Need Help with Strange Vibration
 

Trying to help a computer illiterate buddy of mine with his '00 Boxster. He's been telling me about a vibration that comes and goes, and I was beginning to think he was sniffing glue until the other day when I was in the car with him. Sitting at a light, he said "there it is, there it is," and I can say is that it was certainly noticeable.

It's a base car with tip, less than 60K miles. IMS was done around 2 years ago. It was idling perfectly at a light, and then what I would call a cyclical vibration started. No change in RPM's, no CEL, nothing, just a strong, cyclical vibration that you can almost hear, but definitely feel. Leaving the light, the car ran perfectly.

Any clue what this might be?

motor mount.

Back in 2011, I bought a 98 base that needed some love. One item of note was a bit of a rough idle. After changing plugs and filters, along with cleaning the MAF, I felt no difference. One day while at a light, I noticed that although I still had a rough idle, the tach was rock steady. I replaced the mount and the car felt like new. Worth a shot as I'll bet your buddy may have not yet replaced his yet.

Yep, as above. Absolutely no doubt the motor mount is shot in the car. Relatively inexpensive repair. Not too tough to do DYI but takes some knowledge and a couple of semi-special tools makes it a little easier. A lift is helpful too.

Thanks guys...What puzzled me is that the vibration isn't there 100% of the time. I've been in the car several times trying to replicate it, and it never appeared, which is why I questioned him to begin with. This time, I was in the car with him for over an hour, and for nearly the entire time, there was no vibration at all. I was about to refer him to a psychiatrist, when the vibration just appeared out of nowhere while sitting at a light.

Would a bad motor mount cause an intermittent issue like this, or would the vibration be present 100% of the time?

If you notice the RPM isn't steady when the vibration happens, try cleaning the idle control valve. It is on the side of the throttle body. But as I reread your post it appears the RPM's were steady, so I will third the motor mount.

Need Help with Strange Vibration
 

This sounds more like a fuel issue to me. Sitting at a stop, there is no suspension or road issue at hand. So, odd vibrations are motor related. This leads me to poor fuel quality. Could be a knock sensor timing issue.

Depends, if the MM is shot (I promise you it is with that mileage if it’s original) it’s possible that another issue causing a vibration is felt more. So no, most likely an intermittent vibe isn’t going to be caused by the MM but a dead MM can potentially be felt for sure at idle. Especially if the center rubber has completely detached, which is very common. Basically the front of the engine is sitting low, and on a slightly damped chunk of metal.

I'm wondering if perhaps the motor mount is just worn to the point that the motor might shift a bit to metal on metal contact on occasion, causing the vibration to be pronounced. When leaving the light, it shifts again, perhaps back into normal position on the rubber portion, which eliminates the vibration.

Any idea how many hours it takes to replace it?