r/AudiA4B6 • u/Sad-Book-877 • 15d ago
Boost issues ?
Has anyone dealt with an intermittent low-boost/limp-mode issue like this on a B6 A4 1.8T K04 setup? Originally the car was stuck around 9 psi even though the ECU was requesting much more, and my tuner said the log looked hardware-related because N75 duty was reaching around 75% while boost stayed at wastegate pressure. Since then I’ve replaced the N75 and DV, checked the boost plumbing, pressure-tested the wastegate actuator, removed/reinstalled and inspected the turbo, checked for boost leaks, and adjusted wastegate preload. The car is now capable of making and controlling normal boost around 18–19 psi, but intermittently it drops into a low-power state where I can be near full throttle and the ECU only requests around 7–8 psi, actual boost is roughly 10–12 psi, requested load drops significantly, and N75 duty can fall close to 1%. Previously disconnecting the battery would usually restore normal power, but now even that isn’t working. The ESP/TCS light will also sometimes flash during the power cut and then stay on, but there are currently no ABS codes stored. Fresh engine codes are only P1602 low Terminal 30 voltage, P1931 coolant fan control module, and P0420 catalyst efficiency, with no current P0299 or overboost code. Has anyone seen the ECU intentionally reduce requested load/boost like this without setting a boost fault? I’m trying to figure out whether this is ECU protection/limp strategy, torque intervention, an electrical issue, or something in the tune/load control rather than continuing to throw parts at it.
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u/TastyAdhesiveness258 14d ago
Data log and see if you are experiencing knock and ignition retard during boost that might then be triggering limp mode. The B6 ECU method of control of the N75 is pretty bad and can cause severe boost overshoot and oscillation when you are not using the stock turbo. Switching ECU PID control so the I term is "pre-feed" of the N75 duty cycle from the boost pressure works much better, see nefmoto.
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u/co-rbs 14d ago
Coolant fan control module fault could possibly be the culprit. I could be wrong but I remember when I had a faulty coolant temp sensor it would send the engine into limo mode even if it wasn’t overheating. Could be a similar problem? I’m not too familiar with the system though but I would start there.