Dunno how the people in the office upsell these but it happens, yes, we disable your fuel pump, and d/c te hose in your engine bay and basically run your car off a can of some sort of cleaning agent. We did this at work once, car idles great on it, put it back so the car ran off the fuel in the tank, and it was terrible! Took a fuel sample, the amount of crap that was in the tank, we had to clean the injectors again and call em back saying what we found.. Idled ok before w e sold the product, ran good on the product then ran crap back on their fuel, as u can see why the cust must of been upset with this.. When they didn't notice a problem in the first place But as mentioned about replace ur fuel filter before adding anything to the tank, & u need to get someone to get a scanner and check ur op of ur o2 sensors, if their fluctuating lean and rich or it's dead...