An oil usage question
 

Background: I recently finished up a rebuild following a spun rod bearing 'event'. Stock '05 STi. Rebuild included JE skirt-coated pistons, new OEM Sti crank. complete valve job. Car runs fine, has ~2000 mi on the rebuild. I'm using ~1 qt/500 mi on Rotella-T 15W-40. No smoke out the tailpipe or on cold (SoCal) startup. No apparent blowby when removing the oil filler cap. In other words, no clear outward signs of oil usage. The exhaust pipe tip has some black 'dust' deposited on it.

I was (and still am to a degree) wondering about the PCV valve. Thinking that maybe the crankcase might be getting pressurized if the PCV was not working right, I did an experiment. I drilled and tapped the oil filler cap for a hose barb, and connected tubing and a compound pressure gauge. I placed the gauge on the driver's seat where I could see it while driving. Then I drove around. Some WOT, boost, partial boost, downhill coasting. The pressure gauge showed virtually no change in pressure, either positive or negative. In other words, by direct measurement the crankcase is not getting pressurized, no blowby, no boost getting into the crankcase...

So here's another thought. After my rebuild my oil pressure is high-ish. On a SoCal cold startup with the Rotella-T 15W-40 the oil pressure climbs to ~85 PSI at idle, where I assume it is probably hitting the relief valve pressure. After full warmup I get ~82 PSI at 3000 RPM, 25-30 PSI at idle.

Question: Is it possible that at sustained 80+ PSI some oil is getting by the turbo oil seal? Enough to consume the quart in 500 mi? In my reading about PCV, turbo oil seals, oil pressure, and so on it seems this is a possible scenario which could explain my oil usage. I'm considering trying a 'lighter' oil like maybe a 10W-30, and see if this lowers the oil pressure, and if that leads to less oil usage.

I would appreciate any comments and suggestions on resolving this problem. Thanks all!!

First, I'm not sure a pressure gauge on the oil cap is going to tell you the crank pressure. It's going to tell you the driver's side head pressure. The only time the heads will see pressure is when the crankcase breather can't keep up with the crank pressure and it pushes oil into the heads. So seeing nothing on that gauge is good, just not a sign that there's no crank pressure.

However, since you just did a rebuild, I'm doubting crank pressure is the culprit, unless those new forged pistons are really loose. Now, you would expect a little extra oil burning w/ the forged pistons, especially on cold startup (are you hearing any piston slap?), but probably not enough to account for your issue.

You might be on the right track regarding the oil weight. I only ran 15w50 when I had a built motor that I was racing in high-temp/high-altitude conditions. I wouldn't run it on the street, I'd stick w/ a good 5w30 recommended by Subaru, like RedLine or Royal Purple. But those oil pressures don't sound unusual. IIRC, that's around the pressures I'm seeing... then again my oil pump is oversized and shimmed... so it might be apples to oranges to compare.

As far as oil in the turbo... what sort of turbo do you have, and how many miles are on it? Is there oil in your intercooler? Do you have a catch can? Is it filling up?

A proper catch can setup on the valve breathers, and second on the PCV/crank breather will immediately tell you if your issue is blow-by/crank pressure, since all the oil you're burning will instead be in the can(s). A check of the I/C for oil will tell you if there's oil getting past the turbo seals... at least on the cold side. If the oil is getting into the exhaust via the hot side of the turbo, I'd expect to see oil smoke.

Hey Steve...

I bought one of the Grimmspeed AOS to go on my rebuild so I never have to deal with oil in the PCV/intake again.

That oil sounds very heavy, even for break-in. if you are still in break-in, I'd use Valvoline 5-30 Dino oil, possibly with the Custom Cams additive"

How noisy are the pistons at cold startup? Do you know what P-W clearance your builder used? <=> .0025"

Thanks for your reply.
I don't see smoke, but possibly at sustained 3000+ RPM I might not be noticing some smoke if it is happening.???

Hey Dean!
Good to hear from you.
How's everything going? By chance are you going to be at Pahrump? I'll be there...
-Steve