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Q: 04 sonata V6 shakes violently between 40 and 60MPH on 2004 Hyundai Sonata

This problem is driving me nuts, i have replaced the tires and had them balanced. i think the problem is in the engine, while drining at highway speeds i shifted into nutral and the shaking stopped, shift back to drive and shaking starts again.

could this be the spark plugs or wires.

car has 90k
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Check enging light on? It should be if engine at faullt.If light is on,have computer scanned.
This should find the problem.
The check engine light is not on and I have never seen it come on since I bought the car in 09. I thought that it might be a CV axel but it doesnt have any of the common tell tale cv axel failer indications like clunking while turning a corner. I have checked the cv axel boots and the are in good condition.

Also if it were a fouled spark plug it would cause in-complete combustion and trigger and O2 senser correct?

also could this be a bad torque converter or harmonic balencer?
If it is an axle joint , it would be inboard , not outboard joint with excessive play. Check for excessive movement at final-drive. Does it matter what gear you're in ?(besides neutral).
It seems to only happen in fourth gear. the reason I tried shifting to nutral is because in my mind if the problem is in the engine or power train that the problem would go away when the load is removed. if the problem remained while in neutral then that would lead me to believe that the problem is in the front end.

also when I am traveling over 70mph the shaking goes away. I dont know how to rationalize that.
The faster you go the higher the 'frequency' thus going faster than would normally be noticed.uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuhhhhhhhhhhhhhhhhmmmmmmmm , JUST A THOUGHT , but a while ago we had an Accord in the shop with same issue and the way we were able to verify it was to have one guy up in the car on a lift and one (and about 3 other curious techs),underneath watching as the one in the car ran it up to 45MPH and then applied the brakes at the same time as giving it enough throttle to duplicate the vibration. That ended up being the best way to verify were the vibration was coming from....It was a little tricky to keep the right amount of load on it and you don't really have much time to prove the cause of the vibration before you start overheating the brakes , so it's best the tech in the car has driven it on the road so he knows the conditions he trying to duplicate. Good luck........Regarding the comment 'it doesn't have any of the tell-tale signs' , if it did it would've been fixed already.
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