Misfire Cylinder 2 - Please Help
 

Hi Everyone! New guy here hoping for some advice.

I recently purchased a 2008 Q7 4.2 FSI and it’s a great ride but only a week or so after buying it I suddenly have a problem. Overall the vehicle runs great but suddenly I’m getting a CEL for misfire on cylinder 2.
The misfire doesn’t happen when I really romp on the throttle. Hard driving it feels like a race car and never has a hiccup.
The misfire seems to happen when I’m going about 40ish mph and I slowly push into the throttle or under moderate load such as going up hill. But again if I smash on the throttle I don’t get a misfire at all, no shudder, nothing.... Seems to only occur with semi-normal driving.

I have replaced all 8 plugs and replaced coil pack 2 since that would be the obvious first thing to check. I cleared the stored code and went for another drive and almost right away with just moderate acceleration I could feel a little vibration right around 3k rpm and then the CEL came right back on. Went home and pulled the code and it was exactly the same... P0302 Cylinder 2 Misfire.

My Q7 has 150k miles on it and runs like a champ! With the exception of the misfire now... Only other issue is what I believe is a bad battery. I can drive around for hours and when I come home and check the battery I’m only getting 50-60% charge on the battery. If I let it sit for a few days the battery just can’t hold its charge and I’ll have to jump start it. I ordered the OEM AGM battery with the BEM code and I’m having my local dealership swap the battery in a couple days. I already read about the need to supposedly program the battery into the system by way of the “BEM” code because of something to do with how the vehicles electrical system is regulated. Personally I’ve never owned a vehicle where simply swapping out a battery becomes a dealer only process but oh well, so be it. Definitely hoping that replacing the battery fixes that issue and I don’t have some type of bad parasitic drain somewhere. I’ve never had the red battery light come on that would indicate an overall voltage issue such as the alternator but I continually get the yellow battery low charge light almost immediately if I turn on the radio without starting the engine. My assumption is that the battery has never been changed so I’m giving a new battery a try.

My highest concern is definitely the misfire on cylinder 2.... Aside from what I’ve already done, what more can I do? I’m not a mechanic by any means, I know just enough to be dangerous if that makes sense. I am definitely not qualified enough to start tearing down engines, manifolds, stuff like that.

Any suggestions are appreciated. Thank you!!

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Hi. well first thing to know about cars, when you have a misfire in cylinder X. you don't run and buy a coil pack and a plug because you have no idea if they are even bad, so why waste the money?
You swap the coil and plug with another cylinder, If the misfire follows the coil and spark plug then you know one of those is bad usually the coil....
However if it stays on cylinder X, then you know nothing is wrong with either...
The only good news I can think of is you noting your battery isn't getting charged fully.
This causes the car to freak out and start faulting things that are not bad.
replace the battery clear the code and see what happens.

Also the idea the battery can only be changed by the dealer is a silly one. The "BEM" code doesn't mean anything. The car doesn't use it. It just needs to change so the car knows the battery got changed.
Ive written a few posts on this, I made a youtube video about it. this is a battery that is it. its not an iphone battery with circuitry on it that the regulator can talk to and think bad things about you.
It's simply a battery. You need to tell the car what the CCA of the battery is so the regulator knows when the battery is fully charged. It doesnt decide full charge on battery voltage, it tests the batteries CCA and if it cant meet what you told it it was or exceeds it the regulator starts freaking out and refusing to fully charge the battery and as noted, when the battery isnt up to the correct voltage all sorts of crazy random faults start happening... and god forbid you take a car that is faulting from this to a repair place. they will start throwing part after part after part and will never ever fix it till they code the correct CCA of the battery and change the bem so the regulator thinks a new battery was put in.

Honestly an OEM battery for me was like $15 more than a generic off the shelf, so just paid for OEM.. Ive had a VCDS since 2003 so its paid for its self in spades, but an OBDEleven can do it or you can find any of the VCDS Registries and find a local VW/Audi guy who'll give you some time on theirs to perform the needed changes.

I agree, the car electronics go stupid with a low battery.. dont throw money at anything until you correct that.

Hey guys, thanks for your responses. Battery was replaced today and so far no check engine light has come back on. I can still feel a slight vibration around 2500rpm on moderate acceleration and that’s usually when the CEL would kick on and start flashing. When I felt that vibration today I just backed off on the throttle and the light never flashed or stuck on.... Checked the computer with my Autel computer and nothing shows pending in the ECU.
The battery I bought was a OEM from CarID.Com and it was $377.49 with shipping. They offered a free return shipping label and box to send back the old core and will refund me $18.00 for the core.
I paid $206.29 for labor at the local VW dealership here in Washington State to remove the drivers seat, swap the battery, and set the “BEM” code to tell the computer that a new battery was installed.
So all in all it will have cost $565.78 parts and labor to have the battery replaced.

As as trying to diagnose the misfire before replacing the battery, yes I suppose I was wrong in doing that. However in my defense I’ve never had any vehicle misfire on the same exact cylinder because of a low battery. I am by NO means an advanced mechanic but that’s a new one for me... My Mk4 Jetta misfired like crazy and it did have a crappy battery but the battery had nothing to do with it at all..... it was bad wires. Swapped the wires and plugs and the issue was gone, regardless of the battery being beyond its end of life.
I guess the sophistication of the computers in the Q7 is far more touchy when it comes to any level of low or high voltage? Maybe I misunderstand the concept of the system but it seems like the voltage regulator would correct or compensate by putting more or less load on the alternator. But to have one random cylinder misfire out of everything else that could go wrong with low or high voltage just sounds so strange to me!

