Oh crap. What should I do?

[blue2000s]

Could be a spark plug or coil, it could also be that you lost a wheel weight. Does the vibration come with a loss of power?

[KevinH1990]

I'm pretty sure that your check engine light would illuminate if you lost a coil pack or a spark plug. I had a CEL illuminate when one of my coils went bad.

I don't think it is a wheel weight. The vibration from a wheel weight would be dependent on the car speed, not the engine speed.

[Jake Raby]

Its not internal. If it was you would have scattered the engine on the 2.5 hr trip home..

Probably a coil pack died or came unplugged and no the CEL won't illuminate in some of these instances. You also could have fouled a spark plug..

Wheel weights come into play when high wheel speeds are high, so if the issue happens in first gear above 4K RPM you can bet its not a wheel weight.

[eightsandaces]

IMO, a wheel weight can play havoc at any speed depending on a myriad of factors including how heavy said weight is. The key is a lost wheel weight may be dependent on a particular speed to experience but will not be intermittent in nature in respect to repeatability.

[Mike_Yi]

Thanks for the input guys. It is definitely not speed related, but it is definitely rpm related. A coil pack makes sense. The engine does really feel like one cylinder isn't firing at all above 4k rpm. I think a plug wire would be intermittent. I'm going to pull the plugs today and see if any have a different look to them or smell of unburnt fuel. I may do a spark test on them too.

Thanks again for the input. I'm glad Jake thinks it's not internal.

[Frodo]

C'mon, Mike...is it speed related or not??

[Mike_Yi]

Originally Posted by Frodo

C'mon, Mike...is it speed related or not??

Fixed.