S4 Sputtering, Knocking and Backfiring on WOT

[nb1117]

Hey every. Having a problem with my 2019 S4. It’s been sputtering, cutting out and even backfiring at 5.5K rpm to redline every time I go WOT. No CEL or codes have popped up. First time it happened was just a pull merging onto the highway from 30mph, slowly getting on the gas, when it cut out and sputtered at 6k ish rpm. Sent logs to 034, which admittedly were not the best of quality, to analyze but they said fuel/ignition and boost all look fine. Logs did show -1 to -3 degree of knock across all cylinders during these sputtering events. My brother was driving behind me one of these times and said some dark smoke came out and then some very load pops before a small flame.

I am stage 2+ E85, and have been for 5 months, but ran 93 in the winter months, just swapped back to E85 before this problem started. I’m at 86k miles as well. Oil changed every 5K miles and new Brisk plugs gapped at 0.024” and 034 coils 8k miles ago. Ethanol testing consistently at E70-E73. Yesterday I put in a new set of oem plugs, and still using the same coils. Car still sputters/backfires but not nearly as bad as before. Will be putting back in oem coils and logging again.

Anyone experienced this before? Some help/ guidance would be appreciated.

[GoCougs1234]

I had the same symptom in my APR stage 3 SQ5, including no DTCs or CELs. Turns out I was overboosting and what sounded like backfiring was actually boost getting cut super quick. It happed only near/at WOT in the upper RPMs of gears 1-3.

APR had tuned stage 3 for the stock intake however I had installed an aftermarket intake. Due to the freer flowing aftermarket intake, boost was building faster in the lower gears than the tune could control and the overboost safeties would kick in.

APR at first was miffed since there were no DTCs or CELs, but other stage 3 customers started having the same problem, and APR traced it to the aftermarket intake. APR fixed the tune (more feedforward gain on boost control) and all was 100% fixed.

Probably 1% chance that's your issue - but that's my input!

[handspin]

Second the over boost cut thing on lower gears with E initiated from WOT. Not 93 or via holding downshift / auto-select lowest gear for accel. New tune revisions may be seeking more boost with built in safety cuts for fallback. Fortunately those are not overridden!

[TJay74]

How many miles are on the S4? Has the water pump been checked for failure and or coolant migration?

[nb1117]

Wow. That could be it cause it’s been happening on cool nights with really low DAs. I have a guy near me who I’m gonna have check it over

[nb1117]

How many miles are on the S4? Has the water pump been checked for failure and or coolant migration?

I’m at 87k miles. Did the water pump, thermostat, shut off valve and the solenoids 3k miles ago

[nb1117]

Second the over boost cut thing on lower gears with E initiated from WOT. Not 93 or via holding downshift / auto-select lowest gear for accel. New tune revisions may be seeking more boost with built in safety cuts for fallback. Fortunately those are not overridden!

Yes. It’s never happened on 93, only on E. Every time it’s when the car kicks down or I go down to a lower gear and go WOT.

[nb1117]

Will be swapping back to OEM coils tomorrow from the 034 coils. New symptom or issue happened last night tho. Had a Fuel Rail Pressure Too Low code pop, giving a 3.5K rpm limit. Cleared the code and car is driving fine. I think it may be due to running E in near freezing temp. Temps got real low the past two days

[nb1117]

So I decided to do a log on the way home from work just to get some data. Logs show knock across cylinder 3,4,5,6 with 5 being the worst when I’m just cruising. Knock was mostly >1 degree with one spike at 3 degree It was another cold night but new coils will be in tomorrow

[griffyn211]

Had this same exact issue on my ’19 S5, after looking at logs determined it was fueling related so I removed my HPFP and found scoring on the 034 piston and found it was completely worn out only a few thousand miles after install. Contacted them and they sent out a free replacement which fixed everything. Had the exact same symptoms as you.

[nb1117]

Huh. I will check it out. I was getting a tapping from the hpfp noise so I took out my cam follower and it had some scoring and the plate that the hpfp spring presses onto was wiggling around. New oem cam folllower got rid of the tapping. I went from e85 to 93 octane tune currently. Much less cutting out, indicating fueling issue as well