Tomorrow May1st we are leaving for a week trip to the Ocean and we live in Enumclaw, WA. Comes out to about a 2.5hr Drive there and 2.5hrs back. I imagine this will be a good test for the system to see if the light comes back on and if it throws the P0302 code again. I’ll have to report back on the results in a few days.

Final Note- The dealer put my car up on a lift because they noticed the oil leak under it. They were kind enough to take me out to the service bay and let me see what they were looking at. The entire under side of the motor, plastic sound/skid plate, is all covered in oil. It’s hard to see where it’s coming from because it just seems to be leaking in multiple places around the engine but not a drop around the oil drain plug. When I park the car I’m getting a pretty good amount of oil dripping out onto my garage floor, about 1-2 ounces of oil or so on the concrete when I go to leave the next day. Sooooo this will take me into my next research area to try and investigate where this is coming from. My fear is that I can see where some oil has run down from under the intake manifold down the front of the engine and is dripping onto that plastic skid plate/sound shield. From what little reading I’ve done it sounds like this is a pain in the A&& Repair which requires removing the entire upper intake and replacing several oil seals.... Not a task I would be comfortable taking on myself and I’m not sure what such a job would cost P&L wise at a local mechanic.
I know that I’m not spraying oil out or losing significant pressure but still, the leak seems excessive and if the misfire issue does not return then this will be my next thing to address.

Ill report back back in about a week when we come home with an update on how the Q7 performed and if it throws a code again.

Thanks!

cost me $230 for an OEM battery at my local audi dealer service counter, seat dont get "removed" you just take out two bolts and it lays over.. they are tripple square which you likely dont have unless you took the axles off your MK4 for any reason, but after that its exactly same as changing any other battery.. well except for the next part.

It has a sophisticated Battery Management System that is tied into everything, it uses this to know when to shut down computers, keyless entry, navigation (gps lock for quick location), and much thats normally in a 'suspended state' so they come online quicker.. it also ages out the battery and over time starts under charging it, this is because batteries loose capacity with age and its trying to prolong its life since it does so much more than just start your car.. this is a fine strategy but it needs to be reset when putting in a new battery, or your new battery will never get fully charged and lead based batteries need to be fully charged for longevity.. under charging it persistently will quickly degrade it to the state the old battery was in (which was a gracefull derating over longer time).. When I reset my BMS the alternator voltage went from like 12.8v to 14.4v, quite a difference.. now as voltage drops amperage goes up for a device consuming same wattage, this can induce noise in sensitive sensors.. likely with the low voltage state w/the crap battery, your knock sensor was under powered and detecting that seemingly typical shutter you get as a misfire.

If your going to spend near $600 to have someone change your own battery, then chasing down an oil leak on this is likely far beyond your capabilities.

VW of Puyallup, WA gave me a quote of $446.00 just for the battery not counting the labor. That’s why I ordered mine online and brought it in.

Yesh, find your self a nice indy shop.. the pacific northwest is full of hippy VW shops qualified to work on your vehicle.. they can get OEM parts at near cost, which woulda been closer to $230, probably $150 of that online battery was shipping it.

a vehicle of your vintage has no reason to ever visit a dealer, thats just fastest way to hate your Audi really.. bet you a Touareg battery at same place, with same parts and same labor woulda been cheaper.. those 4 rings cost yeh.

Appreciate the advice. I was very hesitant to take it to a dealership. Yes, they are qualified, I don’t doubt that. But at what cost....
I was recommended a shop to take my Q7 to in Tacoma, WA called Mouse Meat that specializes in German vehicles to investigate any further issues I have. I know nothing about them other than a recommendation from a mechanic friend.

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I would definitely take my vehicle to Mouse Meat before a Stealership, no sarcasm here.. If one mechanic recommends another its a great indicator, they hate fixing other ppls problems and would never send you to a problem mechanic unless they just dont like you..

If they do german vehicles, the'd of had the diagnostics equipment on hand to change your battery serial number.. this is basically what makes a german specialist, each german company, BMW, VW/Audi, Porsche, Mercedes, Bentley, etc all use their own in-house deep diagnostics systems and any non-german shop will just be confounded and ran around in circles w/using generic equipment. and if they been in business they know all the quirks, have all the special tools and have likely worked on countless 4.2L Audi engines as they were very widely used for quite a while.. its like the 1.8T of the VW world, but back then very few Audi buyers would even look at a 4cyl.

Dealerships deal mostly with warranty work, customer dont pay.. mothership pays, they dont care if they throw 20 parts at it.. if thats what the checklist said to do then they will get paid.. When its not warranty work, they dont know how to do real diagnostics and just start going through checklists til your broke.. the'll gladly put $5k in parts in it and not even fix the issue.

I will say that I completely douched the engine bay in tons of engine degreaser because down near the front of the motor it was so caked in oil road sludge that you couldn’t track an oil leak if you even tried. I got that scrubbed off and cleaned up but the underside of the motor is a hot mess and without a lift to pressure wash the underside of the motor I’m not sure how I’m going to clean that mess.
Maybe ramps, a pressure washer, and a good tyvex suit for myself lol. I have a hard time justifying someone to clean the exterior of the motor when I feel like that’s something I should be able to do myself somehow.