My 99 318 is burning a ton of oil
 

hello,

No have a 99 318 4x4 and it's burning a couple quarts of a oil every couple weeks. I checked it today and there is practically no oil on the dip stick and 3 weeks ago I added 2.5qts of oil. I can't find any external leaks and I don't have the plenum gasket anymore.

Replace you pcv valve. And I've heard get a dealer pcv because they have the right flow rates? Don't quote me, but try a new Pcv.

Did you upgrade the intake for one that doesn't have a plenum? Or just fix the plenum?
I'd double check the pleanum. Just to be sure, look down the throttle body. When you did the pleanum though, did you use the aluminum plate, like the Hughes kit, or reuse the steel plate? Did you replace the pleanum bolts? Known for being too long.

+1^ on the new (factory) PCV.
You might want to add a catch-can to get anything the PCV misses.

I had the same problem. Horrible oil consumption, but, no clue where it was going. The engine would smoke a bit on startup first thing in the morning, but, other than that, nothin'. No oil would ever make it to the ground either.... So, I guessed that some was leaking from the valve covers, and then burning off on the exhaust manifolds, and the rest went out the tailpipe........

Do a compression test. Dry first, then wet. See what your numbers look like.

mine did this when plenum let go 1.5qt in less than 200 miles

I upgraded my manifold to one that doesn't have the plenum gasket, so it came be that. I'll Definetely look into the pressure test and occasional valve.

sorry I missed the upgraded manifold part, if you haven't done it yet pull the plugs and look for fouling that will at least help you isolate what cylinder/cylinders

No leaks means it's burning it. A lot. What weight are you using? Mine used to drink 10w30. Switched first to 15w40 diesel oil but settled on 10w40. Now uses 1-2 quarts per change. Which is normal for me. Had an old mechanic tell me to use straight 30 with high mileage engines. Never did but interesting. New trucks come with oil that has the same weight as water.

And I second the Mopar PCv. That also worked for me.