Symptoms:

My 99 GMC Z71 truck is shimmy when I am driving at 20-40 mph. This occurs only if the engine is cold like after I parked it overnight. As soon as it gets warmed up, the problem disappears. I have brought it a lot of times to the dealer for front alignment check but they cannot find anything wrong with it. The tires were balanced a few times and just to eliminate the tires, I have tried replacing them with a new set but the problem is still there.

I am really confused with this because it was not doing this the first couple of years I owned it. What I am wondering is why it would do it only when it is cold?

Tire Problem Advisor:

From the way you described the problem, it seems like it is related to the steering system of this car, which is electronically controlled. Sensors are mounted under your steering column and power steering pump and monitored by 2 modules. The purpose of the system is to improve road feel and stability while driving. If you have these symptoms where the steering is out of control when cold, you can disable the EVO (electronic variable orifice) sensor located under the steering column by disconnecting its 3 terminals. If the problem stops, it is a sign that the sensor is defective and needs replacing.

Notes:

For good maintenance practice, replace also the orifice sensor located under the steering pump. Normally, if the sensor on the steering column goes, this one at the power steering pump is not far behind.

Above repair is only a recommendation and you do this at your own risk. Always have your vehicle align after replacing the tires to prevent repetition of the tire problem.