I can’t give legal advice here because I’m not a lawyer, but I suggest you file a complaint with the Better Business Bureau and your states Attorney General. If you have documentation for all of this, or can get it (like a signed and written statement by the mechanic that told you the fuel injectors weren’t repaired or installed correctly), then I would consider filing a suit in Small Claims Court for all damages. It would be really great if you can get your mechanic to come into court with you as a witness too. Small Claims Court will take some time but its cheap to file a suit there and the defendant has to pay all damages and court costs if you win. You also win if the defendant doesn’t show up in court although collecting from them can be a hassle.

Sorry for the troubles and hope that helps a bit.