I've done compression/leak downs on a few GTS's, one that had bad oil consumption and several that were low to average and the results were all close. With in 5% if I were to take a guess at the comparison. I didn't see anything in them that would leave me to believe that was the issue. I also found it odd that the one that used the most oil had the least amount in the intake throttle body. We have quite a few '93's here and two of them have the DR provent set up and the one that used a lot of oil, still used a lot of oil and the one that didn't....well, didn't. Both are '93's. Another '93 we had here used 1 qt every 400 miles or so and the intake was dry. He's since sold it so I can't look any further.

Of the '94's we've got around here or I've played with, 3 of them didn't use oil but another one (low mile) was eating a lot of oil and then one day it just stopped using it. I can't make any sense out of it. I've only seen three '95's and they were never driven much so no idea on the oil usage.

Aside from the personal insults being bounced around this has been a good thread to read.

 

Huh, ok, thank you I saved time instead of reading the last 16-7 pages.

 

Joe's car had a variance between 2-5% leakdown (tests done at 100PSI, only two cylinders were at 5%, average was 3%). Well within the standards of a healthy motor. His compression test results were within 5psi between the cylinders and showed healthy results too.

 

So interstingly Malcolm said something similar of his '92 (~equiv to '93). It was using considerable oil then after a long drive to a USA event it suddenly stopped and hasn't used any appreciable oil since...

So we are looking for something that maybe can be working correctly and oil consumption be OK, fail and oil consumption goes to crap without much other effect on the engine and can potentially be intermittent (like something sticking in a failed mode but possibly getting freed up somehow later...)

So those are the clues - but what is the answer?

Alan

 

Could be a stuck second piston ring or some other ring problem, which might then allow too much oil thru. If the top ring is not stuck, then leakdown might still look fine. I am just speculating, but a bunch of hard driving for many hours might simply clear the grooves. Pure speculation, but would fit some of the symptoms.

 

But certainly in my case the issue was primarily oil ingestion through the breathers into the intake - classic symptoms initially (oil puddle in the throttle body and mess inside intake) noticeable detonation events on hard accelleration (sometimes) but always on high-G turns when fully hot. I'd expect ring issues to be an insidious continual low level consumption issue - probably not noticeable in dynamic engine performance results (just long term gunk) - no?

Anyway my later breather configurations with a provent back to the intake and then later to atmosphere showed it was indeed huge oil losses that way (stuck a rubber bung in the drain port to see how fast it accumulated - too fast).

Other ideas?

Alan

 

Alan - I suspect that GTS oil consumption is not caused by just one issue. Tuomov's stuck ring idea may have validity in some cases and not in others. Your solution seems to be working in your car, but might not in others. From the several threads on this issue, it appears that the 928 engine in general, and the GTS, in particular has a weak oiling system. It is plagued by excess windage, a poorly designed breather, too shallow crankcase, too few or too small oil drainage channels from the intake to the sump, etc. When any portion of the system runs at less than 100%, the rest becomes overwhelmed. From a diagnosis perspective, the problem is compounded by the fact that one issue is apt to create several others. Excess oil in the intake may foul the rings -- causing a stuck ring that increases oil consumption. Fixing the original problem may not solve the oil consumption issue unless something is done about the stuck rings.

Given that, it seems entirely possible that Greg's windage screen kit will solve the oil consumption problem on some cars and not on others. Colin's approach, too, might help some cars and not others. The same is likely true for all the other approaches in addressing this issue. To the extent the approach has any validity at all, the approach is unlikely to be a universal cure. The only universal cure I can think of would be to address all the weaknesses of the stock system -- drilling out the piston rings, increasing the crankcase depth, increasing the oil return capacity from the intake, installing windage screens and/or crank wipes, etc. And yes, I think a moderate level of vacuum may have a role to play.

 

David - I don't buy that argument simply because there is too big a difference between cars that seem to have been well maintained otherwise. Much of what you talk about may be true - but they are NOT really variables in play here: drilled pistons*, shallow oil pan, breather configurations, head oil drain size/config (all are the same on all these cars). Excess windage may be a variable result - but the rotating stuff is exactly the same - so WHY is that?

*undrilled piston cars have many case of large & ~no oil consumption

I don't buy the simple ring argument either: If is an oil control ring issue you'd expect excess oil loss be to the cylinders directly, if is a compression ring you'd expect more differential leakage when tested...

Still looking for something else...

Alan

 

This thread would be about 2 pages long, if everyone here had had a GTS engine that uses oil apart...

Since there are no oil return holes in the pistons, below the oil control ring, the only return path for the oil is down the sides of the pistons. The sides of the piston and the oil control rings get completely "coked up" with this thick sticky black crap. I'll grab a GTS piston and take a picture, of this, if I can find a piston that has not been cleaned.

My guess is that once this nasty thick crap is heated up or reduced by an oil additive, plus the stock breather system is improved, the oil consumption problem, past the rings, goes way down.

