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, 8 and 10 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 really good 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 breather valve 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. Also, continuing to drive with a misfire problem will surely damage the catalytic converter.
First an explanation of how carbon buildup will eventually lead to an undrivable car. Crankcase gases and oil get into the intake manifold through the PCV breather valve and onto the intake valves and valve seat pocket areas in the cylinder head. A little bit of oil in the intake air may also come from the turbocharger. Because the injectors spray right into the cylinders, there is nothing to help wash the funky oil off the valves. It bakes into gooey black carbon from the engine heat. Once the carbon builds up until the valves and pockets start looking like little black teddy bears in black mossy caves, the swirl of the intake air gets very disturbed. TFSI means Turbo Fuel Stratified Injection. Stratified means that that between the intake swirl and the direct injection into the cylinders, the mixture is only rich enough to combust correctly right near the spark plug. Further down in the cylinder, the mixture is too lean to explode well, but will burn along with the combustion flame front. The building carbon progressively disturbs the swirl characteristics until a misfire problem begins.
At first, a customer just notices that the cold idle is not quite right, and eventually a random misfire DTC shows up (OBD-II Code P0300 Misfire Detected, or VAG code 16684 – Random/Multiple Cylinder Misfire Detected). If a particular cylinder has a worse carbon issue than the others, then a code for that cylinder misfire may show up as well (OBD-II Codes P0301, P0302, etc). Soon after, the stratified mixture starts to get wrong enough to produce misfires from the incomplete combustion when the engine is warmed up. What follows within weeks or months is the nightmare. The spark plug fires, but the mixture does not explode, it just burns like a blow torch. The piston descends with no real power, and continues back upward throwing burning fuel out the exhaust port. The next downward cycle intakes some air into the flame. Then when the piston comes up on the compression stroke, the flame is still burning and the injector fires at 60 degrees before top dead center into a flame front. The noise is tremendous as the piston gets pounded backwards in the bore. The sound is like a jack hammer, way worse than any pinging or detonation that you have ever heard. The chain driven motor (C***) is smart enough to sense the wild knock sensor voltages, and instantly shuts off the injector in the offending cylinder. A Diagnostic Trouble Code (DTC) is generated for cylinder disabling, sometimes without even turning on the yellow check engine light (Malfunction Indicator Light or MIL). That would be a OBD-II Code P130A Cylinder Disabling. The earlier BPY (B**) belt-driven engine has no such cylinder disabling function and so just beats itself to death. The only way to move such a car under power is to disconnect the coil for that cylinder. It runs much better as just a three cylinder, which is to say badly, and cannot be driven. A motor with that level of the misfire from a carbon issue would break something within minutes if driven.
Diagnosis that may lead to carbon cleaning begins with a DTC code check. We also review the Measuring Value Blocks (MVB’s) for other parameters of your fuel system to see if there are signs of other problems, such as a fuel filter clogging or the engine running too rich or too lean. We find misfire codes. We log into the MVB’s 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 from cylinder to cylinder 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 the other cylinder along with the spark plugs or coils, 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 or coils are old or damaged, we replace the plugs and/or coils before we give the car back.
Replacing ignition components fixes the misfire problem maybe half of the time. 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 that the misfires are not due to a lean or rich running issue. One consults the measuring value blocks for mixture adaptation, looking for excessively high or low percentages. Lean misfires can be due to faulty crankcase breather valves (PCV), or any leaking gasket or seal, and may be accompanied by the OBD-II Code P0171 or P0174 System Too Lean. A smoke test may be needed to find the problem. Rich misfires are generally injector failures, a little rare but possible. Evidence would include dark and/or contaminated spark plug electrodes, and combustion chambers wet and discolored with excess fuel. The OBD-II Code P0172 or P0175 System Too Rich may be found when one scans for DTC’s.
A necessary step is to verify that the engine is not damaged. The misfires might be coming from low compression. The likelihood of the engine having low compression is very rare, usually the damage can be traced to bad machine work during a valve job or to bent valves which got damaged during a timing belt or chain job. But a compression test can also confirm a carbon issue diagnosis. The carbon buildup obstructs the incoming air causing the compression readings on our gauge to start out low and increase very slowly as the engine is cranked.
If there is no sign of other causes of misfiring, then we move on to carbon cleaning. We may look inside the intake with a borescope, and sometimes take pictures of the carbon. But we have found that it may be difficult to evaluate the amount and effect of the carbon visually. Remember that the essence of the carbon misfire issue is disturbing the correct swirl of the intake air flow, which is not always a visually obvious thing. We have found that it is not uncommon to find engines with worn or damaged ignition parts, and lean misfires from a leaking PCV breather valve or gasket, and carbon issues. It is about a quarter to a third of the time that more than one problem has to be repaired to stop a misfire issue.
So on to the carbon cleaning process. The intake manifold and injector fuel lines get removed. The injectors get capped off to protect them from dust. Both the cylinder head and the intake manifold will have carbon build up and must be cleaned. Two cylinders are cleaned at a time after soaking in a chemical carbon cleaner bath to break down and dry out the carbon. 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.
On rare occasions, we may find the engine continues to misfire slightly due to differences in the injector spray patterns. This issue can be masked by the larger effects of the ignition, mixture or carbon-based misfires. Unfortunately, this means another time that the intake manifold must be removed. Due to the injectors sealing directly into the combustion chambers, the injector seals are a tough Teflon design, and require special tools to change them.
The injectors operate at 30 to 110 bar (450 to 1650 psi) depending on the load of the engine. In contrast, earlier fuel systems ran pressures of 2 bar (30 psi) for EFI systems or 5+ bar (80 psi) for CIS systems. The extremely high pressures mean that even slight damage can cause significant injector spray pattern and volume changes, which can lead to misfires. Injector failure of this sort is slight and rare. Very occasionally, we have seen catastrophic complete injector failure where the engine crankcase actually fills up with fuel.
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 during the early 1980’s. 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. Audi started using walnut shell cleaning for the 2.0T carbon cleaning jobs this century, and then stopped.
We find that walnut shell blasting is a very effective method for carbon cleaning. The 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 or 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.
Cleaning by soaking and scraping alone always leaves behind scratchy looking debris. These debris patterns form the seeds for building up carbon much more rapidly than a truly clean surface. It is true that high oil consumption with surely lead to an early carbon misfire problem within 40,000 to 75,000 miles. But even without oil consumption issues, many repeat carbon issues are caused by poor cleaning techniques.