NefMoto

I picked up a basket case B5 S4 with Frankenturbo F21's a while back - unknown history on the turbos and engine. It has definitely seen some neglect, but I've been working through hardware issues (boost leaks, incorrect maf housing, worn out stock clutch) and have got it behaving consistently now. The car has an 034 tune so I haven't made any software changes.

It seems to run fine, but when boost comes on it will consistently peak, drop, then build back to peak. I can't find anything in the logs that indicates the ECU asking for less boost/load during this period - no reduction in wastegate DC, throttle plate angle, or requested load and no overboost condition. For hardware troubleshooting I have tried another N75 that behaves well on a 1.8t with no change, sprayed charge piping with soapy water and pressure tested good, checked wastegate movement - both gates are closed at ~5 psi and fully open ~12 psi. My pressure regulator only has markings every 5 psi so hard to tell exactly where they crack open.

Attached is a short log with a couple instances of the typical boost fluctuation. I don't have a ton of experience with ME7 so let me know if I am missing any important parameters (2 logs spliced together - going to a store then going home, hence the time reset and jump in temps). Wondering if anything points to hardware issues. Any guidance is much appreciated.

You have no logs attached.

Trying to figure out why I can't attached files. Getting a timeout error when I try, but text posts seem to go through alright

Link to log file as a temp workaround:
https://www.dropbox.com/scl/fi/87fy15ckd1x16xdcgkqrp/b5s4_20240803_combined.csv?rlkey=r88bx9fjci0q14eq5wcc0rdrd&st=vjiogedf&dl=0

Boost calibration needs work, you have an overshoot and the pid is pulling duty out to try and manage the spike and the result is a loop of overshoot and under.

Thanks for taking a look. Something interesting I saw was even when boostactual doesn't exceed boostdesired there is the same oscillation and minimal change in wastegateDC.

The car was a basket case mechanically when I bought it so I'd like to convince myself it's not a hardware issue before getting into the software. Possible hardware causes could be bad N75, leaks between N75 and wastegate or boost reference, wastegates sticking, boost leaks. What else am I missing?