Oil Consumption (sad face)

[RennSport911rsa]

She's always been driven hard, and this is a built motor, not stock. '91 C2, Engine has ~50k on her now. The original oil the engine was built around was RP XPR 20w50. Always changed within 3k miles though, without fail and it never consumed more than maybe 1/4 quart and thats with track days tossed in. Always used standard size K&N filter or OE Porsche.

In an effort to save a few pennies, I began (after talking to the engine builder) experimenting with some other oils. He suggested VR1 as well as Mobil 1. Tried both. Mobil 1 is when I started seeing some out of the back which was verified on the dipstick...she was beginning to get thirsty. Tried VR1, found it to be worse. From there, began trying various conventionals....GTX 20w50, Rotella T (non synth) 15w40, etc. Thirst got worse. I also changed it up to Mann, or Bosch (as consumption increased and I always putting fresh oil in the more she gets used, the latter 2 as they are cheaper).

Then I got nuts, I took the red pill, and now Im in the damn rabbit hole and I am exhausted. MMO, Auto RX, and most recently STP. All used in conjunction with conventional oil.

When I say thirsty, I mean it. A quart in 300 miles if I am on it. Maybe 500 miles if I take it easy.

No smoke generally on startup. No smoke when it is warming up either, no smoke during the first 10 minutes or so on the road. Then I can see it. Starts around 3000-3500 rpm. Below that, not visible. This is where it gets very odd (or not?). If I get on it good, a few solid floggings back to back to back all of a sudden...no smoke out of the back anymore. AKA the "Italian Tune Up". Well, except for a bit of black way up in the rpm range because shes too rich up there (seeing high 11's on the afr...no good for a non turbo!). Then if I go back to cruising, or get stuck in traffic for a bit, whether I open her back up full tilt or even just easily pass 3000-3500, smoke returns.

Of course the first thought is piston rings. I am really, really hoping it's not because of the downtime and the season we're now entering. And the fact that I tend to explore more rabbit holes during a full tear down. Could it be the valvestem seals, even though I am not seeing any smoke at hot or cold idle, no smoke at startup, regardless if she is left for several days at a time, or overnight.
The other thought I had, and perhaps foolishly, are there spark plug seals?

Or, am I just being a toolbag, and I need to yank this engine out and freshen her up. No odd sounds, I check the oil religiously to ensure things are ok, she's never down more than a quart (I have a larger pan, etc so total capacity is greater than stock to begin with)

Just yesterday I tried the STP stuff for the first time since she was a quart low. Havent done more than 5 miles so far, so no reports yet.

Does it pay to wait 500-600 miles, drain it all out, dump fresh conventional in there (nothing additional, run for 100 miles, re-drain, and go back to the RP? Or go to a different top line full synthetic? Or am I being way more optimistic than I should be and should I dig the hoist out to get her on the operating table?

[porterdog]

I've read that an injudicious valvespring/cam change can lead to destroyed valve stem seals.

A leakdown test will vet your rings.

1 qt/300 miles is a bit much to blame on incorrect oil.

[RennSport911rsa]

I hear ya....we're talking spyhunter levels when it gets bad, the freaky thing though is that after repeated hard runs, there is no smoke.

It always runs strong too, no loss of power. The leakdown test I suppose is next up to do, try some ATF during the test, see if the offending cylinder(s) pressure shoots up

Runs SS cams and springs, stock valvetrain otherwise, heads have sealing rings machined into them, JE pistons, stock race-prepped rods with ARP hardware throughout

[RedCoupe]

I would try "dry" and "wet" compression checks. You do the normal dry compression check, write down all your readings, and then do it again after squirting oil in the cylinders. I have a pump oil can that I can put a small hose on that works for this -you have to be somewhat clever because of the long reach to the plugs! The oil makes leaky rings seal better, but doesn't make any difference with valve stem seals, so if you don't see any noticeable difference after squirting oil in, you can assume the leakage is valve guides or stem seals.

