Vented gas cap vs non-Vented

[gfwong]

I just purchased my 1970 280 SL and am trying to understand if I have the correct gas cap. It is of dull silver metal with Mercedes Tristar emblem and a bar going across the diameter of the cap to assist in twisting. It has no lock.

More importantly I need to understand if it should be VENTED or NONVENTED. 

Initially I drove it home for 3 hours from the place of sale with a non functional fuel gauge. (now repaired)
But because of this I initially stopped every 20 miles to top off the tank & upon arrival to my home garage the fuel odor was prevalent.

Checking inside the trunk for a fuel system vent and charcoal filter behind the spare reveals NONE.

Do these symptoms of gas odor … plus a noticeable suction when I loosen the gas cap indicate the need for a different cap?

What cap is recommended for this vehicle’s fuel system?

[lreppond]

The correct gas cap is unvented.  Although your current cap has the MB logo, it is not OEM and may or may not be vented.  If vented, that would obviously be one reason for the smell of petroleum.  However, the 1970’s USA models (others may as well) have a vapor recovery system that may be leaking as well.  If the smell is on the outside, it’s the vented cap.  If the trunk has a strong smell of gasoline it is possibly the vapor recovery system that is faulty. As for the suction that you experienced while removing the cap, it may be due to simply a change in temperature of the air space above the gasoline in the tank.   Use this forum’s search engine to get more info if needed. 


Oem cap is pictured.   Note the center key slot which was correct for the one key system introduced in 1970 models. 

[Pawel66]

If it is a US car you should have a relatively large canister on the left wall of the trunk, not behind the spare wheel.The small canister on the right was for Euro cars. No charcoil filter there. Everything is described in the Technical Manual.

You will also get a lot of advise if you use the search function. Write in “fuel smell in trunk” (with parentheses) or “fuel odor in trunk”, you will see possible causes and ways of repair in the Member’s posts.

[gfwong]

Thank you for this clarification.
I do see several NV caps on eBay...but they are not chromed and some don't even have keys.
One non chromed w key "OEM with blue factory box" does not  have MB emblem!

That is to say, this is a confusing array of choices, which I'm sure w my relative  inexperience ... i will likely choose the wrong one.

Also if i DO purchase a used cap with NO key...I have a local locksmith who claims he  can very likely make a new key...but not sure if this will match the ignition key.

There is a beautiful chromed, locked, w keys, (not sure of the vent)= over $1000.00!!! (seems a bit pricey)

What is your experience (s) with this first world dilemma?

[lreppond]

You’ve just brought home a 55 year old car.  It’s going to require plenty of sorting to get it running well, safe to drive and dependable.  Those should be your top priorities.  Ascertain the cause of the petroleum odor.  If it’s the cap, I’d go to NAPA or PepBoys and get a generic non locking unvented cap for the time being.  It’ll will do just fine while youre working through other issues the car will inevitably have.  Once everything is in order, you can go with either a locking chrome cap sans the logo (around $150) or if your pocket book will allow it, go for the $1000+ chrome OEM cap.  There’s a reason why cars in top order command top dollar: it’s all about parts and labor and neither are cheap!

[gfwong]

Very good insight. Much appreciate the encouragement for being patient with the expectations with this machine.

I told my freinds & my wife that it is like adopting a teen age kid into your existing family… there’s lots of baggage that’s come with it. A lot of mystery. But a lot of potential enjoyment down the road.

Thanks again.

[Pawel66]

Let me throw in my 2 cents.

Fully agree with Ireppond that, depending on budget constraints (or preferably lack of them), I would prioritize topics to be addressed in the car and have a plan to tackle them one by one. As for the cap, again, taking budget into consideration and the end game you want to reach - buy cheap for now and then the targeted one or, if budget is not a constraint buy a targeted one, make sure you can fit the right key to it or check the availability of a cap with key cut to your VIN.

However

I am not sure if the cap is your issue. If you experienced suction while opening the cap, it would suggest the cap is sealed and tank ventilation system is clogged. The gasoline smell can be the result of clogged ventilation system or of many other issues - there are plenty of places that may be leaking with fuel level sensor area being one of the suspects. Getting to the bottom of this and addressing fuel smell/leak is, actually, a priority, I think.

Search function will help.

If your cap was vented, which I doubt, and you screw on non-vented one, it may turn out that the engine will starve for fuel if the venting system is clogged.

[Lori]

Have a look here

https://www.sl113.org/wiki/Fuel/VentingSystem