Vehicle Lead Diagnosis and Correction

DIAGNOSIS AND TESTING-VEHICLE LEAD/PULL
To assure correct diagnosis, it is important to follow the steps outlined below in the order shown. Road test the vehicle before and after each step to verify that the lead condition has been corrected. When evaluating a vehicle, always drive the same road in both directions to get a feel for the effect of road crown and cross wind. A neutral vehicle will exhibit a small amount of drift on both right and left crowned roads (normal crown sensitivity). A vehicle with pronounced lead/pull may have one or more of the following conditions:
1. UNEQUAL TIRE PRESSURE. Adjust tire pressure to the pressure stated on door placard. Make sure the tire pressure is equal on all four tires and evaluate the car. Also verify that the tire size and type are correct and match each other. If the car still has a lead condition go to step (2).
2. TIRE CONICITY. Excessive tire conicity is one of the more frequent causes of vehicle lead. Cross-switch the front tires and evaluate the car. If the car still leads in the same direction or gets worse, return the front tires to their original position, then go to step (3).
3. SUSPENSION ALIGNMENT. Check and record the wheel alignment settings including caster. Nonsystematical front caster or camber can sometimes cause a lead condition or can be used to fix a lead condition. To bias the front suspension caster and camber alignment settings to correct or minimize a lead condition, perform the Vehicle Lead/Pull Alignment Bias repair procedure. If the car still leads after the alignment bias procedure, go to step (4) or (5) accordingly.
4. TORQUE INDUCED LEAD. Torque induced lead on this vehicle is a condition that is generally noticed at speeds above 50 km/h (30 mph), and usually gets worse with mild to moderate acceleration. To repair a vehicle for torque steer, perform the Vehicle Lead/Pull Engine Mount Shim repair procedure. If the car still leads and all of the previous steps have been performed, go to step (5).
5. STEERING GEAR VALVE IMBALANCE. Steering gear valve imbalance can sometimes cause a vehicle lead. Although there is no quick test or measurement that can be performed to verify a good or bad steering gear valve, generally the steering efforts will feel much lighter in the lead direction and heavier in the opposite direction with an unbalanced valve. Replace the steering gear only as a "last resort" to solve the problem.