Carbon build up in the intake system has been an issue ever since bad fuel was invented. Carbon used to just rob power and mileage. With the advent of direct injection in 2005.5, the Audi and Volkswagen 4 cylinder 2.0T TFSI and 6 cylinder FSI engines seem rather prone to carbon build up that will eventually lead to misfiring and finally a breakdown. FSI engines inject directly into the combustion chamber, and so do not have atomized fuel in the intake air to help keep the intake valves and ports clean.
Understand that there is no way of seeing carbon inside a motor, so the procedure is a process of deduction. We start by consulting with the customer about oil consumption. If the oil consumption is above a quart per 2000 miles, we recommend replacing the PCV valves during our procedure. Even if your current misfire issue is not due to carbon, if your engine is consuming large amounts of oil then you will eventually have to do a carbon cleaning process. You do not want to be caught out in some Podunk town if the car becomes undrivable. Continuing to drive with a misfire problem will surely damage the catalytic converter.
Diagnosis that may lead to 2.0T carbon cleaning begins with a DTC code check. We also review the measuring value blocks for other parameters of your fuel system to see if there are signs of other problems, such as a fuel filter clogging. We find misfire codes. We log into the MVB and watch the engine recording misfires in front of us. We may or not see misfires. But the codes don’t lie. If the misfire codes point at a particular cylinder or two, then we carefully switch ignition coils and spark plugs in a particular order, clear the codes and try again. Usually the codes come right back, sometimes requiring a test drive. If the codes moved to to another cylinder, we replace the spark plugs and/or coils as necessary. Sometimes the codes do not come back, and we release the car to the customer for further driving. If the plugs are old or damaged, we replace the plugs before we give it back. If the coils are original in an early engine, although most have been replaced, then we would replace them before returning the car for driving. Replacing ignition components fixes the misfire problem maybe 30% of the time for these engines. When and if the car returns with a misfire, or if it just continued to misfire in front of us, and the codes stayed on the same cylinder(s), we then check compression and leakdown to verify that the engine is not damaged. The misfires might becoming from low compression. The odds of the engine having low compression are very low, usually the damage can be traced to bent valves which got damaged during a timing belt job.
If there is no sign of damage, then we move on to carbon cleaning. The intake manifold and injectors get removed. Both the head and the intake manifold will have carbon build up and must be cleaned. Two cylinders are cleaned at a time, sometimes after soaking in a carbon cleaner bath. The injectors must have their seals changed and the tips cleaned. This requires a special tool set, or the injectors will very likely be damaged. Injector damage will only show up after the job is done, and the engine still misfires. The only injectors we have seen go bad were on 2.0T carbon cleaning jobs after some other shop had tried to repair the problem.
We utilize a walnut shell blaster to clean the carbon from the head. Unlike sand blasting or glass bead blasting, the walnut shell cannot damage the engine internally should any debris get into the engine. Once the engine is reassembled, we clear the codes, and test drive the car. We then log in again to check for codes, and review the measuring value blocks for any signs of misfire. And we consult with the customer about oil consumption again.
Walnut Shells: The Blast From the Past
The owner at Karmakanix hails from the day when the dealerships did carbon cleaning on the old Audi 5000 model. At that time, we removed the manifolds and performed cleaning with crushed walnut shells shot through a blaster. Audi would pay for the repair under warranty, even though the issue clearly came from using poor quality fuel. Likely this was related to Audi trying to make a come back from the Unintended Acceleration Hoax. A brief explanation from Wikipedia about the conflagration. A bit funnier version from The Truth about Cars. Volkswagen had similar carbon buildup issues, but would not warranty the carbon cleaning job. Rightfully so, since the carbon came from using cheap gas. And VW did not approve of the walnut shell blaster cleaning fix. We would remove and completely disassemble the cylinder heads to clean the carbon. Frankly, Audi walnut shell blasting was really messy. Audi started using walnut shell cleaning for the 2.0T carbon cleaning job this century, and then stopped.
We find that walnut shell blasting is a very effective method for carbon cleaning. Some carbon is somewhat oily and needs soaking in a cleaner to dry it out. Care and protection must be exercised to avoid getting debris inside the engine and fuel system. All bolt holes and other ports must be covered with high quality duct tape, and the engine bay covered with a drop cloth. Carbon cleaning is a job best left to qualified professionals who are trained in both motor repair and fuel systems. The cost of repairing after an amateur can be more than triple.