02-25-2020, 03:46 PM
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Join Date: Apr 2011
Location: Herndon/Reston, VA
Posts: 1,916
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Bore Scoring....cause found?
Hi all,
ICYMI, here's a posting (from the Cayman Register) about bore scoring on recent engines. Note that there is a link to a paper by LN Engineering.
Quote:
A number of us guys attended the 40th annual Tech Tactics yesterday and I learned something that I really wanted to share with the group. Charles Navaro from L&N Engineering gave a presentation on Cylinder Scoring, and what we learned is a real serious threat to our aging Caymans. This was the first time I have met Charles. I was VERY impressed. This guy really knows his stuff.
We learned that Porsches equipped with Lokasil (M97) and Alu-Sil (9A1) bore crankcases are starting to suffer terminal cylinder scoring. Cylinder scoring is degenerative. It starts with excessive oil consumption and will ultimately leads to misfires, catalytic failure, detonation and will require complete engine replacement. We also learned that the chances of this failure can be greatly reduced or even eliminated with some preventative maintenance and targetted inspections.
Charles has written a very in depth paper on the problem, delving all the way down to the manufacturing process and the metallurgy of the crankcase and pistons. Not exactly light reading, but good information. You can read it here.
I don't mind giving you a spoiler on this one. If you want to skip to the conclusion, Charles believes that these failures are caused by "too much fuel" in the cylinder. Too much fuel washes the oil away and leaves the bore without adequate lubrication, leading to a failure of the piston to cylinder friction points.
What causes too much fuel...? Several things can contribute but Charles believes the real threats are in this order:- Fuel Injector Failure (caused by age and ethanol)
- Vacuum Leaks (caused by age and modifications)
- Failed Oxygen Sensors (caused by age)
Fuel Injectors:
If fuel injectors fail, they will leak their fuel down into the cylinder after we turn the engine off, filling it with lubrication destroying solvents. Today's ethanol enriched fuels are very corrosive on our fuel system and it's parts. The current 10% ethanol is beyond the 5% that many of our cars were engineered to withstand.
Charles pointed out that in more than 90% of the vehicles that he's seen cylinder scoring in, the fuel injectors of the failed cylinders were found to be leaking. He shared a story about another shop that replaced an engine with a short block because of a scored cylinder 2. Short blocks don't come with injectors. The shop reused the injectors from the old engine and the new engine suffered scoring of cylinder 2 in short time. A subsequent inspection showed that injector 2 was leaking. The shop essentially transferred the problem from the old engine to the new.
Vacuum Leaks and Oxygen Sensors:
A vacuum leak or bad Oxygen sensors can cause the DME to think that the engine is running lean when it really isn't. What does the DME do when it thinks the engine is running lean...? It richens the mixture (adds more fuel) .
Our cars can develop vacuum leaks in many places. The most common place is the vent line for the Air Oil Separator (AOS). It's a cheap plastic pipe that hardens and becomes brittle with age. It can crack and because it's out of sight we would never know. There are many other places that can also cause leaks.
Charles points out that if there is a vacuum leak, or if the O2 sensors are worn, the "Fuel Adaptation" readings from the PIWIS (not sure if we can do this with Durametrics) can provide advanced warning. The fuel adaptation shows a factor that the DME is applying to the fuel mixture. 1.00 would be perfect, no adaptation. A number greater than 1.00 shows a rich adjustment. Charles says that a number of greater than 1.05 likely indicates a problem.
How can we protect our engines...?
The good news is there are several steps we can take to protect our engines and prolong their life. Most are simple and/or inexpensive. Some are a little more pricey but still a lot cheaper than buying an engine.- Don't ignore a check engine light. When a CEL is on, the engine defaults to a rich condition.
- Keep an eye on Fuel Adaptation Ratios. If you see a value of 1.05 or higher, find the cause.
- Be vigilant and look for vacuum leaks whenever you have the engine cover off.
- Proactively replace potential suspects. AOS vent tube, O2 Sensors, fuel Injectors, etc... ($$$)
- Seek an Oil Analysis that can identify fuel in the oil. If found, find the source (likely an injector) and fix it immediately.
- Consider using an oil that contains Molybdenum or add a Moly based oil additive. ($)
I have recently replaced my O2 sensors. Next up for me, I'm going to inspect all vacuum lines, get my fuel ratio and start shopping for new injectors. My car is 13 years old with 84,000 miles. A fresh set of injectors should bring me another 10+ years of confidence.
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2016 GT4
2012 Cayman R
2005 Lotus Elise
1994 RX-7 R2
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