Originally Posted by paulri
For those of you who have caught major problems before they became prohibitively expensive, through a UOA--I am just wondering (because I'm not a mechanic) about the thought processes here.
OK, you see high iron, and you say, OK I'm going to disassemble my engine until I see lots of wear scars? High copper and lead combined, would lead you to dig right through to the bearings? I don't trust myself to dig into the innards of an engine (I chose not to do spark plugs on the Sienna; book time for that job was 3 hours)--but could I take the car to the mechanic and say, look for a source of lead, or copper, or iron that is wearing abnormally?
If this is something that I could pay a mechanic to do, this would probably be enough to get me to start doing UOAs on both of these cars. The extended OCI would pay for most if not all of the UOA, and the lead time for a potential major problem could definitely come in handy.
This is exactly the right kind of question to be asking: What are you actually going to do with the info from a UOA?
UOAs make sense for tracking wear when there's a likely failure mode that:
1. Shows up in a clear and predictable way on the UOA, and
2. Is worth trying to catch before it produces other symptoms.
In practice, that's almost never the case for passenger vehicles.
For most of the engines on the road, the most likely wear-related failures will be compression loss, head gasket failure, etc. -- all of which can and should be diagnosed by other methods before being addressed. They can show up on a UOA, but would have to be verified by those other methods anyway, and in most cases aren't even worth addressing until they cause noticeable functional problems.
If your engine is prone to something like excess bearing wear leading to seizure or something with no other symptoms, that's another story. But very few engines will ever have that kind of problem, and of those, only some will produce wear metals that might show up on a UOA.
This is why I stopped doing it on my E36 M3, why I didn't bother on my RX-8, why I'm starting back up again on my current car, and why I'd advise almost everyone not to bother with UOAs for tracking wear.
What UOAs are really useful for in most cases is finding other issues (e.g. dirt contamination, fuel dilution) and seeing if your OCIs are too long. But that's another story.