Steering wheel trembles on highway

I have a 97 318i with auto transmission and 103k mileage. Yesterday I drove it to the highway for 15 miles and noticed the steering wheel trembled sometimes. The speed was around 50-80 mph with AC on. When this trembling got stronger, I could feel the whole car trembling too.. I think the following may be possible reasons:
(1) something wrong with the engin, which is the worst case..
(2) it's normal when you turn on AC which consumes much power.. (should not be this case because I don't have this problem before)
(3) tires should be balanced
(4) something loosened in the drive train which resonates the car at certain speed range.

I'm going to have a long trip with this car so any help would be sincerely appreaciated.

Was it worse when braking? My guess (without driving it or seeing it) is worn front control arm bushings.

no it remains the same when braking...

Sounds like a steering/suspension problem. The A/C shouldnt bother anything. Have it checked out.

same problem with my 320i. i have to refill my ATF fluid every now and again to make sure the steering is tight as i think it is leaking. after refilling, the car is more stable. My brother used to have a 518 with the same problem and he had the slave cylinder replaced which sorted the problem. When you are stopped turn the steering wheel and if a grinding or rubbing noise is heard, you should check your ATF fluid as it may be low/leaking or both. Check the ATF fluid level anyway as this provides the lube for the powersteering.

I would suggest to start out with trying the easy stuff, i.e. tires balanced, check for steering fluid leaks etc, then if it persists you can eliminate those from the list.

Originally Posted by spiral

I have a 97 318i with auto transmission and 103k mileage. Yesterday I drove it to the highway for 15 miles and noticed the steering wheel trembled sometimes. The speed was around 50-80 mph with AC on. When this trembling got stronger, I could feel the whole car trembling too.. I think the following may be possible reasons:
(1) something wrong with the engin, which is the worst case..
(2) it's normal when you turn on AC which consumes much power.. (should not be this case because I don't have this problem before)
(3) tires should be balanced
(4) something loosened in the drive train which resonates the car at certain speed range.

I'm going to have a long trip with this car so any help would be sincerely appreaciated.

Usually this means your ball joints, tie rods are bad, could have a bent wheel, out of balanced, tires. jack your car up and see if you can wiggle the front tire up and down or side to side.

Hope this helps. If you need any parts just let me know

Yes, this happened to me, it feels like the steering wheel is going to rip out of your hand? I replaced my ball joints and it took care of it. I would suggest changing the lower arm and get both ball joints (inner and outer). You may want to do the rear as well but at least make sure you do both sides in the front. That should fix your problem