RallyDogRacing

I think I have exhausted my internet & forum searches for ideas, and just about completely exhausted the realm of DiY actions before giving up and taking the S4 to the shop. Quick background: 2001.5 S4, 116k miles, just purchased dirt-cheap as a some-day track car.

Current status: when test driving the car, as soon as WOT is invoked, or boost pressure exceeds 3psi-indicated; the torque output of the motor falls-off and boost pressure ceases to rise past 5psi-indicated. Audibly the exhaust exhibits a fluttery noise, accompanied by the boost-gauge needle bouncing/fluttering between 4 and 6psi. Driving the car at 3psi or less (including vac) at all RPM ranges is very smooth, excellent response.

VAG-COM codes: none in normal operations. Intermittent cat warm-up out of range, which when cleared does not return with any regularity.

Trouble shooting steps performed:
1. Pressure tested intake track. Plugged PCV line from intake Y-pipe, plugged vac-source to vac-cannister. Tested with constant source to 12psi-indicated on the boost-gauge. No escaping air noises. Definite oil-gurgling noise in oil-pan.
2. Verified WG actuator rod movements. Confirmed opened and closed positions. Default is closed position.
3. Verified N75 function. Tested with N75 to WG feed disconnected / plugged. 12psi-indicated was achieved - but flutter and torque drop exist pass 4psi as before. Boost ramp degraded over N75-attached state. Returned to normal function.
4. Rebuilt & re-checked Stramosphere BPVs. Units were internally dirty and slow to respond. Rebuilt after cleaning. Confirmed with MightyVac that pistons moving and holding vac.
5. Bypassed N249 valve. Per some older articles about possible BPV wierdness due to N249 valve, removed and re-routed lines to eliminate N249 interferance of BPV operation. BPVs functioning on manifold pressure directly. No appreciable change.
6. Replaced PCV-spider-hose, driver's side valve-cover gasket. NOTE: did not locate actual PCV valve to replace if not integral to spider-hose.
7. Replaced plugs w/new iridium. Granted I used cheapie Autolights, but that did eliminate all misfire codes that existed when car was first purchased.
8. Replaced air filter with OE unit
9. Oil & filter change to synth 5w40
10. Swapped Stratmosphere BPVs with known-good N710 units from a friend's S4. No change, returned N710's to owner and re-installed Stramosphere units.

Last ideas for next-steps before I give up and take it to a dealer.. All opinions are welcome.
a. Pull exhaust after cats/down-pipes and test-run to check for exhaust clog.
b. Pull driver's side cat/down-pipe and replace with known-good. NOTE: pass side cat was replaced by previous owner and shop noted a recommend to replace driver's side based on warm-up DTC.
c. Pull intake piping from inlet of turbo cold-side housings. Check for linear travel as evidence of bad seals / split shaft.

If anyone has other recommendations - I am definitely up for trying things. I'm pretty much out of ideas.

Rob D.
Houston, TX
'84 UrQ - 20vT
'07 Touareg - V10 TDI
'01 S4

jamesdmanley

oh cool another houstonian! wish i could help but my s4 was wrecked before i got a chance to dig into it. still waiting on settlement and looking for a replacement

rktskicar

What sort of ECU? Can you run a block 115 VAG log (3rd gear, 3K to redline) and post results? Sounds like limp mode or valet mode if you have a multiprogrm ECU.

Bruce