Politely ask the shop owner to have a senior mechanic perform a compression test, then install the correct spark plugs. I know it's hard to accept, but every failure happens some time, and that time just might be when it's sitting in the shop. Also, have you actually sat in the car to feel what he's talking about. He might be pointing out a problem that existed for a long time and you just didn't notice it. His idea of "vibration" might be a whole lot pickier than yours.
Spark plugs do not "burn out". They gradually wear out. We know what he meant. It's time they were replaced. But the unprofessional terminology is what causes a lot of information to be lost in translation between the mechanic, service advisor, and customer. If something happened to cause an old spark plug to misfire, it came in that way so you should have noticed it. Thats why, if it seemed to be running properly, "worn out" implies time for a maintenance tune-up; "burned out" implies not working at all. Again, I might be picking at nits, but you can see that unprofessional communication skills can lead to at least the appearance of unprofessional skills.
Since the Check Engine light is on, there WILL be at least one diagnostic fault code in the Engine Computer's memory. That code will lead him to the circuit or system with the problem, not the specific part. The code could be "multiple cylinder misfire" for example. This is where a compression test will help. If some cylinders are significantly lower than others, a cylinder leak test will blow air into each cylinder, one at a time, then he can run to the tail pipe, air intake, oil fill cap, and radiator looking for the escaping air. I'm not familiar with the design of the cylinder head's combustion chamber, but I suppose a spark plug that is too long could be hit by a moving valve. The resulting bent valve would cause very low compression and very high leakage at the tail pipe or air intake. If that was caused by the old spark plugs, the problem would have occurred long ago when they were first installed. I hate to second guess a professional who isn't here to defend himself, but I would suggest that if the engine has bent valves now, it was likely cause by the mechanic installing the wrong spark plugs. Again, that's assuming it really is bent valves. There are plenty of other things that can cause a misfire and vibration. The place to start is reading the fault codes and performing the compression test.
Sunday, February 28th, 2010 AT 9:54 AM