EcoBoost shudder is a knocking or vibration that can be felt when the engine is under load. It is caused by the EcoBoost system injecting too much fuel into the cylinders, resulting in an uneven combustion. This can happen if the EcoBoost system is not properly tuned, or if there is something wrong with the injectors. EcoBoost shudder can also be caused by using lower-quality gasoline. If you are experiencing EcoBoost shudder, it is best to take your car to a qualified mechanic who can diagnose and fix the problem.