Ways to diagnose rings as source of oil consumption

[zwede]

So I have this 454 in a C3 Corvette. Doesn't see many miles, drive it around on nice weekends and to some shows. Runs OK but burns a qt of oil every 700 miles.

The engine doesn't have a lot of miles on it since a complete rebuild and has been burning oil the whole time. I suspect the rings never seated, but not sure.

Cranking compression is good at 200-205 PSI on all 8. I did a leakdown test before first start and had under 6%. No leakdown test since then though (borrowed tool). Pistons are Probe street-series forged. CR 10.15:1 (measured/calculated).

I've checked and replaced intake gaskets, valve seals (positive seals), sealed the rocker studs, added PCV catch can, all the usual suspects. No change in consumption.

So I've seen some "rules" that will tell you for sure it's rings:

1) If rings are leaking oil will turn black quickly. Well, this is true on mine. It will be black within a few hundred miles.
2) Looking at the pistons there will be clean/shiny areas close to the edges while the center is sooty. This is also true, I checked with a borescope through the spark plug holes.

Question: Are these sure signs of non-sealing oil rings? Are there other things to check to tell me for sure I'm dealing with bad oil ring seal?

And to throw another thing out that confuses me: I used to have lots of oil fouling on the plugs, with the rear 4 cylinders being the worst. 1,500 miles ago I switched from full synthetic oil to conventional thinking maybe it will let the rings seal better and the plugs have gradually improved to where there is almost no oil on them anymore. But the oil consumption seems the same????!!! I don't get that one at all.

[SupStk]

Your #2 is what I usually use as an indicator. Judging from your post, it is a ring sealing issue.

[crazycuda]

Out of curiosity did you initially run synthetic in the motor, or wait 1500 miles before making the switch. In the 90's Jag had a tsb that stated do not use synthetic oil in any of the motors till they passed the 1500 mile mark or ring seal problems would occur. My old boss didn't listen and on a xjs he did a rebuild and ran le synthetic from the start and the motor had nothing but oil consumption problems. Had to pull the motor give the cyls a quick hone and fresh rings and bearings etc and reg oil. Problem was gone.

[Belgian1979]

I hard a mechanic swear by Forte top end kit. Seems to help rings to reseal. Haven't tried it myself though.

Personally I don't think any of the things you described let you determine how good the rings seal. I have a similar problem that I'm still chasing. Some said that this type of oil consumption was caused by a second ring to be mounted upside down. However I did check that upon assembly. Compression is good on all 8 as well on mine.
As far as synthetic goes : factories these days fill their engines with synthetic. Don't think that's the issue.

Currently I went from the Mishamoto oil catch can to a Moroso unit which seems better built. Still have to try it out but weather is bad out here. If it doesn't help I'm going to take off my heads and have the guides redone.

[rebelrouser]

Lot of manufacturers are now going to a blow by tester to measure leakage by the rings, since it is a dynamic test while the engine is running it is the most accurate. I check my engines with both leakdown and blow by tester, results seem to match, low leak, low blow by.
Oil is supposed to be scrapped off the cylinders by the oil control rings and the second compression ring where it is then returned to the oil pan via holes drilled in the pistons behind the oil rings. I have noticed a lot of modern piston designs don't have very large return holes. Manufactures tell me clogged oil return holes is now the leading cause of oil consumption on new style engines. Many times they recommend carbon cleaning an engine before tearing down for ring replacement. Excessive bearing clearance may overwhelm the return holes, due to more oil than normal being thrown on the cylinder walls. If you reused your old pistons, were the ring lands and oil return holes properly cleaned? And excessive clearance on the ring lands and the piston rings will make them use oil as well. rings seal fine, the oil is pulled around the backside of the ring due to excessive clearance. I don't recommend it, but back when I worked at the GM dealer they had us under warranty, put Borax down the carb at speed to reseat the rings, never seen it work.

[Belgian1979]

a small comment to that. My piston pin portrudes into the oil ring. That hole is big enough to get rid of any oil. However I've also suspected it from letting oil pass to the piston top because of it.

I don't know when the OP sees it smoking. Mine seemingly does it specifically on decell, very high vacuum conditions. Most point to valve guide clearance.

[BOOT]

I changed from norm to full synthetic in my 4.3 09 Silverado at about 45-50k and by about 70k I noticed it was burning 1/2 qrt every 1000miles, swapped back n it slowly got better until it pretty much stopped by 80k.

[user-23911]

Soot on the piston means too rich....bad tune.
Oil turns black through excess fuel (soot)......not leaks.

That might lead to bore wash........as in fuel washing the oil off the bores.
It just gets worse from there onwards.


How does the oil smell?
Like fuel?

[zwede]

Thanks for the replies. Answering the questions:

I've never seen it smoke. It's possible there is some at WOT but it's hard to tell (600 hp in a 3,000 lbs car with a suspension designed in the late 50's keeps me busy.

Yes, I'm guilty of using synthetic oil from the start. And that's also why I switched to conventional to see if it helped. Like I mentioned, plug fouling has decreased significantly, but it seems consumption has not. Although I haven't driven up a qt of oil yet. Currently I'm at 400 miles and it's a bit below 1/2 on the stick.

Since it runs OK and is not a daily driver, I'm leaning towards just keep driving it and see if more miles will help. I have maybe 5K miles on it since the rebuild.

[zwede]

Soot on the piston means too rich....bad tune.
Oil turns black through excess fuel (soot)......not leaks.

That might lead to bore wash........as in fuel washing the oil off the bores.
It just gets worse from there onwards.


How does the oil smell?
Like fuel?

If it was that rich I'd think the oil level would go up? But no, it's not that rich. I run EFI. However, there is something weird with cyl 7 & 8 where I see some soot on the plugs. Not all black, still clean at the tip but sooty at the base. I haven't figured out why those cylinders run richer. I moved the injectors around and it made no difference.

[zwede]

Let's see if I can attach some pics of the plugs

First one was Jan 2017:

1357_2.JPG

And this one is Oct 2017, maybe 800 miles later:

1357.jpg

[statsystems]

The upper plugs are rich at cruise. The lower plugs are better but still rich.

Can't see WOT from that angle

[zwede]

And here is the #8 piston:

8_2.jpg

[user-23911]

6 of your cylinders are burning oil.
2 are not.
You should be able to work it out?

1 of them is running extremely lean.


So it's EFI and therefore got the usual EFI problems.
You'd really want to flow test the injectors.


edit, so I didn't read properly.
4 cylinders.......all broken, different plugs, different type, different time.

Got boost?
got detonation too?

[zwede]

The upper plugs are rich at cruise. The lower plugs are better but still rich.

Can't see WOT from that angle

Could be. I had it set a bit rich at the time. I think target AFR was 14.2:1 at cruise. Didn't think that would be enough to foul plugs, though? I have since changed the target to 14.7:1 for cruise & idle. I need to check the plugs again.