Issues: Oil Consumption and Cold Piston Knock

A hot topic amongst C5ers, especially those active on the Internet, is high oil consumption by LS1 and LS6 engines. We asked John Juriga about this and, in this exclusive to the C5 Registry On-Line and the C5 Registry Newsletter, he confirmed there’s a problem, but not one as widespread as some people believe. He also explained the fix GM Powertrain has developed for it.

"We have seen a greater percentage of complaints than we’d like about oil consumption," John admitted. "The condition under which we get that oil consumption is high-rpm, light-load–like if you drive in a city schedule but never take the car out of second gear. In that situation, the piston rings can get into a flutter condition and that’s when the oil consumption takes place."

Piston ring seal depends on a balance of four forces: combustion pressure, ring inertia, the ring’s radial expansion pressure and crankcase pressure. Ring flutter is uncontrolled oscillation due to an imbalance of those forces. Once a piston’s rings go into flutter, their ability to scrape oil off the cylinder wall as the piston moves downward is impaired, blow-by increases and oil consumption rises dramatically.

The combination of high rpm and the low crankcase pressure typical of low engine loads causes those four forces to become imbalanced. The small amount of LS1s and LS6es that see regular, high-rpm, light-load operation may suffer high oil consumption.

"The severity of this problem is specific to the driver," Juriga continued. "You can take a car that is a major complaint for one customer and give it to another customer who’ll have (different driving habits and) no complaints and get 5000 miles to a quart."

The common sense is that high-speed and light-load is not a duty cycle typical of normal use, even for an engine in a car like a Corvette. Who drives around town running 4000 or more rpm at part-throttle?

"It’s not the way most people normally drive," John agreed, "so it has not been a substantial part of our normal durability schedule.

"It is a substantial part of our schedule, now.

"This particular problem is not something you see as a wear issue, either. You can tear apart the engine and find nothing. In fact, that’s why it was so difficult. Someone says, ‘I have an oil consumption problem.’ We give the car to our guys who put a thousand miles on it and oil consumption is within limits. When we drive it aggressively, but in a more conventional manner, there’s no problem. We tear down the engine, everything looks fine–no wear, no scored bores, no ring gap alignment problem, nothing to explain the oil consumption.

"This issue has become very pronounced on the Internet. People are saying, ‘Oh–we’ve got a problem with oil consumption.’ but the vast majority of customers don’t have any problem. There are a few who drive like that–and they’re entitled to, that’s why they buy a Corvette. They are the ones that have trouble and we want to try to help them."

Internet conspiracy theories, urban legend and rumor mutate and spread rapidly. While the core issue might have factual basis, it quickly becomes exaggerated and laced with disinformation. To objectively verify a problem like this and develop, then test a successful fix is difficult and time consuming. This is why General Motors has seemed slow to respond.

GMPT contacted customers experiencing the problem. This group was asked specific questions about driving habits. Once GM acquired data pointing at the difficulty, the second task was to devise a test schedule that could be run under controlled conditions and would include some high-speed, light-load operation. Once GM did that, then tore down engines and found no wear, materials or assembly trouble; ring-flutter-driven, oil consumption was identified as the cause.

"We went back to our ring supplier and worked with them in developing a fix," Juriga explained. "We are going to change the ring pack. We’ll use a higher tension oil ring. We go from a nine pound ring to a 13 pound ring. We’ll also change the second compression ring to a ‘Napier ring’ design which has a very pronounced scraper profile on it. The current second ring uses a conventional oil scraper design.

"We’re going to get that done for the start of production (MY02) on LS6 and within a couple weeks afterwards, it will go into the LS1, so it will be across-the-board on both."

 If you download the larger view of this image, you can clearly see the unique scraper face of the Napier profile, second compression ring. Image: Author.
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Above is a side view of the scraper face on a typical second compression ring. Drawing: Author.

Above is a side view of a second compression ring having a Napier profile face.
 Drawing: Author.

