While driving on the main highway from Farjado, Puerto Rico to San Juan, our minivan rental started making loud scraping noises and could barely get up an incline on the road. We stopped by one of the gas stations by the side of the highway (I don't recall if it was a Puma or some other local brand you see all over Puerto Rico) to see if there was anything under the car that might have caused the scraping sound. We didn't see anything and decided to add $10 of gas since we were at the gas station. My husband paid in cash inside, so that we could pump gas outside. After we got back on the highway, the car continued to make loud noises and have difficulty going uphill. We made it to a car rental office right off the highway that we saw, which was technically closed but had several personnel. The manager went outside with my husband to look at the car which at that point, wouldn't start at all now. The manager arranged for a replacement car to be driven from the airport location (about 10-15 minutes away) to us so that we could switch cars and that was the end of that. Well, we now get a call from the hotel location where we rented the car originally about 5 weeks later saying that their report shows that we put diesel in the car and that we have to pay for all the repairs. He did not have the amount of money they say we owe, but said that we would get a formal letter from their risk management asking us to pay. There is no way for us to confirm or deny which gas we put in the car since we paid in cash, other than my husband recalling that he selected the cheapest gas with the lowest octane from the pump. Somehow, I don't think diesel has 3 different price ranges, nor is diesel cheap compared to regular gas, nor would I think you could fit a diesel nozzle into a gas tank, but perhaps it's different in Puerto Rico. My Amex loss damage waiver does not cover the damage in this case because I had to have rejected damage waiver coverage, but the car rental place only had 2 insurance packages available, basic coverage and premium coverage. We chose premium, but I think both coverages included damage waiver. I could not pick and choose the type of coverage in each package offered, even though we asked the question at the time of rental and were told that we could only buy the basic or premium "package". Any ideas as to how I can get out of this mess? If somehow we pumped diesel into the gas tank, we did not do it intentionally. In fact, the hotel car rental agent said that in his 5 years of working there, he has never seen this happen before.