Justin suggested starting a thread about FSI fuel injectors and the potential problems caused by carbon buildup on/in the injectors. There’s not much information specific to the RS4 available on the subject so this is a good place to start.
I’ll start with my recent symptoms and the diagnosis that led me to cleaning the fuel injectors.
Symptoms:
Surging idle - Idle was constantly fluctuating
Rough cold start - The cold start cycle was extremely rough with RPMs approaching 2000
Other possible symptoms:
Engine misfire
Performance loss
Increased Fuel consumption
Engine damage - Possible in the event of a fully clogged injector. There have been a few documented cases of this.
Causes
In our cars the fuel injector heads are exposed to the combustion chamber so they can get carbon on/in them just like our intake valves. Fortunately the buildup is not near as dramatic as the on intake valves. However, when you have a tolerance of 1 micron any amount of carbon buildup can affect the injector flow in a big way.
When I had my injectors cleaned, the guy said he found sludge inside the injector filter baskets. I’m still not sure on the cause of that or if it should be expected on our injectors.
Diagnosis
Fuel trim data is a good place to start if you think your injectors might be clogged. Look at VCDS measuring block 032, which shows your long term fuel trims at idle and load. Short term fuel trim can be monitored in block 033.
A clogged fuel injector will inhibit optimal fueling in the motor. The ECU will see this lack of fuel over time using the O2 sensors and call for a permanent fuel trim correction. Looking at measuring block 032, a clogged fuel injector will cause a positive number to show up in the Partial field(Partial means at load). In my case it was showing between +5% and +6%. While these numbers are in spec, we really don’t want to be running lean. Lean is bad for performance.
If you have positive long term fuel trim at load(block 032 Partial field), you can be fairly certain your fuel injectors are not flowing optimally if your car meets the following conditions:
Good O2 sensors - Bad O2 sensors can cause inaccurate fuel trim readings
Good fuel rail pressure on both banks - Check measuring block 106
No vacuum leaks - Check the Idle field in block 032, it should be slightly negative.
Normal short term fuel trims - Your short term fuel trims need to be within +/- 10%.
Solutions
Clean your damn injectors! Like with soap and water dummy. I wish it was that easy.
You’ll need to either:
Buy a new set of injectors, which is a minimum of $800. Part number 079906036D
Send your injectors off for cleaning, should be around $200 + shipping. Look for someone with an ASNU machine. I used https://www.racecityinjector.com/ but it’s best to find one in your region to minimize shipping time.
If you opt to buy a new set you will minimize down time and have the option of reselling the old ones after having them cleaned.
Here’s a data sheet showing injector flow results before/after cleaning. Two of the injectors were pretty bad off. I’m guessing those were both in bank 1.
http://i.imgur.com/Moyirs0.jpg
Conclusions
Please share data. I would like to see long term fuel trim data from other RS4 owners. If you have any of the symptoms listed above, please consider servicing or changing your fuel injectors during your next carbon clean. If you have a positive LTFT >10% on either bank you might not want to wait that long.
Technical gurus, please add to the topic.