B8 S4 random misfire (really random)

[HOB0HUNTER]

I've got a 2010 S4 with just shy of 115k miles that's giving me a really random misfire. It tends to happen one time and then nothing, sometimes for a few weeks to a month. The only codes I'll get are the random cylinder misfire code as well as 4-5 other cylinders all getting a single misfire at the same time, the most recent time I saw every cylinder except for cyl 5. I never see any dashboard indicators flash when this happens and the only reason I even know it occurs is seeing those codes present when scanning to solve some other issue. Looking at the logged time of the misfires it seems to be happening on cold start.
I've already replaced plugs and coils, plugs were startlingly bad looking compared to any others I've ever replaced but this is the first DI engine I've owned and they actually looked just like a number of B8 S4 plugs I've seen pictures of on a google search. I haven't cleaned the intake valves yet but if the carbon buildup on my plugs is any indication they'll be pretty gunked up.
Last year I had the supercharger off to replace the hoses that go from the valve covers to the PCV and I didn't have new gaskets or that little tube that goes up from the PCV itself, I wasn't expecting to have to pull the supercharger when I did and didn't have time to wait on gaskets before I needed to drive the car, so I took that risk. I didn't have any misfires for probably six months after putting the supercharger back on. Maybe my problem is related to these gaskets and just didn't manifest for half a year?
The only engine mod I've done is an APR intake with the stock intake tube, but I'm pretty confident that wouldn't cause this issue.
I think I'll poke around the engine bay looking for vacuum leaks and try logging some data with OBD eleven but would definitely appreciate some pointers if you guys have any ideas. Other than that I plan on swapping out the PCV this summer so I'll have a chance to swap out those supercharger gaskets and take a peek at my intake valve condition.

[SwankPeRFection]

Misfires will usually happen in two different scenarios and each one is an indicator of what could be going on.

1) Misfires happening at WOT with EPC and cylinder deactivation happening around 4500-5400 RPM on this platform. These are either old plugs that need to be replaced or CAT’s starting to clog up and restrict exhaust flow out of the cylinder when the engine is under extreme load/RPM. These misfires will also happen up in the 5K range if you’ve got an intercooler brick leaking. Cylinders 3 or 6 will usually be the first and repetitively happen each time since the bricks tend to start leaking at the back. Losing coolant and having to top it off fairly often is your number 1 indicator that this is an issue. Also, white frosted spark plugs is also a good indicator of this. (Spark plug may also look like the porcelain is brand new and somewhat clean on affected cylinders when you know the plugs have been in there for more than 3000 miles.)

2) Misfires at startup or idle. These are usually goofed up plugs, can be clogged CATs, especially if you don’t drive the piss out the car and don’t redline it and haven’t had a chance to see if it misfires at WOT. These misfires can also happen due to bad fueling issues due to fuel trims being all over the place for some reason. (Bad O2 sensors, leaky injectors, clogged injectors, etc.) Leaking intercooler bricks will also cause stumbling or missing during cold/warm starts intermittently but these tend to be felt rather than the ECU logging them.


A bit more info would be good to know about the car and your “other” engine issues you’ve been experiencing that you’ve had to scan the car for. Sometimes those other issues are a good indicator of related problems you wouldn’t think are related.

FYI, depending on how you drive the car and mileage and how long you plan on keeping it, it might be good to go ahead and replace your widebands if you’re seeing stumbly drivability/fueling issues and if you see wacky readings on the lambda readouts in VCDS.

Also, I know people will disagree with me on this but I’ve run the CRC GDI cleaner every 15k or so miles on my car since early on and I’m over 180k on it now and the valves have remained non-caked up. If you’ve never done anything to do a carbon cleaning, it may be worth running a can through it and then again in 3-5k miles once you fix any intake contamination issues (leaky IC cores if you have that issue). You can spray it in via the PCV vacuum hose on the intake pipe right in front of the throttle body if you’ve got an aftermarket intake… super easy. Just follow the cam instructions to a T, it’s important the engine is hot before you start and you don’t left it sit more than an hour after to heat soak before you start it back up and take it for a minimum 30 min spirited/highway drive. Just read and understand the can instructions. This stuff has worked for me very well.

[SwankPeRFection]

Also, and this is important. If you do the CRC spray cleaner, when you start the engine back up after and hour, it will smoke out the back. Let it idle for a min or so and then do small revs before you start driving it. Then for about 5 min, don’t drive it hard. Don’t redline it until a good 5 min in or a few miles and slowly work your way into redline pulls (increasing rev range little by little with each shift without a lot of accelerator load until you’re ready to go WOT on subsequent ones). If there’s a ton of shit on the valves that had to come off, this gives the engine enough time to knock all that junk out without a ton of load/stress, which could introduce a misfire or bad stumbling. You’ll get a feel for it once you do it and feel the engine is smooth enough to romp on it a bit and then continue driving it at highway speeds for a bit.

