When my car has more than half a tank of petrol the engine surges and jerks, however with less than half it is fine. What on Earth could this be?
Thank you for writing in. Without more information it is difficult to determine the direct cause of your problems. That being said, there are a few things you may want to consider. First, the car is likely jerking due to a disruption in the gas flow. What immediately comes to mind is the tank's purge valve. If this sticks open it will cause an upset with the vehicle's fuel trim, and possibly flood the engine with fuel. Have it inspected by a professional with the right equipment for confirmation. Good luck!