I've suggest to Colin (three times now) that his vacuum pump system, which I've carefully warned is destined to cause damage when used for extended periods of time, might be a better short term tool, than a long term solution. If you can get the rings cleaned out and get them to "bite" into the cylinder walls, a passive breather system that allows the crankcase to breathe better is a much smaller risk.

 

I was trying to speculate about the reason why some GTS's burn a huge quantity of oil while others burn much less, and why some people GTS's have begun to permanently consume less oil after a very long, hard drive. This despite no difference in crankcase breather setups before vs. after. My speculation was that maybe the second ring is stuck or has carbon in the groove that prevents it from working. It's possible that a hard, multi-hour drive can clear up the problem. The second ring being stuck or not sealing properly might not show up in leakdown test, since the ring gap of the top ring may be the main determinant of those readings regardless of what's going on with the second ring. Two caveats. First, this will obviously not explain the oil in the intake manifold that makes it there from the breather system. Second, this is just speculation and not based on actual knowledge.

 

Dat ****...

 

So a little more on this topic - 2 things of note:

1) I opened a 5.5mm port to the crankcase on my vacuum pumped car to bleed vacuum. At up to ~3K rpm (cruise) this bleeds vacuum down to 1" - 1.5" Hg from ~5"Hg a pretty significant effect, at higher rpm flows it won't beed as much off - I didn't measure too much there.

2) The dual stock ports to the throttle body are each approx 12mm diameter, one is the eductor (center) and the other just a flat port - the flat TB port likely has more area - but not in it's outlet portion.

So a total of over 75.4mm^2 vs 17.3mm^2 on the port that I used.

Now there are other restrictors in the stock configuration in both the cam vent elbow and the filler neck port (so the net increase may be only a little over double my case here) but as a result I'd expect that the crank pressure will equalize to the throttle body within about 0.5psi below ~3K RPM. I have no idea what the throttle body is at - I'm sure it varies dynamically from the ambient but I'd guess not by very much at all in this same flow range...

You don't need a very big port - stock the net port sizes are too small - smaller than they could be in the exact same config to reduce pressure (maybe they were worried about too much oil laden flow). I'm now pretty convinced building pressure makes the consumption a lot worse - so using these restrictor/orifices may have been very counter productive.

Alan

 

Just for Ss and Gs:

Stole Greg's bore gauge and measured my four GTS rod bushings- all measure identically, they are all 0.0001 to 0.00025" 'taller' than they are 'wide', so either they're made that way or they've ovaled out a tiny bit over 100K miles.

He also gave me a set of used R1 GTS rods from a non-blowed up motor, will measure them too but need to clean them first.

Width, zero'd out:





Height: + 0.0002":




Measuring the absolute ID of the bore gauge when zero'd out in an outside micrometer:




0.9452", more or less, giving a pin to bushing clearance of 0.0002-0.0003. (?) I'm not as confident about the absolute bushing measurement, it's hard to zero out the bore gauge while getting it perfectly perpendicular in the mic while turning the mic anvil to zero everything out. And take a picture at the same time...:

EDIT: Never noticed it in the tech spec books, S4/GT/GTS Con rod bushing spec is 24.018- 24.028mm, or 0.9456 to 0.9460. Piston pin spec is 24.000 +/- .004, or 0.9447 to 0.9450. So my used GTS wrist pins are still in spec, if a tiny bit worn in the middle.

Con rod bush to piston pin radial clearance is .018 mm (new) to .032 mm wear limit. So that works out to clearances of 0.0007 to 0.00125. So my absolute measurement must be off.

 

I think that if the small end bore was made intentionally oval, like it is for some rods for other engines, then it was made oval in the other direction, wider and less tall. This is to distribute the load evenly when the rod small end distorts at TDC. Because of this, my guess is that the small end bores were round when they came from the factory but those cheap *** GTS rods have deformed over time. This, I believe has nothing to do with oiling, however.

So far, I don't think anyone's posted any evidence of the GTS wrist pins being underoiled. We've learned some other interesting things, though, like the GTS rings jamming up.

As a digression, I don't think Mahle makes any pistons anymore with a round wrist pin bore. Instead, they compute the thermal and other distorions to the piston and then bore the hole to be round under those conditions. When you hold a new Mahle piston in your hand, the pin bore is irregularly shaped - good luck checking whether those are within spec. I have some new Mahle Subaru pistons so I should try to demonstrate this when I am back home again (currently skiing in Vermont as I type).

 

Datum from a very different engine that may or may not bear on the discussion...

I rebuilt a Corvair 140hp engine, using new jug sets from GM. After about 20,000 miles, the oil consumption went from nil to a quart every sixty miles in less than two weeks. Compression went from 140-145 psig to 195-210psig.

Problem turned out to be bad oil rings - of course, GM refused any action, since I got 20k out of them...

So, bad (or stuck) oil rings can cause high consumption with no other symptoms, and neither compression nor leakdown tests will be useful.