Mercedes-Benz Forum banner

1 - 8 of 8 Posts

·
Registered
85 Mercedes 500 SEC
Joined
·
217 Posts
Discussion Starter #1
Hi

Have a 1985 500sec..Im having a missing issue which I reckon is fuel related but also everytime I take the cap off its a strong smell of fuel and there is fuel all over the cap and it drips onto the ledge when I take it off. Why would that be? And whats the nozzle just inside the tank? I would've thought the return line would go back straight into the tank. Do I have an issue?

Thanks in Advance
 

Attachments

·
Registered
1989 W124 260E
Joined
·
1,527 Posts
Do you hear an hissing sound as you remove the filler cap as it sucks air in to the tank . If it is you dont ,,then replace the cap .This will be a start if you replace it and i could stop your fuel vapours exiting the neck of your fuel tank ,.There sould be pressure build up in the tank this is normal .Without the pressure build up might be why you have engine missing
 

·
SuperModerator
1986/1990 W126
Joined
·
13,765 Posts
Too much pressure or vacuum is a fault and is easy to remedy with a new vent check valve. It's just a little plastic valve in the general area above the rear axle in the vent line that goes to the charcoal canister up front.

Of course the line itself can get blocked too. I would strongly recommend searching the threads already done about this as all the details you need including part numbers and the spec for tank pressure are there.

Could be that sometimes you have too much vacuum and hence the misfire. Although in my case the pumps just imploded the fuel tank a bit and the car still ran great.
 

·
Moderator
1991 500SEC 50K mi. 1987 560SEC Now 153K mi. 2020
Joined
·
4,401 Posts
Hi

Have a 1985 500sec..Im having a missing issue which I reckon is fuel related but also everytime I take the cap off its a strong smell of fuel and there is fuel all over the cap and it drips onto the ledge when I take it off. Why would that be? And whats the nozzle just inside the tank? I would've thought the return line would go back straight into the tank. Do I have an issue?

Thanks in Advance
After driving, it is normal to have a whoosh of fuel smelling air released from the tank as the cap is opened, especially if the fuel level is low and the ambient temperatures are high. My cars both do this as the filler cap is removed.
I expect your current weather in AU to be very warm these days, as by comparison, we here in the Boston area freeze watching the snow falling. :)

Under the above conditions, a suction would be considered abnormal and could lead to fuel tank damage, as Ian has pointed out.

The line inside the tank filler is to vent displaced air upstream of the fuel pump nozzle discharge as fuel is pumped in.

Here are some fuel system related files on the tank design & venting from the w126 MBCDs.

Good luck,

MBL
 

Attachments

·
Registered
85 Mercedes 500 SEC
Joined
·
217 Posts
Discussion Starter #5
There doesn't seem to be any vacum when opening the cap. I have just replaced it with a benz cap. What's the nozzle that just sits in the entrance of the filler. It must spray fuel on the cap.
 

·
Outstanding Contributor
350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
Joined
·
3,170 Posts

·
Registered
Too many to list
Joined
·
8,808 Posts
The vent check valve is a common problem and if you don’t fix it , it can collapse your metal fuel tank

change the check valve
 

·
Registered
1990 560SEL
Joined
·
30 Posts
I suspected the same issue on my 1990 560SEL and changed the check valve today. The part is less than 20 USD and really easy to reach and change. No need to lift the car and you only need a pair of pliers for the spring clamps. I changed the purge valve under the hood as well which was even easier.
 
1 - 8 of 8 Posts
Top