2002 Mazda Millennia Shaking over 55 mph

I was driving on the highway one day at 70 mph (the speed limit here) and all of a sudden it felt and sounded like I was crossing onto the shoulder - a frontal vibration and mmmmmmmm. I have had the tires balanced and the front end checked. I have even just had two new engine mounts replaced. The problem is still there!

What is it? How can I check it?

By “front end checked” what does that mean? Did the tech check for damaged brake rotors, or loose wheel bearings or failing CV joints or loose steering or damaged bushings or all of the above? Was the rear checked, too? Sometimes and issue you think is at the front is really from the rear. It sure sounds like you threw a (big) wheel weight off at speed. Always consider a “horse” before a “zebra” but you said you had the tires balanced… all? or just fronts?

Have you had the engine itself checked? Are all the drive belts in place? Loose alternator, loose power steering pump? Loose air conditioning? Does turning AC on and off make a different? Did you shift the transmission down into a lower gear so the engine revs higher? Did the vibration change?

Driveline vibration changes with car speed, engine vibration changes with engine RPM, transmission does both (manual trans or automatic?)

There just isn’t enough info to diagnose it without a little more help on when and where it occurs. Post here and we’ll try to help.

I went to two mechanics.

  1. The first place is my local tire company that do tires and front ends. I told them about the problem and to please (a) balance all the tires and (b) check the front end for any problems because of the vibration.

  2. I took it to a second place to see if they could fix it. They found that I needed two engine mounts replaced - which they did. However, the vibration continues. I was going to bring it back to them and ask them to give it another try at diagnosing and fixing the problem.

The vibration occurs while driving at CONSTANT speed above about 55. I can reduce and almost eliminate the vibration by varying the engine speed - accelerate and coast, accelerate and coast.

Have someone check intermediate halfshaft bearing.

The intermediate halfshaft comes out the right side of the transaxle, and then is mounted to the side of the engine with a bearing. Then the right side halfshaft connects to the intermediate halfshaft.