Car smokes. Gets worse over years. Miller cycle engine. Assumption: Oil leaks into cylinders. After many months, starts running very rough. Worst when warm once it starts running badly.
According to mechanic, when it starts running rough, spark plugs have fouled out (due to oil, presumably) and need replacing. So this time it started running rough, we replaced fouled out spark plug and (expensively) replaced all valve cover gaskets. But this time, unlike all other times spark plugs were replaced, it still runs rough.
If mechanic was right, replacing the spark plug and sealing the gasket better should have fixed rough running, even if only briefly. But it made no change whatsoever. (He now says “Engine is tired. Replace the car.” Of course, he also sells cars.)