too. However, upon thinking about it the bearing should fail before the seal.

<nitpick>Not necessarily. They are a mechanical seal, and while not easily replaceable, they can be. Usually though, the cost of a reman water pump is about the same (or less) as a seal and labor so its not really worthwhile.

Thanks for the info, racer72, I didn’t know all that.

My comment about the cap related to this: the radiator cap was cocked a bit. I didn’t think a slightly cocked cap would cause enough pressure increase to take out a pump seal.

Thanks all - some good ideas.

Bleeding air from the system - the shop has said they’re careful to do this thoroughly, and said that doing too quick a job of it could cause trouble - but that’s not what they’re doing.

Belt alignment - this was one of the first ideas. They said that the belts wear out quickly when their alignment is bad, and the belt here shows no extra wear at all. Also none of the pulleys or brackets look damaged. Finally, they say this hurts the bearings before it hurts the seal (I am hearing both versions of this point here and don’t have any independant evidence).

Radiator cap - they replaced the radiator and the new one came with its own new cap. They said they took it off and pressure tested it and both its operations (allowing overpressure to escape and allowing vacuum to pull fluid back fromt he reservoir) occurred at the right pressures. But they also put a new one from Subaru on since the last pump replacement (so no real evidence yet on whether this matters). I keep wondering if being at an angle could make it overpressurize the system, or else somehow make the system operate under partial vacuum to try to suck air in through the pump seal and run it dry. Somebody said it’s a mechanical seal, but I don’t think it is - I think it’s an elastomer rubbing against the shaft (shop said sometimes this rubber gets dry if the pump’s been in a warehouse too long, which is obviously not a mechanical seal).

Keep the ideas coming! Thanks!

You are infering too much. A mechanical seal generally has a nitrile element in it as well. And when it fails your pump will leak like a sonnabitch.


That is what an “average” waterpump seal looks like.

Seals don’t just “dry out” from getting old either… well, thats not entirely true. They have a shelf life of around 8 years and even then they are usually still OK. If you see one that is dry/britle, its usually because it either got too hot or was exposed to a material it wasn’t meant to.