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Cars That Sit Need Clean Fuel, Too

by Classic Motorsports

Posted on 11/5/2024 at 9:00 AM

Sports Car Under Wraps



Think fuel cleanliness isn’t important for classics that don’t get driven much?


If anything, explains Zachary J. Santner, senior quality specialist at Sunoco, a Top Tier fuel that’s high in detergents might be even more important for an older car that spends a little extra time in the garage.

Our story starts back in the 1990s, as new minimums for fuel additive standards allowed gasoline retailers to decrease the levels of detergents found in pump fuel. Soon after, though, the problems started: increasing reports of engine issues due to combustion chamber deposits.

So, in 2004, a group of automakers – the biggies, including GM, Ford, Toyota, Volkswagen, BMW and Mercedes-Benz – released its own standard for gasoline detergent. It was called Top Tier Detergent Gasoline, and retailers could voluntarily join the program. (A current participant list can be found at

A 2016 study by AAA aimed to see if Top Tier fuels really kept combustion chambers clean. Both Top Tier and non-Top Tier fuels were run through the same engine for 100 hours to simulate about 4000 miles of real-world driving. The engine was cleaned and freshened between fuels. Parts were weighed and inspected. 

According to AAA, the deposits from Top Tier fuels averaged 34.1 milligrams per intake valve, while the non-Top Tier fuels left behind a significantly higher average of 660.6 milligrams. 

Those deposits found on the valves, Santner explains, come from fuel and engine oil. Fuel can contain residual gums that stick to the valves, and oil can travel past valve seals on a worn engine.

Picture an older car that’s been sitting, he continues. As it sits, a little bit of oil seeps onto the backs of the valves. When the engine is then fired up, some of that oil is burned, some is washed away by the fuel, and some gets coked onto the backs of the valves. 

A fuel high in detergent can wash away that carbon before it does damage, Santner says. “There’s enough detergent to keep things clean,” he adds of Top Tier fuels. “At Sunoco, all of our grades are Top Tier.”