Many stock turbo cars "piss" all sorts of oily crap and contaminants into the intake path and a lot of it gets caught in the intercooler. On my car, for example, the valve cover breather vented into the intake tube, charcoal canister vented into the intake tube, EGR into the intake manifold, pcv into the manifold, pretty much everything in my intake path was filthy inside from the inside of the intake tube to the intake ports in the head- I had to scrub them with a wire brush to remove an 1/8th inch buildup of gunk. The intercooler took 5 rinses with gasoline before it stopped pouring out black... cleaning it improved the efficiency a bit, I'd bet .