This past weekend my 2003 G35 sedan turned 105k so I changed the spark plugs. I have oil in one tube but otherwise things went well with one exception. When I started the car, the engine was missing. An OBDII scanner told me it was cylinder #1 and the problem was quickly traced to the coil pack connector not being all the way connected.

So after I fixed that I started to notice a very clear knock. Weather was nice here for the last two days so I drove with the windows down and could hear the knock sound very clearly when next to other cars, next to highway dividers, etc.

Given my recent work I start thinking the worst --- what if I dropped something down the spark plug tube....

So yesterday I changed to oil - knock still there.

Using a giant screwdriver as a ghetto mechanic stethoscope I isolated the knocking to the alternator/idler pulley area. In my experience when those items go bad they make a grinding or screeching sound. I decided to remove the belt to spin the alternator and idler pullies to listen for noise or feel play and noticed that my belt was trashed.

The belt had a 1" section of grooves missing and was "slapping" the idler pulley each time the bad section passed over it. The slapping sounded just like a knock!

It's always good to find an easy problem when you fear the worst but I had never heard a belt make a noise like that.


thought you should know
Brian