Weren’t you going to wash your car? I’m going to. I’m still going to. I will. I still will. I am (not completely wrong, but not as common. It’s a little ambiguous. Does it mean “I am going to.” Or “I am washing it now”) For the other question, it’s exactly as Laura says.

Usually, if a question is with "would" you'd answer with "I would" or "I wouldn't"

Wouldn't is, as Laura said, hypothetical. I don't think that is what you mean. It sounds very odd without a conditional. I think what you mean to say is weren't you going to wash your car.

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Q: "Weren't you going to wash your car?" (means: I thought you were going to wash your car but you didn't) A: "I was." (means: I was going to wash my car but I didn't for some reason, like maybe it started raining.) - or-- A: "I am." (means: I still am going to wash my car I just haven't done it yet.) Q: "Wouldn't you wash your car?" (This is a hypothetical situation with "if" implied. For example, 'Wouldn't you wash your car if it were as dirty as mine?') A: Yes, I would. ---OR--- No, I wouldn't. (You wouldn't say 'I am' or 'I will' because we aren't asking what you are actually going to do. It's only hypothetical.)