Monday, March 15, 2010
Oil Analysis Reveals Lead Wear Trend
As you may recall, my previous oil analysis
identified a slight rise in lead from an average of 4 PPM to 7. As the lab
technician then indicated, and I agreed, that change did not warrant concern
because it wasn't that far from the statistical average.
Yesterday I received the results of the sample taken last week and unfortunately it shows that lead has more than doubled to 18 PPM. The change has been identified by the lab technician as the likely start of a trend of increased bearing wear. The lab technician remarks that iron has also increased but that's only relative to the most recent samples. There seems to be some variability in the levels of iron over the previous five samples and the highest of 8PPM was found over 20K miles ago. As a result, I do not see iron at 10 PPM to be statistically significant.
I must admit that I am somewhat more skeptical of this sample than I would otherwise be because of the nearly 40% longer sampling interval. Some of the increase may very well be related to the increased time in service. The lab technician recommends additional changes with a shorter time in service and I plan to do that. I'm inclined to "reduce" the interval to 4500 miles so I can get the sample amounts back to a baseline to better identify the rate of change. However, doing the next change at 3000 miles would also serve to indicate the slope of the trend -- if the numbers are equal to or greater than this sample given half the time in service it might indicate the need to pull the engine sooner rather than later.
To better analyze the results and, I'll admit, do some daydreaming about what good could come of an engine rebuild I called Jim, owner of Metric Mechanic. He was kind enough to spend the better part of an hour over lunch this morning discussing the issue with me. I'll try to summarize it with a few points:
- He was appropriately skeptical of the oil analysis but admitted the engine is nearing "end of life" so anything is possible.
- If the bearings are wearing it's most likely a rod bearing since those take an absolute beating. In the same breath he said it's very likely NOT a main bearing because, relatively speaking, they're "just along for the ride" on a stock engine.
- The stock bearings consist of a sandwich of three metals: a substrate of steel, copper on top of that, and a top layer of lead about 2 thousandths thick. I took away from this the observation that an increase in copper might dictate the need to pull the engine before it starts making metal in a big way and destroys my core.
- If you're lucky and take care of it (meaning, do all required maintenance and don't abuse it at the track), the average BMW engine will last about 250K miles. And when the engines "time out" it's usually due to a loss of compression that is the result of unavoidable damage to the top compression ring groove and the associated wear at the top of the cylinder bore. But based on his experience many engines fail for other reasons (head gaskets, cracked heads, etc.) so they never get to a point when they simply "time out" in this manner.
- Their primary business is building high performance versions of these engines rather than simple overhauled versions of the stock engines. They can build the engine to whatever spec you desire but they specialize in boring and stroking to 3.2L, using custom lightweight pistons and connecting rods, porting the heads and in general using materials that allow them to build engines to far tighter tolerances than would be possible on the BMW production line. The price for this work? About $10K. That's not exactly petty cash but it's probably one of the better choices if the intent is to stick with a BMW engine rather than swap it for a bowtie (LSx).
Of course, this is nothing more than the official start of the analysis and I intend to continue to use the vehicle without reservation. I'll likely schedule the next oil service and analysis in 3000 miles which translates into 2-3 months given my current driving habits. Stay tuned!
Mileage: 189777, Labor: $22