Oil leaking into throttle body from valve cover breather

[E30is90]

About 6-months ago I had someone replace my valve cover gasket, rocker shaft plugs, valve cover breather hose and do a valve timing adjustment. This past weekend I opened up my air filter box to clean the filter and found oil all over the inside of the box, the air intake bellows and traced it back to the throttle body. It was a big mess. In the 21-years owning the car, I've never had oil leaking into the throttle body and air intake prior to me having the above parts replaced. I don't want to immediately blame the guy who did the work, but it seems more than a coincidence that oil is coming in from the valve cover breather the first time I open up the air filter box after this work was done.

What could have happened/gone wrong with doing that work that could cause oil to be sucked up from the valve cover breather and then spill into the throttle body? Is there an easy fix or something to specifically look at? Any help would be greatly appreciated.

Thanks!

[jlevie]

Oil in the intake is normal on a higher mileage engine. It results form increased blow by past worn rings and from worn intake valve seals. However, any leaks into the crankcase (dip stick o-rings, oil filler cap, valve cover gaskets, or oil return tube seals) will aggravate the issue.

[nando]

check for vac leaks. Specifically the drain/vent tube below the intake manifold. There's a couple o-rings that can go bad, or the tube might not be seated properly into the intake manifold.

[E30is90]

Oil in the intake is normal on a higher mileage engine. It results form increased blow by past worn rings and from worn intake valve seals. However, any leaks into the crankcase (dip stick o-rings, oil filler cap, valve cover gaskets, or oil return tube seals) will aggravate the issue.

Thank you for your reply. I read about "Blow-by" from worn piston rings happening in older vehicles and that is a possibility. I know that a compression test could be done to check for this. But, I didn't have a drop of oil coming into the throttle body and dripping down into the air intake, prior to me having this work done 6-months ago. I have an AFE air filter that I clean out every 6-months and never had any trace of oil before this work having been done. I'm still thinking that something wasn't quite put back together properly when these parts were replaced. Any ideas of where/what to check would be appreciated.