Engine Misfire- Fuel Cap

[Mike Webster]

Scholars,

Several weeks ago I noticed a small amount of fuel on my rear bumper so I installed a new fuel cap gasket (from Bud's). After driving about five miles with the new gasket my engine began sputtering (at 3000 rpm) and would not maintain idle (at 800 rpm). Prior to this, the engine was running perfect. Is there a relationship here?

Mike in Detroit
1966 230sl silver/red

[nick350]

Im keen to find this out too as I too have gas on my bumper / fender and the car stinks. I have had the car tuned from running badly and a relationship between the two gas caps I have ( one ebay locking...or not ) and one supplied after market type would explain quite a lot.

Nick350
r107 SL red
w113 SL white

[ted280sl]

Mike,
  I would not consider myself a scholar, except when it comes to the study of w113s. Our cars have a vented gas tank. The gas cap is supposed to seal. The breathing lin exits the top of the tank inside the trunk. There are meatl lines the go to a breather contraption which should mount on the right rear fended. Another metal line should go from there and exit below the car. I suspect that some of your lines are eithr clogged or damaged. Having a compressor to blow air through the lines would be real helpful in determining where the problem is. All of the lines are fairly easy to access. remove the spare and you can get to the breather contraption. Remove the guard inside the trunk by the gas cap and you can check the lines from the tank. My breather contraption was damaged. The replacement was not expensive and included plastic tubes. The installation was very easy.
Good luck,
Ted 1969 280SL 4-speed

[Sven]

This brings up a simple question.  I am in need of a new gas tank cap (mine is brush painted to match the car and looks like a 1st grader did it).  I am looking to buy a new chrome one, should I get a vented or sealed cap (the vendor I was looking at sells both)?  Ted states they are supposed to seal, but is any harm done in getting a vented one to ensure plenty of breathing?

sven

[J. Huber]

Guys, you might try a search for vented gas caps -- there has been a lot on this over the years. I believe the short answer is our cars take the non-vented type. In fact, I recall one person having to add a second gasket to make a tighter seal.

James
63 230SL

[n/a]

Mike

I believe Ted280sl has the right idea.  The cap should be sealed.  The condenser that is mounted inside the right quarter is to catch any vapors from the tank.  When the vapors condense, the small amount of condensation is supposed to drain back into the tank.  There are two small lines that go from the condenser to the fuel tank and a larger line that is vented through the floor to the outside.  This line to the outside is to equalize the pressure between the inside of the tank and the atmosphere.  The larger line should have a "rubber" bell shaped piece on the end that is exposed dto the atmosphere. I am not sure if it is to prevent dirt and trash from clogging the tube or if it is to provide a low pressure area. Either way it should be there.  If you do not have or cannot obtain the original bell shaped piece, a boot from an ignition coil wire makes a good substitute.  I believe the boot on the coil end of the wire is the right size.  

What you experienced when you put on a new seal was what some people would call vapor lock.  The engine will run fine until a vacuum is created inside the tank and the fuel pump can no longer deliver fuel to the engine.  If you loosen the cap and the engine runs fine then you know you have a problem with one or more of the three lines to the tank and condenser or with the condenser.  Of course the fix is to make sure all the lines are open andd the condenser is working properly.

Good luck

Iverson

[Jonny B]

Remember that the later cars (don't remember off hand when) have the fuel conservation tank in the left rear fender, and the venting set up is different. The cars with the small evap tank in the right year, vent directly to the atmosphere (funnel shaped vent thing). The later cars vent back to the engine.

Jonny B
1967 250SL Auto

[Mike Webster]

Thanks scholars for all the help. That new gasket and a new fuel pump (replaced last year) worked so well it managed to suck all the rust and other crud though the system and now everything is pluged up. Replace with new fuel tank/blow out lines/clean screens/filters etc.

[nick350]

OK, so what happens if the seal is not sealing. Does the car run OK? Does the mechanic tune the car assuming that the seal is intact? Does this account for the stink of petrol at the rear of the car?

Nick350
r107 SL red
w113 SL white

[Cees Klumper]

I believe that fuel smell in the trunk is usually from bad hoses running to/from the venting canister. A bad gas cap seal, OR a deformed (bent) fuel filler neck, will cause spilling onto the bumper; neither is a good thing and can be easily fixed.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic