For a long time, most car engines were using multi-point port injection that sprays fuel directly at the intake valves. Today, engines are increasingly commonly direct injected. A problem of direct injection engines is that the intake valves often suffer from carbon deposits. Apparently the port injected engines used the gasoline as a cleaning fluid to remove carbon deposits from intake valves. As a response to the problem of direct injected engines suffering from intake valve carbon deposits, some manufacturers like Toyota don't have only direct injection but rather dual injection so an engine has both port and direct injection at the same time.

Do traditional carbureted engines have the same property that gasoline cleans the intake valves from carbon deposits? Or is the carburetor so far away from the valves that all liquid gasoline has already evaporated by the time the airstream reaches the intake valves?

  • Engines using exhaust gas recirculation (EGR) valves meter exhaust gases back to the intake manifold to mix with the incoming fuel/air mixtures. Exhaust gases are fed back into the intake to lower oxides of nitrogen for emissions control during acceleration and cruise. The egr valve is closed at idle and wide open throttle. Exhaust gases contain combustion byproducts and one of them is carbon that freely deposits anywhere in the intake manifold, intake and exhaust valves and piston tops.
    – F Dryer
    Aug 2, 2022 at 23:14

1 Answer 1


Not carbs per-se but the process of having relatively cool fuel coming into the intake port and cooling the intake valve stem. Both carbureted and throttle-body fuel injected engines have no issues with carbon deposits.

The deposits happen in "direct injection" engines where you have only air coming into the intake and the fuel is sprayed into the cylinder either before or during the compression stroke. The carbon is from oily blow-by gases that hit the valve and "coke".

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