[knock.out]

Have you considered the catalytic converters? Common issue on our cars and usually manifests early on with misfires before potentially significant failure.

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[HOB0HUNTER]

Awesome, thanks for the reply! The other engine issue I've been scanning for is a vibration or shaky feeling in the engine predominantly at low speed and idle. It feels a bit shaky but RPMs are steady, fluctuating around +/- 3RPM at idle. I've had the scanner on multiple times while this is happening and the engine isn't detecting any misfires during this. I've looked on the forums about this and there seems to be some consensus that these engines can feel a little shaky. I had a B7 S4 that I put ECS motor mounts on and this feels somewhat similar but stronger. I haven't remembered to look and see if the motor mounts were replaced with stiffer aftermarket ones.
I've attached a picture of my plugs, there's definitely some white buildup on them so it looks like I'll be replacing the intercooler bricks. I'm not sure how old the plugs are, they're AUDI branded so it's possible they've been in the car for a good while. I don't know enough of this car's maintenance history to know if the intercoolers have ever been replaced, but I'd bet they're original.

I'll definitely look into CRC GDI cleaner, thanks!

[HOB0HUNTER]

I'm hoping they're not the issue but I'll look into how I can tell

[B7TitaniumA4]

At 115k have you had your chains, guides and tensioners replaced? Do you have cold start rattle?
I didn’t see it mentioned in your OP, on cold start you could be slightly out of time if your tensioners aren’t fully engaged by oil pressure and it might be just enough that sensors aren’t catching a timing issue yet. Just a thought, at 97k miles I had start up rattle and when I opened my engine to replace the chains all of my guides began to disintegrate and I found many pcs in my oil pan and pickup tube from before I opened it.
Again just a thought.


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[HOB0HUNTER]

To my knowledge none of the chains, guides, and tensioners have been replaced. So far I don't have any rattling, whether at cold start or normal operation. Did you have any codes before you replaced yours or did you just have the rattle?

[SwankPeRFection]

Yep, looking at your plugs, you’ve got some leaking coolant from your IC cores in the supercharger happening on whatever bank the plugs on the right came out of. See that white frosting on the grounding side? That’s coolant. The shaking you’re feeling is the slight misfires the engine is dealing with at startup and depending on how bad the leak is, during normal running. I’m surprised you haven’t said anything about having to top off your coolant reservoir from time to time? Do you see that level going from max to min within a week or a few days driving? Have you had the car long enough to notice that? Other bank might be leaking a little too, but not as much and looks more somewhat normal with a lot of miles and some typical blowby.

[HOB0HUNTER]

I've only had to top off the coolant once since I pulled the supercharger off last spring, I just assumed I hadn't bled the cooling system enough when I'd put it back together. I've had the car for just over a year, so it must be just a tiny leak. I'll order both intercoolers and change out the supercharger oil while it's off this time.
I appreciate the help!

[SwankPeRFection]

Yeah, that is weird. Did you happen to look to see if there was any residue on the runners when you took it off? Usually you can see it. If it’s a fairly steady loss, that runner can actually look cleaner than the rest unless you’ve got a lot of blowby mixing with the coolant in which case it can look a dark orange/black caking.

Do you normally drive the car hard or not? With a leak in the cores, it kind of acts like a carburetor does… the higher the flow rate of air across the leak area will actually suck the coolant as it passes that spot… depending on where that spot is of course.

[HOB0HUNTER]

I didn't notice any residue on the runners but I didn't give them much of a lookover. I'll definitely take a close look when I take the supercharger off this time.
I've been fairly soft on the car lately, we just recently got out of wintery weather.

[B7TitaniumA4]

To my knowledge none of the chains, guides, and tensioners have been replaced. So far I don't have any rattling, whether at cold start or normal operation. Did you have any codes before you replaced yours or did you just have the rattle?

Just noise and leaks so I pulled the engine to refresh.


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[HOB0HUNTER]

So, I finally got my replacement intercoolers and swapped them in. The old ones didn't look too bad, no obvious leaks. I had a real fun time getting the 12 mounting bolts off that hold the ICs in. It took a 1/2" ratchet to get any of the bolts out, and one of them sheared. It took awhile to extract the sheared bolt, it was on the lower inner side by the casting for the throttle body, not quite the worst place it could have happened, but close. Extractors wouldn't work until I'd drilled all the way through the bolt. It didn't look like any were cross threaded, but they must have had some serious corrosion binding them up. I retapped all 12 holes, and the one with the sheared bolt got a little mangled right at the opening but the threads were solid a few mm inside the hole. Got it all back together this afternoon and bled the coolant so we'll see if my intermittent misfires come back.
The engine still feels a little shaky, so my guess for that is probably just bad motor mounts but it's just a guess at this point since there's no motor mount codes being thrown.