okay well i got a 99 SI. And just recently (on my drive back home from california, 4000 mile round trip) and about 200 miles from home out of no where my car started shaking really bad. Like i honestly thought i blew a tire i pulled over and everything to check haha but kept going after i saw i didn't blew a tire or anything. Fueled up at the next gas station checked everything and looked good. kept going. Well didn't happen rest of the way home, but then randomly it will happen again. driving down the highway it'll start for 20 minutes then it'll quit.

My tires were replaced before leaving so there brand new. and aligned.
Now the weird part. We checked all the usual things that we though would be wrong. Axles were fine, very little play. Ball joints. Wheel bearings. We yanked on the tires..no play again. lug nuts are tight. No clue. Thought maybe my motor mounts..but we tried to move it and it didn't move one bit. But i got a spare set of mounts im gonna install.
Today im gonna rotate my tires, put the front to the back and back to the front. Might help.
I dont know, ive asked how many people and everyone is clueless. ive read some things about spark plug wires or possibly an injector. But when my car does it..my car runs fine.

Any suggestions are greatly appreciated!

 

Does it happen at a certain speed?

 

A wheel weight could have fallen off.

 

Agreed, or the technician that installed them could have balanced them wrong. I would start by balancing your tires, even thought they are brand new.

 

happens at all speeds. from acceleration to i hit my speed. then it'll continue if i need to accelerate.

 

but it dont happen all the time...which is the weird thing.
i'll have my tires rebalanced today. see if that'll do that trick!

 

Ya, I'd start there... I had the same symptoms and a weight had fallen off.

 

like it just happened once in a while?

i just wanna make sure before i go and get them rebalanced. It can happen anywhere from 20 minutes of shaking...then it'll stop again. then shake again.

 

^^

 

Check all your bushings they maybe torn and deteriorated. When my car started to shake it was my bushings. I don't remember my car shaking at highway speeds though only when i would brake and i think accelerate.

 

X2

 

also check ur lower control arms i had that same problem and i check my control arms they were bad