Ever since the S2000 came out, there has always been reports of the oil cap gone missing. The owner looked one day and it was GONE! They could hear it rattling around but just didn't know where it was. I've seen two such posts this month.
It most likely vibrated off and fell behind the engine where it lodged between the top of the transmission bell housing and the firewall. You'll have to use a coat hanger and go fishing. Meanwhile, your engine bay will be splashed full of oil. Have fun cleaning it up.

Why does this happen? It may be the fault of the owner when doing top ups or oil changes or it may be the fault of the dealer tech when doing an oil change. Mind you, in some cases, it was a simple matter of the oil changer forgetting to put the thing on at all and leaving it sit on top of something in the engine compartment where it will either leave a big dent in the hood when closing or where it will fall off as soon as you start driving.

Here's what the oil cap SHOULD look like:


Here it is with that little rubbery ring out of its groove:


Here is that ring. Note the grooves:

This is the side that touches the valve cover and is the side that you see when it's seated into the cap.

Here is the OTHER side of that ring. Note that it's smooth:

If your ring falls out and you put it in with this side showing, you have put it in WRONG! It will not fit right and will make the cap screw on lop-sided and NOT seal. The cap will eventually unscrew and fall off.

Here is what can sometimes happen when you take the cap off:

That ring stays stuck on the valve cover and if the engine bay is in low light, you will not notice this ring. In the process of adding oil, you will likely shift this ring and when you put the cap back on, it will again be lop-sided. You will think you put it on tight, but eventually, it will unscrew and fall off. Another possibility is that you knock the ring off and never know it. When you put the cap back on WITHOUT this ring, it will not hold and again, will unscrew and fall off.

Always check to see that this ring is well seated in the cap before you put it back on.

 

great info for those who overlook the little things

 

Great post. I noticed that when I changed my OEM cap with Spoon cap, there was that rubber o ring as well.

I wish the spoon cap came with a warning label though. Aluminum sure can get hot

 

uneccessary comment removed.

 

Thanks xviper...

I just visited local Honda dealer and they did not found the cap behind the engine. Maybe tomorrow they get the piece.

The wine cork still resist fine.

 

And your post contributes how?
Tell it to all those who have had to clean up their engine bay after the cap disappeared. And tell it to all those who never discovered this right away and ran low on oil.
This may be insignificant to you but if it saves a couple of people from grief, then it was worth it, huh?
ps. I did this partly with "tongue in cheek" but having seen all the reports of this over the years, a laugh for you and me may end up educating a few others and leave one less thing for "Wisconsin" to have to reset his counter for.

 

Thanks Xviper. It only takes ONE time leaving your oil cap off to learn a lesson, but hopefully this post will save other people the trouble of learning the hard way.

 

what is with people lately?

 

The most likely event was forgetting to put the cap back on. It happened to me a while ago, when I checked my oil one night before we go someplace, I was adding oil and while I was doing that, my wife came into the garage to remind me that we were late. I put the oil bottle away and forgot to replace the oil cap, I then closed the hood and drove away. After about a mile or two, I smelled something was burning, I knew right away that I forgot to put the oil cap back on, luckily the oil cap was on the synbol "H" on top of the engine, so that when I closed the hood, it stayed at the same place and no dent to the hood either. Apparently, there is just enought room between the engine and the hood such that the oil cap is kept in place if it is put on top of the engine when you need to top off engine oil. Another thing, after more than one mile driving with oil cap off, there was very little oil anywhere on the engine bay, but you can smell the burning in the car with top and windows up.
About a month before that incident, I did exactly the same with my LS400, and it was a mess in the engine bay. Nothing like these happened before and then two incidents in one month.

 

Come on, sometimes even the most experienced technician may forget something like this. I can't think of how many times I've gone to close my hood and forgot that one extra tool still sitting on the front frame rail (under where the stock air box would be).