The 2.7L engine is notorious for oil-related failure if the engine oil isn't changed religiously. The water pump is driven by the primary timing chain. Depending on what's wrong with the pump, a wobbling sprocket could cause excessive wear to the chain. Also, there are guides that the chain slides over. Lack of adequate lubrication will cause wear to the guides and the pins in the chain links. One guide has a tensioner built in to keep the chain quiet as it stretches with mileage. A stretched chain causes the camshafts to be retarded, (slightly behind, or late, compared to where they should be in relation to the crankshaft). Retarded cam timing results in reduced horsepower, and therefore, increased fuel consumption and higher emissions.
If there are enough miles on your engine that it needs a water pump, any reputable mechanic would recognize the intelligence in replacing the timing chain too. That is looking out for you, (better performance, and very low additional cost as long as he already has the engine torn down that far). He's also looking out for himself. He wants to do the highest quality repair possible so you'll be happy and tell your friends. He also knows that if the chain gives you a problem in the future, and you find out it's related to the water pump, you will certainly blame him. We call that "being married to the car". Everything imaginable that goes wrong from now on will be his fault.
"Yah, hey, ... You changed my oil two months ago and now my power antenna doesn't work. It must be your fault. What you gonna do about it?)
There are also two secondary chains. Your engine has four camshafts that run the valves and must be synchronized perfectly. The primary chain is most likely to wear, and only runs one camshaft on each side of the engine. A secondary chain on each side runs a partner camshaft. These secondary chains have much less tendency to wear and stretch.
Along with the primary chain, it's likely he will also replace the guides. They are four long thin flat metal rods with a fiber coating. They develop a wear pattern that will not match the new chain. Reusing old guides could lead to excessive chain noise.
Leaking antifreeze definitely is not good, but there are other possible causes besides the water pump. As for your "no Check Engine" light, that has nothing to do with the water pump. The engine computer turns the light on when, by way of its sensors and other circuits connected to it, it detects a problem that will have an adverse effect on tail pipe emissions.
If you don't trust your "suspect mechanic", you should find someone else, but it sounds to me like he's trying to avoid the "Penney wise, dollar foolish syndrome. There are bad mechanics, just like there are bad people in every profession, but a lot of the problem is the car owner's lack of understanding about cars. It's hard enough for mechanics to stay up-to-date. Expecting owners to know how over-complicated their cars are, especially compared to how simple and reliable they were 20 years ago, is unrealistic.
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Sunday, December 6th, 2009 AT 3:56 AM