Originally Posted by
ichi d
does anyone know if carbon build up could hurt mpg?
Carbon build up will impact your mpg, but the 3.0T is not susceptible to carbon build up, unlike the FSI engines (3.2, and 07-09 4.2).
Here's a write-up a did on carbon build-up a little while back.
Things that have a direct impact on carbon deposits are:
- Fuel quality: The crappier the fuel, the more impurities and the more likely there are to be deposits to collect - go to
http://www.toptiergas.com/ for more info
- Driving distance/engine temps: Those hit hardest are short distance driving, where the engine never gets up to temp. Getting the engine hot means that the gunk won’t be able to collect as readily, because it’s baked off by the high temps.
- Poor seating of the cylinder rings: More oil seeps by into the fuel adding to the large amounts of particles recirculated into the intakes. Use of more than 1Qt of oil every 5K miles means higher than average oil consumption usually meaning poorly seated rings, causing excessive blow-by
So all C6 3.2 engines are DI (Audi calls it FSI), they are the first OEM to make ALL their engines this way. DI is really great for making the fuel/air mix ratios and concentrations perfect, it takes less fuel to make the right mix with DI, it takes less fuel to get the same power using DI, and it can also fight knock (pre-ignition, which harms engines) since every injector can operate independently – allowing the ECU to control the fuel to every cylinder individually. It is also used if the engine gets too hot and the coolant cannot keep up by purposefully putting extra fuel in which will evaporate harmlessly but take heat with it. Here comes the con(s).
Non-DI engines have the injectors in the intake channels, and spray the intake valve’s backing as it opens into the cylinder. This happens with each intake stroke, “washing” the intake valve with fuel. You have, I am sure, seen the Chevron with Techron and like commercials about it having additives which keep valves clean. This is great for NON-DI engines. Why the valves need cleaning is due to 1970’s legislation to handle pollution. Exhaust Gas Recirculation (EGR) and Positive Crankcase Ventilation (PCV) are 2 things required by law in the US (and many other countries) to prevent pollution. EGR pipes unburned fuel and particles in the exhaust, back into the crankcase to prevent catalytic convertor congestion and failure (things like unburned fuel, impurities in the fuel, water vapor, CO2, and oil). PCV takes crankcase vapors (mainly just oil particles and the EGR stuff) which is under pressure and pre-1970’s was just vented to the atmosphere, but now is redirected through a valve system which has a rough collection system that attempts to pull the largest particles from the air in this, and separate them out and allow them to drip down into the oil filter to be collected (if bad) or allowed to be cleaned and returned to the oil pan for engine lubrication – the particles too small to collect are mixed with fresh air coming into the engine just above the intake valves and shot back into the engine to attempt to re-burn them properly.
As you can imagine, the particles will collect on all parts of the intake, turning into sludge eventually and oozing into the intake, where it eventually is burned away during combustion. The problem happens with DI engines – the injectors shoot fuel DIRECTLY into the cylinders and don’t wash this gunk off the back of the intake valves. As such the gunk builds up here but since it’s so close to the cylinder, the heat bakes the gunk into carbon deposits which build up slowly over time. These deposits eventually become so heavy that they affect the airflow into the cylinders, making them move in unexpected patterns, or in very bad cases, not enough air is pulled in as the ECU is expecting, either way the result is too much fuel to the air ratio, or uneven mixing of the fuel, which robs engine power, can cause rough idle, and eventually cause Check Engine Lights (CEL’s) due to misfires (especially when the engine is cold). This causes a negative spiral, where the even higher amounts of unburned fuel due to the Carbon Build-up (CB) makes even more unburned fuel be re-circulated and these richer deposits speed the buildup on the backs of the valves further.
A few things are done by people to correct this.
- Carbon Cleaning – the top of the engine is pulled apart, intakes and valve covers are pulled, and the heads removed, then a Dremel or similar tool is used with a fine grain grinding bit are used to grind the carbon off the intake runners, the intake air flaps, and the backs of the valves. This is usually done around every 30K miles by those wanting to be sure they have the power and performance of the car unaffected.
- Catch Can – This is a 3rd party development, which partially bypasses the EPA mandated pollution measures mentioned above. It basically goes in-line with the recirculation of the gases before they hit the intake, comprising of a separate filter system that more aggressively captures and holds the particles the stock system doesn’t handle. The difference here is that the aftermarket solution doesn’t drop this into the oil, it holds it in the can, and you have a drain at the bottom where every 5-7K miles it needs emptying. Due to this being “bad stuff” from inside the engine, it cannot be disposed of easily, and is best poured into old oil containers and taken to a recycling place like Schuck’s or your local recycling transfer station, it’s mainly oil, water and fuel, so it can go into oil recycling.
- Methanol Injection – Typically referred to as “Meth” it’s been around a LOT longer than the illicit drug, and is basically a raw form of octane. What it’s used for is to both cool the incoming air (making it denser and therefore holds more oxygen for combustion) as well as boost the octane rating, which equates to more power. Methanol is mixed 50/50 with distilled water, and uses a special tank to hold it in, with its own pump and delivery system. The methanol is injected into the air intake around the throttle body, misting the incoming air. The methanol mist in the air cools it through evaporation, and since it’s a combustible liquid, it helps take place of the washing that non-DI engines get, by coating the backs of the valves and pulls the deposits into the fuel system rather than it baking and hardening there, as well as giving more horsepower and performance out of the car at the same time. Methanol is not very expensive, but is dangerous due to high octane and ability to burn, hence mixing 50/50. A typical use system will go through a small (1 liter) tank every 1-2 tanks of gas.
One of the things is that Forced Induction (FI) engines (3.0T) are not as susceptible to this, the reason being the much higher compression and thus higher cylinder and valve temps if you do get it out and run it for at least 10-20 minutes at normal running temps.
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