Carbon buildup
 

As noted in an earlier post I just ordered a new allroad. Now I wonder if this is a good idea. I watched a video of a tech cleaning the valves on a VW DI engine. There was a ton of work just getting to and removing the intake manifold. I understand clearly the problem with DI preventing detergents from working and I also know this a problem for all brands of DI. Apparently there is a bit of a fix by increasing valve overlap which was a TSB at some time. I assume this is part of current engines. The best advice I saw was an Italian Tuneup regularly. Obviously the folks on this forum are aware of this and willing to deal with it, but aside from the possible repair costs it seems like the issue would significantly reduce the value of our cars at say 80K miles. Should I just cool my jets and not worry about it?

I thought Audi had fixed that issue a few years ago? BTW, what is an Italian Tuneup?

An Italian Tuneup is driving the car at high rpm for a time to clean things out. I have seen a couple of notes that among VWs the GTIs are less prone to carbon build up because they tend to be driven harder.

There have been several threads discussing carbon deposits on the valves with DI engines. Bottom line is all of them, by virtue of the design, can have excessive carbon build up in time since the fuel ignition does not reach the valves. It's best to change to the best oil available every 5K miles which can slow the deposits down, but not eliminate them. Driving hard won't do it either. Engine performance decreases over time so it's not usually noticeable to most drivers. The process today to clean the valves using crushed walnut shells is not too expensive nor difficult.

I personally wouldn't worry about it. I have an '06 with the first generation of the 2.0 TDI and 151k miles on the clock. I haven't done anything special to it WRT carbon buildup, and it's running just fine.

Thanks for the helpful responses. I'll just enjoy my allroad when it comes.

Huh? Crushed walnut shells? Can you explain?

It's a technique of using a media (sand) blaster with walnut shells to clean intake valves (on DI engines) via removal of the intake manifold. After blasting the shells are vacuumed and manifold reinstalled.

Thanks. Is this something Audi service can do? I can't imagine it wouldn't be expensive, but I'm guessing. Wouldn't using the best of the top tier gasolines also go a long way in reducing carbon buildup?

Not sure how many Audi dealers have adopted this service. I know mine hasn't as of yet. Some indy shops do it and I believe I've seen it done for about $400-$500. There are also DIY kits if you are so inclined.

In a nutshell (pun intended), the design of DI engines is that the fuel is injected directly into the combustion chamber of each cylinder. As a consequence the fuel injectors are no longer placed before the intake valve, but directly in the cylinder itself. The disadvantage of this is that the regular bathing and cleaning of the intake valves by way of the fuel that they are exposed to, does not happen. The intake valves are only in contact with air or rather blow by gases from the crankcase breathing circuit, in which substantial quantities of fuel and oil can be found. Over time, these lead to deposits of carbonized fuel and oil in the intake tract as well as on the intake valves themselves; this is also referred to as carbonizing (or coking). This is not immediately bad for the engine, but over time can have a detrimental effect on its efficiency. If the valves are heavily carbonized, they may not close properly and symptoms such as a bumpy idle, vibrations and diminished throttle response can be observed; it may also contribute to increased oil consumption. While using Top Tier fuel will certainly help with keeping the cylinders and fuel lines clean, it won't help with keeping the valves clean in a DI engine.

Here's lengthy thread worth the time to read that gets into the weeds on DI engines and valve carbon deposits:

https://www.audiworld.com/forums/a6-...gines-2909914/

Also one on walnut shell valve cleaning of a BMW (similar process for any DI engine):

On the contrary, it is expensive. The intake manifold does not just pop off. There is a lot of interference to be removed--fuel lines, etc. it is very labor intensive so probably several hundred dollars. With respct to detergent gasoline it does not help the back side of the valves because it does not come in contact with them. Also it seems that much of the carbon comes from crankcase ventilation.

True, but the cost is considerably less than the old method of removing the manifolds and soaking them in a cleaning solution for a couple of days. That process is significantly more labor intensive and costly than the $400-$500 of a walnut pressure cleaning.

The buildup comes from the lubrication of the valve stems that causes particles in the intake air stream or blowback from the combustion process to stick and burn in minute quantities. Detergent gasolines can't help much since the fuel is sprayed directly into the combustion chamber in DI as opposed to older port injection systems where the air/fuel mix happens just before the intake valves and thus exposing the valves to the detergents. Particulates in the intake air on DI engines come from the crank case vent and EGR, so using a catch can for the CCV and a EGR delete could minimize buildup, though the car would fail emissions.

Audi service can do the work, I believe it's 3-4 hours T&M.

This is sounding a little better. I watched a video on the soaking method which seems quite laborious. I recall from Navy days that crushed walnut shells were fed into a running turboprop engine on P3s. I also read a piece from a UK source suggesting again that operating at 3000 rpm occasionally will help and noting that slower turning V8 DI engines experience carbon more often. Thanks for all the helpful posts.

Just to clarify, the walnut shell process on a DI engine is done without the motor running. In fact, great pains must be taken to avoid any of the shells from entering the open ports before cleaning and after the shells are vacuumed out. I can't imagine what would happen to the valves if there were shells remaining when the engine was started.

Of course. I was commenting hust on the cleaning effects of the shells, not the process. Thanks for clarifying.