Highway Shaking

Im getting constant shakes at highways speed at the same 10-20 second intervals usually going uphill. I recently had an alignment done ( 3 mos ago). And recently purchased new tires which are balanced. Anyone come across this problem? Anyone have any suggestions?

What exactly makes you think that it is suspension related? I'd be more inclined to think you're getting a misfire causing the shaking once you put the engine under load. When I first learned about how touchy my Aurora was about the spark plugs and wires, it behaved like that sort of. I'd only get the shaking at first under very specific conditions- being only at the low end of a gear, usually 3rd, just a smidge under 2000 rpms and trying to accelerate, esp uphill. Then the damn thing would buck and heave like crazy. I could floor it and it behaved normally. If it was at a faster rpm range it wouldn't do it. Eventually it got worse though. If you've got many miles on your plugs and wires, I'd check in to that first.

Your 10-20 second interval piqued my interest however. For starters, let me describe what I had: I noticed this on a long trip on the interstate at first, though I could then pick it out somewhat later if I paid attention. I would get a slight hesitation in the engine coupled with a chirping noise not unlike a cricket. The thing is, it would do it every 12-14 seconds like clockwork. I tried varying my speed from the low 60s all the way to 90. It didn't seem to matter. Now, I think I've cured it by repeated use of gas treatments, but maybe I was just throwing away my $. The odd thing is, while I could feel a momentary loss of power, I never detected so much as a quiver out of my tachometer. I sort of figured it had a delay built in to avoid too much quiver and the like, but maybe it wasn't the engine after all. If it IS suspension related, it's probably just degenerated to shaking all the time on my car.

its not misfiring. Its been going on for a year now and i just changed my plugs 3 weeks ago with no change in the shake. Im inclined to think its the wheel balancing. Im thinking the car is very sensitive to wheel balancing being off just a tad bit.

Probably is that two wheels or tires are slightly out of balance and as they circle around they line up enough to cause shakes. As they continue to rotate the heavy sides rotate away. Much like two circles of differing size rotate at different rates.

If the above is the case, you will feel the slight beginnings of a shimmy, intensity of shake will progress and begin to slowly fade away.

Many times the cause is dirt/mud buildup on the inside of wheel.

I cannot imagine why uphill is more pronounced unless the out of balance is on the rear during weight transfer.
Hope this helps.

Is it possible thaat it is just your AC cycling on and off? Mine can be felt everytime it cycles. At the intervals you talk about, it could be that. Does it go away when you turn the climate control off?
Just a long shot.

I had the shaking problem on my Buick LeSabre 3800 engine several years ago. Particularly up-hill under modest engine load or modest up-hill acceleration. The symptom seemed like the vehicle was shaking. Turns out it was the crank-shaft position sensor. So, the effect was that it felt like shake but in fact it was really more like a real bad timing problem (so to speak). Your symptom sounds hauntingly similar to mine with the 3800. It didn't happen just crusing on straight and level. It only occured under modest load or acceleration, at highway speeds, and on an interval that did not necessarily corelate to the engine firing at 2-3K RPM.

I suppose it certainly could be some sort of suspension problem. You know best the "feel" of the vehicle. But it sounds like you've ruled out (through alignment and balancing) anything obvious. I guess it could be something nasy like a CV joint binding or something like that. But again, only you have the first-hand knowledge of the conditions that precipitate the problem.

There are several sensors that work in concert to ensure timing and firing the RPM and crank-shaft position sensors are among them. It may be best to simply take the bad-boy in and have the Mr. Goodwrench check it out. It may cost a few $$$ but it the hard shake isn't doing you any good either.

Good luck.

What effect did the alignment and the tire replacement have on the condition?

I had a similar problem. It turned out that one rim was slightly bent which in turn wore out the hub bearings. The regular tire balancers couldn’t find the problem. I needed a GM balance to check the rim for out of round and road force balancer. Most tire centers do NOT have these. Check around. I ended up have a new hub & bearing assembly replaced and then bought new wheels (aftermarket are way cheaper and look cooler!). My vibration is gone at all speeds.