AHA! We have a winner!
And your mechanic didn't know to check this? FIRE HIM!
Ok, let me explain. The airbox drains are tapped holes in the block - one on each corner of the airbox - they're "kitty corner".
Their purpose is to allow crap that builds up and is liquid (or semi-liquid) to drain. Now the problem with this is that on a turbo motor there is a LOT of pressure in the airbox when the engine is running; it is exposed to full boost!
Ok, so to prevent that from being dumped (and harming performance) we put a check valve in there. It has a small spring in it so its open under low pressure, but closes as boost comes on.
Except for one problem - these little valves get gunked up and then stick either open or closed. If they stick closed, the gunk builds up until you aspirate some of it into a cylinder, and major damage is done.
If it sticks OPEN, the crankcase is pressurized.
Why? Because Detroit, in its cowtowing to the EPA, ran the returns from these drains TO THE CRANKCASE! Now that's about the LAST place you want something that gets into the airbox to go (it could be soot, dirt if your airfilter bypassed, etc) BUT the EPA had a schniz fit about them dumping it on the road (which they used to do), so that's where they piped it.
What you want to do is REMOVE that crap. Get some pipe plugs (1/4" should be the correct size) and put them in the holes in the oil return area (block and/or oil pan) where the pipes currently go. You may make a mess removing them depending on the inclication of the engine and oil fill level, so be quick.
Now, what you'll have are two open holes in the block which go to the airbox.
You're going to get yourself some Street Ells that thread into those, some hose barbs, some fuel-rated hose (pick - it doesn't have to be Aeroquipt stuff) with appropriate clamps.
You put the Ells into the block and screw in the barbs. Use pipe dope so you don't have any leaks. Aim the barbs SLIGHTLY downward.
Now here's the trick - you want the hose to angle downward - NO UPWARD RUN - all the way to somewhere convenient. Ideally you want BOTH SIDES to come to a "T" somewhere convenient. Run both hoses to the "T", with the "T" open end pointing DOWN.
On the open end, attach ONE of your airbox check valves you removed (you removed two, remember?) after cleaning it with brake cleaner (spray it real good and get the crap out of it.)
On the OUTLET of that check valve you can either (1) run a hose to a crap can to collect the puke, or (2) just run the hose down to where your bilge diapers are under the engine and change the diapers. With the valves working correctly you'll get some gunk out of the hose but not a huge amount.
You should have air coming out of the check valve hose at idle, but as you come on boost there should be NOTHING coming out of the hose. If there is the check valve is stuck open (again)
The valve should be removed at every oil change and cleaned with brake cleaner. Now you know why you relocated it where its convenient - so you can get to it to clean it easily! On lots of these engines one of them is behind the starter and is a REAL BITCH to get at.
If your airboxes are routed to the crankcase I'll bet 'ya $100 the problem is a stuck-open check valve. $50 worth of hose, clamps and fittings will fix it permanently.
