Need to Blow the Gunk Out

Gasoline direct injection (GDI) is becoming more common as new car manufacturers work to wring more power and better gas mileage out of smaller engines. Carbon and other deposits quickly collecting on intake valves and interfering with their operation has been a problem on some GDI engines. The problem was a surprise to me. Its causes are actually quite simple while the solutions vary and are complex.

Old style, port fuel injectors spray gasoline into the intake manifold runners at around 75 psi (500 kPa) or less. The gasoline and air are drawn into the combustion chamber when the intake valve opens. GDI injectors are mounted in the cylinder head much like a spark plug and they spray the gasoline directly into the combustion chamber. They must deliver the gasoline at extremely high pressure, typically around 3000 psi (21,000 kPa), to overcome high in-cylinder pressures and to deliver fuel vapor in a precise, short time period. For perspective, 3000 psi is the maximum pressure generated by a typical pressure washer or the pressure of the compressed air in a high-powered air rifle.



With port fuel injection, the gasoline and its detergents wash off the back side of the intake valve every time the gasoline enters the combustion chamber. GDI injectors send gasoline blasting into the combustion chamber at pressures that could easily take chewing gum off a sidewalk, but the gasoline often does not reach the back side of the intake valve. Carbon, oil and other residues can build up on the intake valve. The valve does not seat properly leading to problems with engine performance, emissions and durability.

Dirty intake valves are more of a problem on some GDI engines compared to others because engine and fuel system design is the primary factor. Using gasoline without enough added detergents and too many short trips around town are a couple of other suggested causes, but to me they just indicate the need for a better fuel system design. It does not matter how much detergent is in the gasoline if the gasoline never reaches the back of the intake valve. I personally like the idea of needing to frequently take cars for long drives at high speeds, but unfortunately, my spouse (and police officers) long ago stopped believing my “need to blow the gunk out” excuse.

Car manufacturers have different, sometimes almost contradictory, theories on how to prevent carbon buildup on intake valves in GDI engines. Some try to leave the intake valves open slightly longer to allow gasoline to reach and cleanse the back of the intake valve. Some think it is better to keep the gasoline off of the valve so soot does not build up on the valve face. Some car manufacturers are creating GDI / port fuel injection hybrids by adding a fuel injector back to the intake manifold to guarantee there is always a little gasoline keeping the intake valves clean. There is likely more than one good solution because there are many fuel system, cylinder head, valve train, fuel injector, etc. designs.

Time and miles tell which GDI engines have the fewest problems with dirty intake valves. A car manufacturer might declare success if an engine does not need the cylinder head removed and the intake valves scraped clean during the warranty period, but the car owner likely never wants to remove the cylinder head. Maintaining the crankcase ventilation system (PCV valve, etc.) and the exhaust gas recirculation (EGR valve, etc.) systems on GDI engines is more important than ever because they are another possible source of oily residue and carbon that could collect on intake valves. With spring approaching, even carbureted and port fuel injected cars might benefit from a tank full of fresh, detergent gasoline and a good intake-valve-cleansing, blow-the-gunk-out drive (while observing the posted speed limit of course)!

Tom Taylor,
RockAuto.com