Dear Tom and Ray:
I recently drove about a thousand miles without my oil cap. It's a 2005 Toyota Sequoia, and I was off-road for about 10 days at the same time. When I got home, no oil cap! (Human error all the way.) I checked the oil, and it was good all the way to the "full" mark, and there's no oil on top of the engine, or even on the underside of the hood. As I drove, I didn't see any drop in oil pressure or hear any weird mechanical sounds, so I'm wondering what damage (if any) I might have done to the engine, driving all that way without a cap. I am changing the oil and filter today. Thanks for your answer, and yes, it is the first time I've ever done that, and the last!
RAY: You are one lucky fellow, Steve. Mostly because of the particular car you own.
TOM: On most cars these days, the oil cap is right on top of one of the valve covers. When the cap is off with the engine running, the oil blasts out of there like the last feeding blasts out of an infant who's been tossed into the air repeatedly by his father.
RAY: You would have noticed that right away, because the oil spews everywhere, including onto the hot exhaust manifold, where it immediately starts to smoke. It's a complete and utter mess. How do we know? We've done it at the shop!
TOM: And if you actually drive around with the cap off on those cars, you easily could lose enough oil to damage the engine. You can lose half of your oil that way in a matter of hours.
RAY: On your vehicle, I believe the oil fill is down between the cylinder heads, which is a less lubricated part of the engine, and it's not where the cam shafts are churning up the oil and tossing it everywhere. That's why little to no oil was lost.
TOM: And it's unlikely that anything got into the oil fill when you were driving off-road. You'd have to be extremely unlucky to have a piece of debris come up from the road and go precisely into that hole. It'd be like winning a game of quarters at 50 feet.
RAY: Even if anything did get into the oil fill, it either would sit at the bottom of the pan and get drained out during your next oil change, or it would be caught by the oil screen or oil filter.
TOM: So this appears to be your lucky week, Steve. You might want to go out and buy a few lottery tickets.