Since this topic comes up frequently and there doesn't seem to be any howto, I thought I'd toss something together based on my experience with misfire and notes I found here and on various forums. Feedback is appreciated, I can continue to update this.
One of the most common problems almost every owner of a B5S4 will face is a misfire issue. Misfires are the common cold of the 2.7T. The Bosch ME7.1 system records misfires and if they exceed a given amount (the factory manual says 15+) in a given time duration (60 seconds?), the dreaded check engine light will come on. If the codes are retrieved, the owner will see P0300 or P0301-306. If you don’t take the time to troubleshoot the issue and either you or your mechanic start throwing parts at the car, you can easily spend $1000 on just parts and not resolve the problem. Sometimes you might get lucky, but the odds are stacked against you, real troubleshooting pays off big when diagnosing misfires.
Before you (or whoever is working on the car) begins, there are a few tools you cannot do without.
- A basic code reader, you can’t know what’s wrong or when things are fixed without at least a basic OBDII code reader.
- Basic hand tools (socket set, screwdrivers, plug wrench)
- A boost leak tester, you can build one yourself using the directions here (note, you need a compressor): http://www.awe-tuning.com/media/pdf/...ure_tester.pdf
- A Vag-com interface and software, this is really required for more advance troubleshooting, if you own an S4 and don’t have one, you need to get one.
- A vacuum guage. A dedicated guage is much better than a boost/vac guage, but if that is all you have, its good enough for getting started.
I don’t care, I just want to try replacing stuff.
There are lots of people that are annoyed at the thought of troubleshooting a problem, many mechanics included. If you just want to “try stuff” go in this order:
-Boost leak test
-Change plugs to Bosch Platin F5DPOR
-Swap coils around
-swap ICM/POS units around (these are the electronic things sitting on top of your airbox)
-move injectors around
-unplug MAF (for testing only)
-replace primary 02 sensors
-buy new coils
-buy new ICMs
What causes a misfire?
Before digging in, you need to understand that there are only three things needed for a cylinder to fire: Correct Air/Fuel Mixture, Compression and Spark. If any of these are not right, you will get a misfire. Anything that can impact the A/F mixture, compression or spark can and will cause misfires. Things like sticking lifters, bad gas, leaking injector seals, old 02 sensors, dirty MAF sensors and even evap system issues can all cause this problem. Don’t ignore anything that impacts air, fuel, compression and spark.
Is the engine in good working order?
Before you start digging into the cause of the misfire, you need to make sure your engine is in sound operating condition. Boost leak testing is the first thing you should do. The linked guide above will show you how to build a boost test kit. Fix any leaks you find before moving on. Next you need to check your engine vacuum at idle. With the A/C off and the car warmed up, you should see at least 19 inches of vacuum and a very steady needle. Low vacuum indicates leaks, either vacuum or a worn engine. If the needle is bouncing, you may have a compression/valve issue with a particular cylinder. Misfires do not make for a bouncy needle! Last but not least, don’t ignore the obvious. Think about anything that has changed or happened recently, this may clue you in to the misfire. Make sure you don’t have any crank/camshaft position sensor errors, if you do, misfire detection is not reliable until that issue is fixed.
Specific Cylinder Codes
Next, you need to tackle individual cylinder misfires before addressing a P0300 random misfire issue. If multiple cylinders are misfiring, you will get a code for an individual cylinder as well as the random misfire code. The good thing about specific cylinder codes is they are much easier to troubleshoot.
Things to try:
-Are the misfire codes specific to one bank? If so, swap ICMs.
-Swap the coil pack AND plug. Does the misfire follow? If so, try swapping back just the coil. If it follows back, its the coil, if it stays, its the plug.
-Swap the ICM (Power output stage) wiring, does it move to the other bank? If so, likely its a bad ICM.
-If neither of the above work, swap the injector. Frequently people forget about the injectors, but a clogged one will cause a misfire, especially at or close to idle.
-Last if none of the above works, re-check your engine vacuum at idle using a dedicated vacuum guage and confirm the needle is dead steady and above 19 inches. If you have any pulsation, its time for a compression test.
Random misfires are as a rule much harder to diagnose. Do not try to diagnose random misfires if you still have cylinder specific issues. Since you’ve already done a boost leak test from above (you did one right???) the next step is to really figure out if the motor thinks it is running rich or lean. Without a vag-com or OBDII reader that streams live data, you are out of luck. Common issues are air leaks but we’ve already eliminated those in our first tests. Next, and fairly common are aging primary 02 sensors, the rears do not have anything to do misfires. As they get old, 02 sensors start reading lean and try to richen up the mixture. Dieing MAF sensors also cause mixture issues, you can unplug it and see if the misfires change. Old/Bad plugs can also cause misfires at idle but you will notice power issues up top also. Various tunes/injectors can also cause misfires at idle.
Above and Beyond
Beyond the scope of this basic howto is datalogging your car. First you will need to capture when and which cylinders are misfiring by logging blocks 010-019. You will then need to find out what the ECM thinks is going on and what the results are. Watching the short and long term fuel trims will tell you if the computer thinks it is running lean or rich. Watching 02 sensor data can show mass amounts of unburned fuel indicating spark issues at high rpm. Even something as silly as a coolant sensor can cause misfires because the computer is trying to compensate for a cold engine.