Keeping oil out of the intercooler
Why I put an IAG air oil separator on the '04 STi, and what the install actually took.
Historically, EJ engines consume a bunch of oil, and the turbocharger doesn’t help with that consumption. And the way the engine is designed, oil can often times get into the intercooler, which can lead to engine failure and, at the bare minimum, lower cooling. My main concern was engine failure — rod knock caused by blow-by gasses reducing potential octane.
What an air oil separator does
A stock EJ vents crankcase pressure — the blow-by that slips past the rings — back into the intake through the PCV system. That vapor doesn’t come through clean. It carries oil mist with it, and on a turbo car that mist ends up coating the charge piping and the intercooler before it gets burned in the combustion chamber, where it drags the effective octane down with it.
The IAG air oil separator sits in the middle of that loop. Instead of routing crankcase vapor straight back into the intake, it pulls the vapor into a can first, lets the oil condense out and drain back down into the engine, and sends only the cleaned-up air onward. The unit is heated by engine coolant, which keeps water vapor from collecting in the can — the failure mode that makes a plain catch can in a cold engine bay more of a science experiment than a fix.
The install
Installation was easy and difficult. Easy because all I had to do was remove the intercooler, some PCV hoses, and the throttle body. Difficult because I needed to route hoses in hard-to-reach places and hover over an engine bay for more than 10 minutes at a time.
Overall the installation was relatively smooth and I’m glad it got done. Hopefully this extends the life of my engine and improves my fuel economy — unlikely, though. Enjoy the pictures below!
Oh! Feel free to view my car and mods on Under the Hood.
