My 07 S6 is having a misfire that I cannot seem to figure out. I’ll cruise to the office and drive very casually and you’d never notice anything was wrong. An example of it misfiring is driving down the interstate at about 60 and then suddenly go to about half or 3/4 throttle loading the engine up. I can feel the car slightly shake from misfiring. I won’t get any CEL codes or anything.
So far I’ve replaced all the spark plugs with the OEM plugs. After that I got very radical and replaced the intake and swapped in cleaned/flow tested fuel injectors from JHM. and also did the carbon cleaning at the same time. The previous carbon cleaning was only 10k miles ago.
Before I was getting a misfire on cylinder 1 quite often so I put a new coil pack on that cylinder so far. I’ve also made sure there is no oil around the spark plugs either.
Anyone got any advice no what to do next? I’m thinking go ahead and replacing the rest of the coil packs since they are probably needing to be replaced anyway due to age. I’ve tried to do fixes that need to be done anyways.
I don’t have a vagcom reader yet to be able to scan anything.
I just went through this and it ended up being my battery. There’s a thread somewhere that discusses it.
Check the age of the battery and get it tested, if it fails replace it. It sucks because it’s a monster of a battery Group 49/H8 size, but I just paid $250 CAD for a new one.
My symptoms were similar to yours, I did the carbon clean ect ect ect. My S6 is also 07, and has about 100k miles.
I also replaced coil packs to eliminate that as the problem (should probably replace yours as well)
But the battery could be the issue, what I did to figure this out is put the battery on a trickle charger overnight and drove the car, no misfires. next day would not put the trickle charger on and magically… misfires. At first the misfires would start after a long drive (1hr plus) and from 4,000rpm up. Driving normally usually kept it in check until finally it the misfires started right from startup.
I had to “recode” the battery with vag-com as per instructions in the online community, but some guys have replaced the battery with no issue. Apparently the battery module adjusts voltage and amperage to the battery as it ages.
And to tell you the truth, the stock 10yr old battery that was in there would always start the car, as long I drove daily. If I left it a week it had a hard time and if it was a week in the cold (-10 or colder) no start.
That sounds logical. My car doesn’t have much trouble starting but now that its colder it seems like its a tad weaker when its starting. I have never taken the cover off of my battery to see if its original or not. I guess it would make sense that it misfires under a load but once the car downshifts and the engine revs up the alternator is generating more power and it won’t misfire.
Only thing that concerns me if that I would think the alternator should be able to supply enough power to give the coils enough power to work properly. I wouldn’t think the battery would come into play when it comes to that.
So replacing your battery fixed the misfire issue altogether?
Yes. No misfires under load, under de-celeration, anywhere.
Justincredible mentioned that the ecu uses the battery voltage at startup to determine spark (the ecu is the amplifier for the coils) so a low battery would get the ecu to underdrive the coils, or so that’s what I took from his write up in another thread on here about misfires.
Worth noting I still have O2 Sensor issues, but again no misfires.
I want to say I’ve read where people would have misfires and would turn the car off and restart it and then it would be fine. Which makes sense based on what you said about the ECU.
Yeah I have the JHM tune and when I start the car in the cold (like -20C here) it runs rough, but if I restart it a few minutes after it seems to run just fine. I have air ride in the car so when I go to start it, my compressors come on that draw a 65amp draw on the battery… So I will be looking at fixing all my leaks so that the compressors don’t fire right as I start the vehicle.
i apologize in advance if this is a total newb question and i’m really not trying to thread jack but how do you detect a misfire? what do you look out for or feel during the course of driving? I know my Vagcom tells me but just wanted to be cool like all you guys. lol
For me when I’m driving and press on the gas in a situation where I know it will mostlikely misfire I can feel the car surging. I let someone else drive once and from the passenger seat it was not as noticeable.
Having more misfire issues. Some rough starts in the morning and some CEL codes.
Can someone explain more about how the ECU uses battery voltage to determine spark output? When does it check the voltage?
I checked yesterday and apparently my battery is original… as in February 2007 original. It still starts the car fine every morning but I suspect the voltage of the battery is low… I checked it this morning and it was sitting at 12.15 volts right after I opened the door to pop the hood which primed the fuel pump and turned on the dome lights. My battery must be barely hanging on.
I would get a new battery and see if that makes a difference.
Modern electronics can be very sensitive to battery voltage it seems. Low voltage in the system can cause the fuel pump to not output the proper fuel pressure and this can cause misfires. If the injectors are not getting enough voltage, it can cause them to not work right, same thing with the ignition coils.
Most batteries have a life of 5 years. Yours is 11 years old. Time to replace it regardless.
I plan on replacing the battery here in a few minutes. Just crazy to me that it checks all of that without the engine running. To me once the alternator is providing current it shouldn’t matter.
Curious what the amp rating on the alternator is anyways.
I’m also very impressed the battery has lasted this long. I’ve never had one come anywhere close. I’m guessing the system takes care of the battery pretty well and I’m sure having it in the trunk away from all the heat makes a difference as well.
