Those designed to reduce knocking may actually do so, but using them simply masks a problem. It doesn't fix it. Engine knocking is a sign that your car needs a repair, so make a trip to your mechanic to rectify the problem instead. Tests on additives designed to clean your fuel system have not been shown to make any significant difference in reducing corrosion or deposits and don't improve performance, while tests on additives that claim to improve gas mileage never have shown significant improvements, the Environmental Protection Agency says. The only gas additive that has proven useful is a stabilizer, but this should only be used in cars that aren't driven very often.
Extended-life radiator coolants
The proper mix of radiator coolant and water is essential to keeping your engine cool in both hot and cold temperatures as well as to prevent corrosion. Some coolants are promoted as extended life, and they use a different type of corrosion inhibitor.
While the claims are valid, automakers designate their cars for one type or the other, so don't assume extended life coolant is best for your car, and never mix the two types as that affects the corrosion inhibitors.
Check your owner's manual to see which type is recommended. Follow the manufacturer's schedule for draining and replacing the coolant as well. More frequent coolant flushes are a waste of money.
With an older, high-mileage car, it's easy to believe the engine is getting tired and needs an additive to improve performance or reduce wear. But oil additives simply don't work and may actually harm your engine, according to numerous independent testing agencies.
In fact, the Federal Trade Commission has gone after several companies that sell these additives for false advertising. The dreaded engine sludge that causes an engine to run poorly or even seize is a rare occurrence and typically happens only when oil changes have been neglected or if the car has been driven a lot with an extremely low oil level.