wierd suspension freeway handling problem

OK, I have a 2001 turbo. When I am on the freeway hard on the gas the car rocks from side to side. If I get off the throttle it is fine, no problem, but it has been so bad at times I thought I would have lost control. I have had this with stock wheels and tires and aftermarket wheels and tires. Stock ride height and lowered ride height (with springs) I suppose the only thing I have not changed is the rear shocks ? Has anyone else had this issue ? Thanks Ed

hey Ed, Not had anything like you describe but on my C4S (similar suspension to turbo) at 55K miles i had a front shock start to 'go' and over seams in the highway the steering wheel would 'shimmy' in my hands. not sure if that's of any help but maybe you've got some worn shocks?

I'd suspect alignment or suspension link bushing.

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The only time I felt a near loss of control in my 03 Turbo was when I was running quite fast out in the middle of nowhere against a motorcycle and had to put the brakes on hard (V1 alarmed). The car veered badly and that's as close as I ever want to come to losing control and it wasn't that close. Still quite out of character. Anyhow, I guess I had a hunch and pulled in to a gas station and checked tire pressures. The right rear tire was about 6 psi low. I topped it up but had to replace it for it had some hardware in it. Hard to imagine in your car's case a low tire, or underinflated tires could have gone unnoticed for this long, though. I trust the car has been properly aligned and the settings aren't way out in left field? An alignment also gives the tech a chance to look for anything amiss. My Turbo is too stiffly sprung to check shocks the old fashioned way -- the bounce test. Have you inspected the shocks/struts for any leaks? You might enlist the help of a friend to drive near your car, follow your car in other car -- but not too close -- and observe your car's action to see if that could be of some help. Ideally it would be someone who knows what to look for. I'm not a big fan of this method though cause something's wrong and until you know what's wrong you really want to avoid driving the car any at all much less pushing it hard to demo the behavior. 'bout the only other thing I can think of -- barring some shot suspension bushing, a damaged/broken suspension member -- would be a bad transmission/engine mount. Sincerely, Macster.

My Turbo handled very poorly and down right scary when I put new Sumitomo tires on the rear with 50% worn Michelin PS2s. I measured the circumfrances of the F&Rs and determined them to be close enough to work very well together. I found out measuring is not the same as running. Fit new Sumi fronts and all was perfect, better then ever since I've owned the car. Your car rocking sounds like a traction related problem where the tires are getting intermittent grip under hard acceleration. Hard to tell without knowing more. I could ask lots and lots of questions, but I type slow. GL!

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Tires is a good idea. I never even though of mix-matched tires. Sincerely, Macster.

Tire mis-matches definitely do this. Can only assume that you have not done that to yourself.

The car has factory 997 take off tires front and rear in 19". Remember, I have had this issue with 3 different sets of wheels and tires and two different sets of springs. A Porsche tech is the one who did the lowering springs, so I am sure a leaking shock was not overlooked. Thanks Ed

this could be anyting from tire pressure, to uneven worn tires, bushing, shocks. Either something isnt "gripping" or something is "moving" or bad alingment. by gripping I mean, worn tires, low tire pressure on one side... First place I would start is Tires. Tread wear all the same all the way around? Pressure as it is to be all the way around? If tires are worn uneven, then I would go get an alignment and 4 new tires If tires are worn the same my next step would still be an algnment. If they cant pull it into spec, there may be your answer. The car may be bent ot may have some bad suspention parts somewhere. Next if it comes into alignment, I would drive it 50 miles and bring it back and have them do it again and see if something is moving. Or take it to the dealer take them for a ride show them the issue, Give them the keys and say "fix it" and wait till they call you

broken half shaft somewhere?

Do you have an LSD? Could be struggling to maintain balance across driveline?

19" tires aren't a very good option for a 996 Turbo. In fact 997 C4S or Turbo wheels and tires are a terrible choice and will cause the problem you describe. The 997 is more forgiving as far as diameter mismatches. I bought a set of 19" wheels for my 996 Turbo and the tire choices were so few that would not cause issues that I sold them and stuck with 18s. F to R tire diameters need to be extremely close of you will have issues. Keep running what is not considered a close enough match and you will tear up your diffs before long.

Personal experience was a transmission mount on the top being completely broken free that had things squirlly. I figure the rear alignment is depends on the transmission not being able to torque out of alignment up and down or to the sides. After the repair the car was rock solid in turns. Must of been the loose trans changing the rear alignment. No idea the mount was broken till it fell out when replacing the engine.

I am no expert and everybody in here has made excellent suggestions.....my .02, since this is happening under hard acceleration, only...have you checked the motor mounts?