Differential diagnosis for a car when mechanic is saying it ran out of oil and it didn't.

I've never done it but I know there are places you can send a sample of oil to have it analyzed. If metal particles are found in it, that points to the engine coming apart. If the oil is perfectly clean, that suggests the engine ran out of oil, then some was added later to cover it up. The metal particles will be clumped on the bottom of the oil pan rather than dispersed in the oil.

It depends too on what failed and the symptoms. You mentioned the differential but that has nothing to do with the engine. The differential is part of the drive axle. Many engine problems can be repaired. It's not always necessary to replace the engine.

On the one hand there is really no manufacturer that is having common engine failures like that so two in one car is very unusual. On the other hand, even with a slow leak, you should have had some indication there was a developing problem if it was due to running out of oil. The engine would make a rattling sound and the oil pressure warning light would have turned on, and at that point there would have still been some oil in the engine so it would typically survive. If there was no warning of the immenent failure, that suggests a problem that was not oil-related.

To put things in perspective, I haven't drained the oil in my rusty trusty '88 Grand Caravan in over nine years, and it's a daily driver that I regularly pull a tandem axle enclosed trailer with that's bigger than the van! There is a very slow leak so I do add a quart about every 1200 miles and that keeps a fresh supply of additives in it. Please understand that what I'm doing is not neglect; it's abuse, and I'm not recommending anyone do that. I use it to show my students what some engines are capable of, (it has over 380,000 miles and has never been rebuilt). Sometimes when I ignore it long enough, it takes three quarts of oil to fill it, and it's only a 4.5 quart system. If my Dodge can hold up to that, I'm certain a Mercedez engine is at least as tough.

Any chance you can find out exactly what failed? Even if I can't tell you if it's due to running out of oil, I can suggest various things that could have happened and possibly how to narrow down the cause.

I can think of three things he might have meant by "mismatched". The obvious one is they were different part numbers. Every spark plug manufacturer makes them with different "heat ranges", and you need to have the proper heat range in your engine. If the wrong heat range is used, it is possible for them to cool off too much during operation which can lead to fouling from carbon deposits not burning off. If a part number that is too hot is installed, the electrode can get too hot and melt away. Either condition will eventually cause a misfire and hard starting but except for rather unusual conditions, not serious engine damage.

A slightly hotter or colder spark plug can be used to overcome specific conditions but that is rarely done any more. In particular, if a high-mileage engine is burning oil, a hotter plug will tend to remain cleaner and not be fouled by that oil.

Your mechanic could have meant spark plugs from different manufacturers had been installed. That is not normal because they are normally replaced in complete sets, but it won't cause engine damage. It doesn't matter who made each spark plug. They all do the same thing. Perhaps someone previously took them out to inspect them and set the gaps and they dropped one and broke it. They could have had the correct replacement in stock from a different manufacturer and used that. Some people are too picky to mix and match but there is no earth-shattering reason that can't be done.

Either of those conditions would not be readily apparent unless the mechanic was removing them for some reason. It's not really something you "just happen to notice".

The third possibility is the air gaps were different among the spark plugs. Every engine manufacturer specifies that gap, and while it's important, it's not exactly critical. What I mean is it is not acceptable to say "this one is close enough" and stuff it in the engine, but if that gap is off by a couple of thousandths of an inch, no one will ever know it by the way the engine runs. The gaps actually do increase a little over the spark plugs' lifetime because the electrodes slowly burn away. The only way your mechanic would know the gaps were wrong or different is he would have to remove them. That is not something that is done during any other routine service except for a tune-up.

Gap