I live in Japan and recently purchases a 1991 C4 with 40,000 kilometers (roughly 25,000 miles). Unfortunately, a week after I bought it, I drove it into a concrete waterway (used to irrigate rice fields) on the edge of the road during a winter blizzard. Although there was little body damage, I'm afraid that I damaged the 4WD steering/suspension system. Now, during snow-ice conditions at about 80kph (about 50mph) the steering wheel shakes. When this is happening it seems that the wheels are wobbling. I live in a rural area and fortunately there is a 70-year-old mechanic nearby who loves Porsches and until a year ago was the proud owner of a 930 series, which he drove and serviced himself for many years. But he is unfamiliar with C4's and has no idea what my problem could be. The other day I drove the car about 250 miles to the nearest official Porsche dealer. The mechanic there thought the problem may be 'electrical' and 'reset the computer controlled calibration?' the someone controls the 4WD function. During the 250-mile return ride home there was no problem. But today while driving in snow-ice conditions, again the steering wheel shake/wheel wobble problem returned - again at a speed of about 50mph. The higher the speed, the more radical the wobble. Any ideas out there about what the problem could be?
UPDATE: After posting the above message, I revisited my local mechanic and resolved the problem. In snow road conditions, snow gets trapped in the wheels (I have RUF 'Speedline' wheels). In cold conditions, the snow turns to ice and remains trapped in the wheel spokes, which alters the wheel balance, causing the wheels and steering wheel to shake at speeds over 50mph. Clearing the wheels of trapped snow and ice solved the problem. I hope this helps other C4 owners living in wintry conditions.