2014 audi s4 engine noise

Me and wife just bought a 2014 audi s4 dsg, stage 2 apr supercharger pully, exhaust system was completely stock with cats, car was drinking coolant and a bit of oil, (only had for about 1000 miles), missfired on 6,5,4 and the engine was making a tapping noise, and the car completely had shut off, constant missfires, had it towed to a shop that works on these cars, Basically, I was missing on 6,5,4, driver side bank, and there was a noise from the engine, started it up after pcv replacement, seals etc, car was still missing and still making that noise, They started tearing back down, compression tested those cylinders, compression was 130/130/129 on 6,5,4, he said he unhooked the o2 sensor and the car ran with no misses even when revving, they boroscoped the cats and they were melted inside, very clogged, badly, with the o2 sensor unplugged, it still has this noise but no missfires, but as of right now the exhaust is only exiting the small bung hole for the o2 sensor, and the mech thinks that it is still pushing back up into the engine, and that is the cause of this noise.

Looking for other opinions on what this noise could be, will be having him take the cats off regardless.

https://youtube.com/shorts/UER2z5nl9Qo?feature=share

If it’s still losing coolant it’s the intercooler cores. Taking off the supercharger the mechanic should have noticed either bad residue (if missing with blowby oil) in the intake runners in that side or somewhat cleaner if it’s just coolant dripping. If you’re losing that much coolant, it’s all going into the engine and it’s not good. Replace them both, even if it’s just one of them.

If the CATs are melted, you need to replace them. Either way, they need to be done.

Supercharger bricks are good, after pcv no coolant loss, more concerned with the sound of the engine.

Very well could be exhaust going back up into the cylinders and causing back knock, but it’s hard to say. If the CAT is truly clogged, then get it out of there and see where you are after that. Running the engine without sufficient exhaust flow on a bank isn’t good. Also taking out the wideband O2 on that bank isn’t good either. It’ll run because the ECU will probably flip it into safety fueling mode (rich), but if it didn’t do that, it could also be trimming fuels back down lean which will compound chasing down the problem. If compression is the same across all cylinders, replace the CATs and see what up once that’s ironed out.

Also, if coolant was being lost via the PCV then you had coolant mixed into your oil. If that happened with a big amount of coolant loss, then you could be running on crappy lubrication which could mean bottom end problems. Check your oil cap for residue and maybe do an oil change to be super safe. If it’s milky and you change CATs and you still have that knock and it follow RPM change/frequency then you know where you are. Hard to believe your coolant loss issue was due to PCV and you didn’t have coolant in your engine oil making things worse. Maybe they didn’t check at all because they didn’t know.

There is also another popping sound that can happen with the latest PCVs when idling for extended periods of time which go away when you remove the oil cap, but it’s hard to tell from your video if that’s the sound or if it’s something else. It’s not very clear.

Also, FYI, you’re on the low limit for the compression test on that side for all cylinders. Like really low. Without knowing what the other bank tests as, it’s impossible to know if that one bank is gone or the engine is just worn. Hard to believe a 2014 has that bad compression without some damage be it bent rods or something else. I will say this, IF and that’s a big IF, your initial coolant loss wasn’t from the PCV (which I don’t believe it would have been unless you can flat out prove it and it doesn’t sound like you’ve driven the car enough after to really tell) chances are good the coolant loss was due to that bank’s IC core leaking into all cylinders and if it was bad enough, you probably hydrolocked it a bit and it’s also what contaminated the CAT to clog it up so badly. Now, with slightly bent rods, your compression ratio is low across that bank. That’s absolute worse case scenario.

If this was mine, I’d do a compression test on all cylinders and see. If the passenger bank is much higher, I’d call it a loss. In all honesty, the bank with the problem could also be washed out by the extra gas the engine was trying to throw at it to stop the misfiring and that could lead to low compression numbers as well, but if you’ve run the engine for a little bit, that should all come back up. All these results all depend on when in the timeline the tests were done, etc. You could just take the exhaust all down, take the CATs out and start the car and run it up to temp for a bit. Then do another compression test and see what’s up. If things are even across all and if the problem banks comes up and there’s no more misfires, then you could be OK. I would do at least that if it was mine as it’s not hard to yank the downpipes and CATs out. Hell, you know they’re busted, so you can cut them to get them out without having to remove stuff to wiggle them out if you’re DSG. That will cut down on time to do that. Evaluate the engine health after that, it could be fine.

The big variable for me here is the problems you’re reporting and the fact that we don’t generally see heavy coolant loss from PCV on the newer revisions which a 2014 would have. It’s super rare and when it happens, contaminated oil is easily seen. I seriously doubt that was your coolant loss issue. I’d be more inclined to say your coolant loss was due to the driver side bank IC core leaking a lot, which will cause misfires, a clogged CAT and if it was leaking really bad, messed up engine due to hydrolocking, which would explain the low compression numbers on that side and engine knocking.

FYI your coolant level is low in the video too, so either they didn’t fill it properly after doing the work or you’re still loosing a lot, which is why I keep saying IC core caused all your problems. Good luck with tracking down your issues and fixing it.

The intercooler bricks were pressure tested, no signs of leaking and appear to have been replaced recently (previous owner probably), the pcv was very bad, coolant and oil when i pulled it out.

The video you posted… did the tapping noise start after you replaced the PCV? Like is this the first startup after that and while you just let it idle? If yes, what part number PCV did you install?

The tapping noise started before the pcv was replaced, no whistling, it was definitely cracked though. I’m unsure of psrt number, was updated one from fcpeuro

could be one of the belt pulley. but yeah, this engine is probably nearly dead, I'd ask to borescope the cylinders as well. all that stage 2 heat with stock cats might not have gone well for these cylinders

I see.