[RennSport911rsa]

I have a similar can, will clean it out tomorow for that purpose, good call
yep, have used similar methods before, certainly helps narrow things down. The characteristics are just so odd, and not leaning one way or the other as far as cause...but the leakdown certainly will you're right

[RennSport911rsa]

Not that it is important in the big picture I suppose, but I also ran Redline 15w50 for 1 oil change (burned super fast) followed by a stint (2 changes worth) of LubroMoly 10w60 which was in between the Mobil 1 and the Redline as far as how quickly it got swallowed up. Mobil 1 was the best so far when compared to my initial XPR. But as suggested, could all just be down to things wearing for the worse, and the engine just getting thirstier

I was praying to the oil pumping gods that this was just a matter of deviating from what I "should" have been using, and what had previously served me well, but it appears there is something mechanical. Will do the compression test next week, but it's also due for an oil change anyway, so struggling with what to use this time around.

[aigel]

Sorry to hear.

I think your oil has nothing to do with it.

I am not following your description when it smokes ... it is a bit confusing.

1) No smoke at start up (the usual flat six puff)
2) No smoke when cold
3) ... smoke under WOT
4) ... smoke at hard decel after WOT (i.e. go off the gas at 5000 rpm)

Can you continue with short statements like this. I lost you at 3).

I'd definitely do a leak down for further diagnose.
If there is no puff of smoke at start up at all and no smoke at decel, I doubt it is the valve stems.

Good Luck!

G

[RennSport911rsa]

no smoke at startup
no smoke during extended idle, whether car is warm or cold
visible smoke at varying throttle inputs once car is fully up to temperature
visible smoke at WOT
occasional smoke at hard decel (rev to 5-6k, let off gas completely)
after several WOT runs have been done (smoke show), on subsequent WOT runs and hard decels, does not smoke

the last statement is where I am getting confused too, because if it were rings, it should be doing at any WOT event? It's almost as if crankcase pressure is too high, and then is only relieved after several WOT runs/decel events. Maybe time to see how my catch can is routed/vented

[porterdog]

You know, this forum doesn't get as much attention as some others. Your problem is curious enough I'd be tempted to try the 911 or Rebuilding forums.

[RennSport911rsa]

will see how this goes first, it's only up a day

[mreid]

I'm still ??? On the "larger pan"? What am I missing?

[aigel]

Rings can be intermittent, causing smoke sometime and not other, depending on how things line up.

Again, a leakdown test is your next step.

I also like the idea to post on the engine rebuild forum. Because that's where you unfortunately look to be heading.

G

[techman1]

Hmm. Not an engine builder, but...
Could the consecutive WOT runs create enough heat to close up the tolerances in worn valve guides?

[RennSport911rsa]

I wouldn't say it's an obession with the type of oil, just something that I have noticed since switching from what was originally run. Oil is certainly an easy thing to change, and a variable that has many possibilities. But with the miles it has on it now, even despite excellent maintenance, it may just be time for a refresh. Not bothered by that, I absolutely got my money's worth out of it over the years. A refresh sure beats tossing a rod that's for sure! The timing just stinks

The whole purpose was just as I said in my first post...brainstorm a bit since it's not doing what I would normally expect an engine needing rings (or valvestem seals) to do.

The larger pan I should have used more accurate terminology - simple aluminum spacer that was machined to allow for 2 extra quarts, with an extended pickup. Super simple.

Leakdown will be done later in the week, will go from there and I guess get the parts book ready, and see if I want to just refresh or use it as an opportunity to make some changes

[RedCoupe]

I'm confused! (Those who know me will say that's a normal condition!) Where would the "simple aluminum spacer" go? The C2 doesn't have a removable sump cover like the 911SC and earlier engines, so how would you get extra oil capacity, and where would you extend the pickup?

[RennSport911rsa]

I will snap a pic for you should it end up coming out and will ask my machinist to resend me the spec sheet on the engine

[sand_man]

I apologize in advance if you indicated it, but where do you keep your oil level? In in my 930 (911 Turbo), I always kept it on the low side. In my 993, I also keep it on the low side. When completely warmed up, so long as I see even a tiny bit of movement on the oil level gauge, I'm happy. Generally speaking, I have found that trying to maintain higher oil levels seems to also increase oil consumption.

[plymouthcolt]

I was burning a quart at 600 miles. It turned out to be worn valve guides.