2008 BMW 335i E90
N54 / MSD80 (I8A0S)
6-speed manual
Stock twin turbos, ~235,000 km
Mods:
- JB4
- MHD
- BMS dual cone intake
- ******* *********
- Upgraded intercooler
- Chevron 94 octane
I'm trying to diagnose persistent overboost and would appreciate some help determining whether this is a boost-control/vacuum issue or a mechanical wastegate/turbo issue.
BACKGROUND
I was originally running the MHD Pump US2 JB4 backend flash with JB4 menu 4/2.
I was experiencing boost surging and overboost. Lowering JB4 menu 12 from 2400 to 2200 to 1900 was my first try which didn’t fix it.
To remove the backend flash as a variable, I have now:
- Flashed MHD Stage 0
- Only selected the aftermarket-******** option and changed idle to 700 RPM
- Changed JB4 to menu 4/3
- Put JB4 on Map 0
The overboost problem still occurs.
LOG
I've attached:
P32_M1_R4_261013_1225.csv
At approximately timestamp 1879.94:
Pedal: 100%
Boost target: ~8.0 psi
Actual boost: ~17.3 psi
Throttle: closes to ~35%
WGDC: ~18%
So the car is making roughly 9 psi more boost than requested even though JB4 is commanding relatively low WGDC and the throttle is closing trying to control it.
I am avoiding additional high-load/WOT pulls until I understand the cause because the overboost is already significant.
VACUUM / BOOST SOLENOID TESTING
1. I disconnected BOTH boost-solenoid OUT hoses going toward the wastegate circuits.
Result:
The car produced essentially no boost, around 0–1 psi.
When I reconnected the solenoid outputs, the abnormal boost returned.
This seems to confirm that the turbos can bypass exhaust with the wastegates uncontrolled/open and that the excessive boost depends on vacuum reaching the wastegate system.
2. Vacuum supply to each boost solenoid:
Approximately 25 inHg at the VAC/IN ports.
3. Solenoid OUT nipple test:
I disconnected the wastegate hose from each solenoid and connected my vacuum gauge directly to the OUT nipple.
Once warm, both were around 10 inHg at idle / during the basic rev test.
4. JB4 menu 10 test:
Both boost solenoids respond to menu 10 and can vary vacuum approximately:
0 inHg → 15 inHg → 25 inHg
Front solenoid was very repeatable.
Rear solenoid initially showed approximately 19 inHg at the same ~1500 RPM/menu-10 position where I expected around 10–15 inHg. After cycling it to minimum and maximum, it subsequently showed around 10–11 inHg.
So both solenoids definitely have the ability to change their output rather than simply being stuck fully open.
5. I also checked vacuum at a disconnected wastegate hose end and got a similar static reading, but I have NOT yet done a proper dynamic test through each complete wastegate circuit individually while revving/driving.
I also haven't yet isolated whether vacuum could be crossing between the two wastegate circuits.
WHAT I'M TRYING TO DETERMINE
Given that:
- Actual boost reaches ~17.3 psi with only ~8 psi requested
- Logged WGDC is only ~18%
- Throttle is closing despite 100% pedal
- Removing vacuum from both wastegate circuits results in essentially zero boost
- Both solenoids respond through a wide vacuum range when commanded with menu 10
- The problem remains after removing the JB4 backend flash and running MHD Stage 0 / JB4 Map 0
I'm especially interested in what test would best distinguish a mechanical wastegate/turbo problem from a vacuum-control problem before I condemn the stock turbos.
Thanks. Attached Files P32_M1_R4_261013_1225.csv (399.0 KB, 2 views)
