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Old 11-12-2013, 11:57 AM   #1
Bender's Avatar
Drives: 2014 Focus ST
Join Date: Mar 2010
Location: Doylestown, PA
Posts: 20,440
Carbon Buildup 101

As more and more cars start to flirt with higher mileage, a higher number of members are encountering carbon buildup. Though carbon buildup is nothing new and older members are well versed in it, it may be of interest to newer members and those buying used vehicles with mileage in the mid 5 figures and beyond.

So what is carbon buildup?

Carbon buildup is just what it sounds like. It is a buildup of carbon material on the top of your intake valves (between).

What causes carbon buildup?

To understand what causes carbon buildup, you first need to understand how our engines work. At its most basic, an engine burns two things: oxygen and fuel. In our case that fuel can either be gasoline or diesel. So, an appropriate mixture of fuel and oxygen are needed in the combustion chamber to explode when the spark plug ignites. In the old days, oxygen and fuel were mixed in the carburetor and passed to the engine over the intake valve. In more recent times, a fuel injector sprayed a fine mist of fuel into the intake port. From there the air/fuel mixture passed over the intake valve into the combustion chamber. This was called port injection (you still see this advertised on older GM cars). Port injection was a huge leap forward in terms of both power and fuel economy. You could much more finely control the mixture, and dynamically control the mixture based on what the various sensors were communicating to the ECU. Though they were very different, both had something in common - fuel passed over the intake valves. This is important and we will come back to that point later.

A relatively recent way to mix fuel and air is called direct injection. Just as port injection is exactly what it sounds like (injection of fuel into the intake port), direct injection is just what it sounds like. It is the injection of fuel DIRECTLY into the combustion chamber. Just like port injection was a step up in both power and efficiency, direct injection is even a bigger leap forward. Both the GTI and the TDI are direct injected. As fuel is injected DIRECTLY into the combustion chamber, it does not pass over the intake valves.

Ok Bender, hurry it up man...

Sorry, I am getting to it, but this is all important for you to be aware of if you want a comprehensive view of the issue. So, now you know the various options for how fuel and air get into the intake chamber, but you still don't know where the carbon comes from. Once the air and fuel mix and are combusted, there is extreme high pressure in the combustion chamber, forcing the piston to move. Most of the exhaust gases go out through the exhaust valve, but some tiny amounts of exhaust (which contains burnt and unburnt carbons) were actually forced past the piston and piston seals into the crankcase. This is called blow-by. This builds up pressure in the crankcase that must be relieved. Back in the early 20th century the would simply vent the crankcase right to the atmosphere. Because this was terrible for the environment and resulted in oil buildup... everywhere... GM invented the PCV (positive crankcase ventilation) system (side note - GM was such a believer in PCV tech they gave it away to other manufactures FOR FREE). The PCV recirculates these crankcase gases into the intake manifold, where they mix with incoming air for re-combustion. This makes lots of sense as it has minimal impact on performance and it results in huge environmental improvements.

So the PCV takes your blow-by gases out of the crankcase and dumps them back into the intake manifold. Back in the old days, this was no big deal. They would be mixed with fuel and off they would go to do it all over again. Now, since the fuel never enters the intake port, they don't mix with fuel. Instead, they start to congregate, and settle, and build up. This is the source of carbon buildup.

Makes sense. What if I used a catch can rather than a PCV?

For a long time, it was generally accepted that catch cans were the solution by many (myself included) on this board. Long term testing proved they had no impact on carbon buildup.

What can I do?

When applying for its patents on direct injection, Audi noted the potential issue and recommended keeping the engine above 3000 rpms, for over 20 minutes, on a regular basis ( I would say that I am the perfect test case for this and can confirm that it works. I did need carbon cleaning, but it was not until 91,000 miles, when many are facing it at 50,000. Drive the car hard, use good oil and good fuel (note, neither has been proved to have an impact, but it won't hurt), and don't worry about it. Its a major maintenance item, but not one to fear.
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