By Chris Haag
First, try to reset your ECU. You can do this by pulling your negative battery cable and holding the brake pedal down for a few seconds. Let the car sit for a couple minutes then reconnect the negative battery cable. Then go do a short wide-open-throttle pull. You would only need to run the car from about 3000RPM - 4500RPM in 3rd gear to hit a peak boost target and then see the boost levels start tapering. If your boost levels jump up to correct levels, it is your ECU that is shutting down your boost control system to limit how much boost your car builds.
If your boost levels do not change at all, it is not tuning related and we are dealing with a mechanical issue with your engine and/or boost control system. Basically your ECU is trying to run higher boost levels, but your engine/turbo/boost control system are not allowing it. Here are a few common things that could cause this behavior on a Subaru:
- The boost control solenoid may have failed. This is the solenoid that the ECU used to bleed pressure away from the wastegate actuator and control your boost levels. When this solenoid fails there is no check engine light or notification of any type, other than your boost levels drop. This failure is starting to become more common as these cars are getting a little bit older. The best way to diagnose this failure is to find another solenoid to swap on your car. If your boost levels come back, then you need to get a new solenoid from Subaru.
- Verify that your boost restrictor pill is present and located in the correct spot. If that restrictor pill has been removed for whatever reason, it would cause your car to run very low boost levels.
- Make sure you do not have any post-turbo boost leaks. Check all connections for the intercooler and by-pass valve, making sure the connection from the intercooler to the throttle body is sealed up. That is a big problem area on these cars. If possible, try to get a pressure test or smoke test done at a local performance shop.
- Make sure you do not have any leaks in the vacuum lines for the boost control system. Since those vacuum lines are constantly heat cycled by the turbo, they become brittle over time and will crack and split. Verify that the vacuum lines are still sealed up and pliable. If they are not, it would be a good idea to get some good quality silicone vacuum line to replace them.
- I have seen clogged catalytic converters in the up-pipe cause low boosting problems.
- If none of the options above fix the problem, it is possible the turbo itself may be failing. The best way to verify this would be to remove the downpipe from the vehicle and spin the turbine wheel by hand. See if it spins freely and that there is very little (ideally none) play in the turbo shaft. If it is difficult to spin or has excessive play you may need to replace the turbo.