On older engines the injector sprays the fuel on the back of the intake valve. Every time the engine runs, fuel washes the valve. On direct injection the injector sits in the combustion chamber and sprays the fuel straight into the cylinder, past the intake valve. Nothing washes the valve anymore. So every direct injection engine gets carbon on the intake valves. Some faster, some slower, but all of them.
What builds there comes mostly from the oil, not from the fuel. Oil mist comes into the intake with the crankcase ventilation, and a little oil runs down the valve stem through the guide. Some exhaust gas also comes back into the port when intake and exhaust valves are open together. On the valve it all bakes into a hard black crust. In the lab, 10% or more of this deposit by weight is not carbon at all. It is mostly calcium, zinc, phosphorus, molybdenum and sulfur, the elements from the oil additives.
This is why cleaner in the tank does nothing for these valves. The fuel never touches them. Audi and VW say it in their own bulletins, their fuel additive cleans intake valves on port injected engines only. Top Tier fuel does not change it either. Its intake valve test runs on a port injected engine, there is no test for direct injection valves.
Which engines get it worst. On BMW the classic is the N54, the twin turbo six in the 335i. BMW made its own walnut blaster for the dealers, and the manual for it was written around the N54. The N55 and the B58 get it too. MINI has a bulletin for the N14 in the Cooper S. Cars over about 35,000 miles, driven mostly in the city stop and go, can get heavy carbon. MINI names fuel quality, low loads and short trips.
On Audi and VW the classic ones are the early 2.0T FSI and the 4.2 FSI V8. The B7 RS4 is famous for it. Two shops put a bad RS4 on the dyno before and after cleaning and got more than 40 horsepower back. The 2.0 TSI and the 3.0 supercharged V6 get it too.
On Porsche it is in writing for the SUVs and the Panamera. Porsche Technical Information from 2018, revised in 2019 and again in 2024, covers Cayenne 958 and 9YA, Panamera 970 and 971, and Macan, from model year 2010. The complaint is knocking under high load. Porsche says the intake valves and ports must be checked before any other work. Only walnut is allowed, with the factory tool VAS 294 011. Dry ice is forbidden. The bulletin does not cover the 911, Boxster and Cayman, but they are direct injection too, from the 997.2 and 987.2 on.
Mercedes went to direct injection on its four and six cylinders in the US in 2012. Volvo Drive-E four cylinders are direct injection too.
Some newer engines got port injectors back, next to the direct ones. Audi did it on the 2.5 five cylinder in the RS3 and TT RS from 2017, on the last version of the 3.0 supercharged V6, and on the second generation R8 V10. The third generation 2.0 TSI got it too, but not in North America. Audi says on the US engine the holes for the port injectors are not even drilled. Audi gives particulate emissions as the reason, but when the port injector sprays, the fuel hits the back of the valve again, like on the old engines. So these engines carbon up much less. At high miles I would still scope them.
What the owner feels. Engine runs rough right after a cold start, misfire codes, rough idle. On the Porsche SUVs, knocking under load. Many owners just feel the car got lazy. It comes so slow that they don't notice.
But not every cold start misfire is carbon. On the N54, BMW has a bulletin where the cold start misfire comes from carbon on the injector tips, with plugs wet with fuel. The fix is injectors, not valves. Mercedes had cold start misfires on its V8s fixed with software or injectors. MINI says rule out VANOS, fuel pressure, ignition and boost faults first, check the intake for leaks and the crankcase pressure, and only then pull the intake and look at the valves. Wrong crankcase vacuum alone can make rough idle and misfire, BMW says so in writing. So first I measure, then I scope. I look at the valves with a borescope before anybody pays for a cleaning. I can also see it with a pressure transducer, from the intake numbers at idle.
Carbon can also keep the valve from sealing. MINI gives 8% as the leak-down standard. If a high mileage city car is way over it, like 15 to 25%, MINI says the head comes off and the valves and seats get lapped.
Walnut blasting done right. The intake comes off. Each cylinder gets turned to where both intake valves are closed, so nothing goes into the cylinder. An adapter seals the port, the vacuum runs the whole time, and crushed walnut shell goes in with air at 6 to 8 bar, about 90 to 115 psi, in short bursts. All other ports and fuel openings stay covered. New media every time, never reused. Picks for the stubborn spots, then the borescope again to see the port is clean. New intake gaskets, and the adaptations get cleared. BMW, MINI and Porsche all use walnut for this, and Porsche allows nothing else.
What I would not do. Spray cleaners through the intake take off only part of it. In the maker's own tests one treatment removed about 12 to 24%. Too much in the intake and the engine can hydrolock and bend a connecting rod. And dry ice, Porsche does not allow it. Best is walnut.
What you get. If carbon was the problem, the cold start misfire goes away, the idle gets smooth and the lost power comes back. If the engine ran fine, don't expect a new car. Cleaning fixes what the carbon caused, nothing more.
And it comes back. Walnut removes the carbon, not the cause. Same engine, same oil, same driving, and it builds again.
What helps. First a healthy crankcase ventilation. A failed PCV valve or oil separator sends more oil into the intake. BMW gives a crankcase vacuum spec for its engines, 17 mbar on the N54, 50 on the N55. Those numbers are for sea level. In Denver the N54 reads about 15, and up here in Salida even less. I measure it, it is a tell. Then the oil. How the oil is made changes how fast the deposit builds, that was measured in the lab. And the driving. Short trips in the cold put water and fuel in the oil. A longer drive now and then gets the oil hot and the water out.
Catch cans. They are sold as the cure. I have not seen one controlled test that shows a catch can keeps the valves clean, and owners with cans still find carbon on the valves. The factories build the oil separator into the engine, and BMW and Mercedes even heat the ventilation so it does not freeze. That matters up here. When the ventilation freezes, pressure builds in the crankcase and pushes oil out at the seals. Some owners also report the can and hoses freezing, and in winter what collects in a can is often more water than oil. And in Colorado, modifying an emission control system is tampering, everywhere in the state, unless the part is approved for that car by EPA or CARB. Crankcase ventilation is emission control. My advice: keep the factory system healthy, scope the valves, clean when it is needed. Factory always wins.
Every direct injection engine gets it. The only question is how much and when. I look first and clean when it is needed. I have the walnut setup in the shop.
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