In my post, Cold Weather Fuel Leak, I mentioned that my Subaru Impreza was hit with the infamous fuel leak problem that seems to be affecting many of these early models. I haven't posted much of an update since then, but rest assured that no self-respecting car hacker would pay the dealer the $400 they wanted to fix this little problem. Instead, the engine bay was properly torn apart over the holidays and the entire fuel system, from the filter to the return line, was replaced with new rails, connections, and steel-braided lines. In doing so, the fuel flow has also been changed from a serial to parallel pattern, the injectors were upgraded with higher-flowing units off a Japanese Impreza, and a few extra engine sensors were wired-in for additional data-logging.
Everything came back together just fine (with a few extra parts left over for good measure) but I'm still re-tuning the fueling maps. The car runs properly while in closed-loop fueling (meaning the time where output from the oxygen sensor is used to determine fueling) but goes ridiculously rich once the boost comes on (and it moves to open loop fueling.) Since I haven't had time to debug this, I dropped the boost limiter down around 2psi, and just drive it on light throttle.
I have a couple theories as to what's happening, and hopefully this weekend I'll have enough time to dive into it further. The first possibility is that the injector scaling capability of the UTEC might not be working properly. The second is that several other people with similar setups have reported that in creating a better flowing fuel system, the base fuel pressure has gone up, which is why the injectors are flowing more fuel then expected. The final theory is that the stock fuel pump is no longer able to keep up with the new high-flow system. If this were true, fuel pressure would actually drop as boost comes on, and with low pressure, the injectors would be unable to properly atomize the fuel, thus causing unburnt fuel to leave the exhaust ports and be seen by the O2 sensor as running rich. This one is particularly dangerous. If true, then there is no telling how lean the engine is actually running.
Fixing the first problem involves software. Proving the second requires a fuel pressure gauge and OBD scanner capable of accessing fuel trim values (neither of which I have.) Solving it probably means an adjustable fuel pressure regulator. The third one is messy. It will also require a fuel pressure gauge, but fixing it will require an upgraded fuel pump and an adjustable fuel pressure regulator. Fortunately a buddy of mine has all the diagnostic equipment we need.
With something like 10 hours of personal time doing the labor, and another 10 to retune the car, there's no way I'm actually saved money (if you count time as money.) But where's the fun in just taking a car to the shop? Besides, I can now tell you what every hose under the hood goes to ;-)
If you're interested in taking a wrench to your own Subaru, I'd suggest starting with the following discussion board sites first: