句1: He waited until she was about to leave.
句2: He had waited until she was about to leave.
哪句是对的？假如有错，错在哪？
我觉得，句1: He waited until she was about to leave. 合乎语法。
句2: He had waited until she was about to leave. 主句用过去完成时没必要。
wait 跟leave 的先后关系十分明确，而且后者还有was about to，问题不在until，重要是没必要用过去完成时。
我重点谈谈你追问的句子：
He waited until she washed the car.
▲主句用一定形式没问题，说明waited 这动作持续到“某个时间”为止。但until从句本身有问题，它没办法表达出你说的3个意思。缘由在于：
延续性动词（如wash, work, study）用在until 句中，假如跟主句的时态相同，则非常难分辨出这个动作已经开始还是结束。这个时候，假如表达“等到开始洗车”就用began to wash；假如表达“等到洗完车”就用had washed；假如表达“等到尚未开始洗车（马上要洗）”，就说was about to wash。分别翻译如下：
1. 他等到她开始洗车。
He waited until she began to wash the car.
2. 他等到她洗完车。
He waited until she had washed the car.
3. 他等到她马上洗车。
He waited until she was about to wash the car.
▲但，对于非延续性动词如：finish, leave, arrive, get to, come, go, open, close, sTOP等，用在until句中，一旦发生就意味着这个动作结束，所以，直接用通常过去时即可。比如：
He waited until she sTOPped the car.
He waited until her car arrived.
He waited until her car left.
He waited until her car got to his house.
He waited until her car came back.
He waited until she opened the door of the car.
▲再举一个延续性动词的例子：
Mother waited until her son began to study his lessons. 等到开始学习为止。
Mother waited until her son had studied his lessons. 等到学习完为止。
Mother waited until her son was about to study his lessons. 等到马上学习为止。
在一个简单句中，谓语动词用通常过去时，或者用目前完成时，一个一次性动作无论延续还是不延续，都表示发生，也表示完成。但在一个复合句中，当两个动作放在一块时，动作就有先后完成或开始的先后顺序。这类需要通过时态表现出来。以下两句都是简单句，没有动作完成的先后和开始的顺序关系。
I washed my car. 我洗了车。
I did my work. 我做完了工作。
