I know there have been posts about checking the vacuum elbows, but here is another reason to check. I got a check engine light and it returned code 4-3-3, "rear knock sensor reading out of bounds". I reset the code and within 1/4 mile, got the code again. I checked my Haynes book and it said I had to remove the intake manifold to gain access to the knock sensors. Before ordering the rear knock sensor, I decided to take a look to see if I wanted to tackle removing the intake manifold or if there was a way to get at the sensor without removing the manifold. While I was poking around, I noticed one of the vacuum elbows coming from under the throttle body cover was all rotted away, causing a significant leak. It had a fair amount of grease and dirt on it which is why I didn't notice it before. (Yeah, I know, I should be cleaning the throttle body periodically.) I figured that the rotted elbow couldn't be doing me any good, so I replaced it with a piece of vacuum tube, reset the code and 100 miles later the check engine light is still out and I feel pretty good about fixing the problem with 2 cents worth of vacuum tubing. If I hadn't noticed the rotted elbow, I might have taken the easy way out and taken the car in to my indy. While I trust my indy to do the right thing, it still would have cost me. The moral is keep that throttle body clean and check the elbows. I think I just going to replace them all. At 125,000 mi, it wouldn't hurt.