This revised ring pack was validated in-part by field use of replacement rings in engines having trouble with high oil consumption under high-rpm/light-load. The increased oil ring tension keeps the four forces mentioned earlier in balance so oil ring flutter is eliminated. While the ’97-’01 second ring had a scraper face, the Napier ring is quite different and results in more aggressive oil control on the piston down stroke.

"We’ve had over a dozen customers with complaint vehicles," John Juriga stated. "We put these rings in and it’s a ‘clean kill.’ It takes customers who are aggressive drivers and who had oil consumption as low as 500-800 miles per quart up to 1500-2000 miles a quart.

"This fix is not out in the service organization at this point. It goes through our technical assistance network on a case-by-case basis. We’re getting these rings out that way because (at this writing) they’re very hard to get. We are going to make up some piston assemblies for service. The way these pistons come into the (engine) plant from Mahle (piston supplier for Gen III engines), they’re already fitted with rings. We’re trying to decide whether to have dealers install the rings on the pistons or supply piston assemblies that include the revised rings. Once we get that resolved and have required amount of parts in the pipeline; we’ll make them available through service channels, I hope in about 4-6 weeks." (at this writing, that’s late-July or early-August).

"It’s on a case-by-case basis because, with some customers, all you have to do is tell them, ‘You can eliminate your problem if you throw it into third or fourth gear instead of riding it in second.’ They’ll be happy to do that and the problem goes away.

"Other customers say, ‘No. That’s why I bought my ‘Vette. I’m gonna drive it the way I wanna drive it.’ If so, that’s fine. If you’re getting 500-800 miles per quart, that’s too much and we’re going to swap the rings out in that engine."

The revised ring package will not increase an engine’s performance. If your engine is not experiencing excessive oil use, there’s no advantage in running out to get new rings. Even if you do have an engine that experiences abnormal oil use due to high-rpm/light-load operation; you’re better off modifying your driving habits than going through the trauma of a partial engine overhaul under warranty. If eliminating most high-rpm/light-load operation doesn’t stop excessive oil use, then ask GM to repair the engine.

Some people involved in the public dialog about this issue have been highly critical of General Motors. Some of the harsh words are unfounded because this problem is not as common as Internet rumor claims. Additionally, it is clear to this writer that, while GM may erred in not making high-rpm/light-load testing as prominent as it should have been, a small group of owners, because of their unusual driving habits, have to share some of the responsibility for this problem.

Going to a higher tension oil ring and a Napier profile second ring solves the oil use problem convincingly. Will the change also result in oil consumption decreases in LS1s and LS6es which are driven normally or driven aggressively, but not in the high-rpm/light-load manner that previously caused ring flutter? There is that possibility.

The two LS6 pistons. Because of the differences in piston-to-bore clearance, they are only interchangeable in one direction. You could use the new piston in a ’01 LS6, but you can’t use the old piston in an ’02 LS6 block. Image: author.
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In mid-April ’01, there was a change in the LS1/LS6 piston which will carry over to MY02. To address a limited amount of complaints about "cold piston knock", there was a small reduction in piston-to-bore clearance and new pistons, having skirts coated with a polymer, antifriction material, were introduced. The LS1/LS6 are first in the Gen III small-block engine family to use coated pistons. Corvette often leads the way with new technology that eventually sees high volume production. In the near future, all Gen IIIs used in GM trucks and passenger cars will have coated pistons–we’re talking millions of engines a year, here, not just 30,000 or so C5 powerplants annually. "When you decrease the piston-to-bore clearance, you’re more susceptible to hot-scuff ‘cause you’ve got a tighter fit. The coating gives us resistance against scuffing," Juriga stated. When asked about possible power losses due to the tighter clearance, he added, "We haven’t seen any measurable hit from a power standpoint because of the tighter clearance."

This piston knock anomaly that has been occurring in some ’97-’01 engines after start-ups in cold weather is not a durability concern. It’s a pleasability issue on which there was enough input from customers that GM made a production change. Like the revised rings, there’s no performance advantage in switching to the tighter clearance and the polymer-coated piston. Those hearing a cold piston knock are better off ignoring it until the engine warms a little, rather than subjecting themselves to the hassle of a dialog with a GM dealer intended to force repair or replacement of the engine.

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