Damage from water pump failure falls into two basic categories. Loss of engine power and integrity due to overheating and damage to the engine block or water pump housing due to the water pump impeller hitting it after the support bearing failed badly and got extremely loose. Internal engine damage can be variable with many scalars including blown head gasket, cracked head and/or block, burned valves and pistons, and melted internal plastic parts for the cooling and valve drive systems. Any severely overheated engine should be checked for the presence of hydrocarbons (HC) in the cooling system vapors. Any hydrocarbons indicate combustion gas leakage, and the engine must be repaired or replaced.
At some time around 2009, Karmakanix began to experience a high failure rate of the German aftermarket water pumps in both the Volkswagens and the Audis. Typically, the issue was leakage, but some made bad bearing noises, which was mostly a problem with 6 cylinder water pumps. About half of the failures occurred with less than 10,000 miles on the pumps, although some of these were over two years old. Of all the water pumps that we replaced between 2006 and 2009, well over 20% failed with less than 30,000 miles on them. The same 3 German brands that we had been using since the 1980’s with almost zilch problems all began to have an unreasonably high failure rate, likely resulting from poor seal material. We speculate that the seal failure could also be the cause of the noisy bearing type failure, as the lubricant may have been washed out the bearing by hot coolant after the seal failed.
Early pump failure can also be caused by flushing with a compound that contains muriatic acid, as it can damage the water pump seal. Muriatic acid also stains the bare aluminum black. Failing to flush out debris from the cooling system can also lead to premature water pump failure. Electrical systems should be kept clean and corrosion eliminated. A bad electric ground can and will corrode your cooling system. Ask any sailor about electrolysis. Check your coolant with a voltmeter with the engine running and all accessories turned on. Hopefully, there is no voltage, and thus no current flow. The limit for normal operation with most systems is .25 Volts between the coolant and the negative battery post. Corrosion will also occur as the coolant becomes acidic with age, meaning that the pH drops below .70 as glycolic and formic acid form from the glycol. Coolant should be changed at least every 40,000 miles to prevent debris and corrosion.
Plastic Water Pump Impellers
Many sources berate the plastic water pump impeller. It is certainly true that the first versions made last century had a relatively high water pump failure rate due to disintegration and cracking of the plastic impeller. Some impellers would come loose intermittently, causing the engine to overheat momentarily, typically during a freeway drive. The pump would then operate normally when the engine cooled down, or after it was shut off for a while. Up until 2009, Karmakanix was among the crowd buying water pumps with only metal impellers to avoid the failure problem. As it happens, the plastic formulation was changed in 2001, and the new version does not break or deform. But the world needed to empty itself of all the older bogus plastic replacement water pumps made before the transition. Many shops have not noticed that the water pump impeller problem abated around 2005, and factory pumps do not have the plastic failure issue anymore. If and when a factory water pump with a plastic impeller suffers severe bearing failure, the engine block does not get gouged out, as the plastic impeller is of course much softer than the metal block. Our policy now is to used only factory water pumps, unless one is just not available due to the age or model of the vehicle. Granted that severe bearing failure is not the norm, but engine block damage is just too serious a consequence to ignore.
Engine Block Damage from Water Pump Failure
A poorly understood fact is the actual power transmitted through a timing belt in action. That power is related to the number of valves and springs, and the path that the belt must follow, but the power increases radically with engine power requirements, not just with rpms. At redline and full power, the timing belt drive system can use between 18% and almost 40% of the crankshaft’s total power output. Most modern timing belt type motors have the water pump driven off the timing belt, and it receives a much higher bearing load as the power increases through the timing belt.
If the water pump bearing fails, the water pump impeller can rotate sideways far enough to hit the engine block and can dig a ditch into the block. The clearance of the pump impeller to the cavity it runs in determines the ability to properly flow coolant, and when said ditch exceeds about 3 millimeters in depth, the pumping ability diminishes significantly. Too much coolant is allowed to recirculate directly around the impeller instead of being pumped through the system. The reduction in cooling system function due to block damage from water pump failure generally is first noticed when the engine is idling. The coolant appears to be circulating, but although the coolant fans keep cycling, the coolant temperature continues to rise, sometimes until the coolant cap relieves the excessive pressure and boil over occurs, i.e.; overheating. Of course, hot weather exacerbates the issue.
Karmakanix initially noticed this problem with the first water cooled cars of the 70’s and 80’s. A really bad water pump bearing failure would gouge out the aluminum water pump housing, and leave metal flake in the cooling system. These housings were separate from the engine and could be easily replaced. Vanagon engine blocks are commonly damaged from the tilting water pump impeller. More than half the Vanagon blocks we see during a water pump job have damaged blocks. Most in fact were caused by previous water pumps, and the gouged out block was simply not repaired. The problem is often caused by mechanics who over tighten the alternator belt to prevent it from squealing when cold. This causes excessive side force on the water pump bearing, which leads to water pump failure and likely block damage.
The AUDI 3.0 V6 evidenced in the photos of this page had its aftermarket German water pump fail at 27,000 miles. All the timing belt adjustments had been done correctly, and the cooling system had no previous deposit problems. The engine ran for less than half an hour after the water pump started making noise. The block was gouged about 4.5 mm deep. The timing belt had worn away about 1/4 of the belt material. The timing belt skipped a tooth on the bank 2 intake cam. The engine was within minutes of breaking the belt or skipping enough camshaft teeth, then “crashing” or bending all the valves. The extra work required included completely removing the front end of the vehicle to access the pump housing part of the block, cleaning the belt debris from the block and covers, cleaning each individual toothed belt pulley, repairing the gouge in the engine block, and finally flushing the engine 4 times to remove all of the metal flake from the coolant system. The gouge in the block had to be scratched with 36 grit, center punched to get sufficient “tooth”, cleaned, then filled with heavy duty 500* F metal-based epoxy and allowed 24 hours to cure. The epoxy was then sanded to shape, and the area cleaned again. Many extra hours of labor were spent because of the failed aftermarket water pump. Leaving the gouge unrepaired would have led to an overheat situation during stop-and-go driving in hot weather.
2.0T Plastic Water Pump Failure
Consistent with the tendency in technology, some water pumps are now made in plastic housings. Specifically, our example here is the water pump for the chain driven 2.0T motor which starts in 2008 in Audi vehicles and 2009 in VW’s. Having a water pump made entirely of plastic is different than just a plastic coolant flange. The water pump is driven by a tiny toothed belt, and power is going through the plastic housing. Generally, the leakage does not occur at the water pump bearing, which is normal for older model pump failure.
We find that the pumps fail by cracking the plastic right at the edge near a bolt hole. We postulate that this indicates that the pump could use one more fastener one the top surface near the large hole. Often, the failure occurs when a chunk of plastic breaks off right next to a slot in the plastic that goes between the outer circumference to the O ring channel. This would appear to be an engineering booboo. Pictured is a failed pump from a 2014 Tiguan with just over 14,000 miles on it. Certainly the issue would not have occurred with a metal housing.
The failure stresses the importance of stress. It may be that the factory assembly procedure fails to tighten the bolts properly. Any part should have all the fasteners installed, then snugged, then tightened in at least 2 stages. If the first bolt is just tightened all the way (or more) initially, there will be abnormal patterns of stress across the part, and a plastic part may not survive for long. Perhaps even more care is needed when dealing with plastic. More likely is that plastic water pumps are just not the best idea. One just two of the water pumps that we have replaced, 2 of the top bolts were longer than the rest, and it seemed that those 2 bolts had bottomed out and mushroomed the tips slightly. We believe that the longer bolts at least contributed to the stress issue that broken the plastic pump housing.