....when I open my gas cap.
Is this normal?

Thanks in advance.

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1976 280C
SLOWER DRIVERS KEEP TO THE RIGHT.
DRIVE RIGHT PASS LEFT

No.

Need more info for any diagnosis. Don't you have the Holley conversion carb?

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Yes, Holley 4 BBL. I believe the model number is 4360. PO installed it before I bought the car over 20 years ago.

When I open the cap it feels almost like a radiator cap but without the water. There's pressure built inside the gas tank. This happens after the car has been running for a while.

Thanks!

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1976 280C
SLOWER DRIVERS KEEP TO THE RIGHT.
DRIVE RIGHT PASS LEFT

It's normal to have a very SMALL amount of pressure, but if it's hissing or being noticeable like a "whoosh" when you open the cap you probably have a purge valve or vent line plugged up somewhere.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I just happen to have an image from the FSM handy (attached), FWIW.

Attached Thumbnails

 

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- Greg -
1973 220D, The Prodigal Benz
1974 240D

Okay, I thought you had mentioned you had the Holley conversion.

A couple of probable causes:

Your venting is incorrect/blocked leading from the vapor recovery tank in the rear boot well. The large plastic line is the main vent leading back to the front of the car. The two smaller lines are fluid recovery lines.

The routing of the vent line leading to the air filter housing is clogged or not connected.

The charcoal canister mounted at the front is plugged or the lines are miss-routed/disconnected.

The diaphragm mounted beneath the passenger compartment is inoperative (follow the steel fuel line to locate).

http://www.peachparts.com/shopforum/vintage-mercedes/238292-m115-t-b-i-conversion-3.html

Post #38

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

What's going on is this....

As you drive the fuel moves around in your tank and the friction from this creates heat. Not a lot but it only takes a little to get a build-up in pressure.

Add to this your car is outside (Duh) and the sun warms up everything even on a cold day. This heat adds to the friction heat.

But not to worry! Your fuel cap is vented, right? Well, yes, but this venting only allows air into your tank so your tank will not collapse from the decrease in volume as the gasoline is used up. Vented caps are designed to not let gasses escape to the atmosphere. So your system is designed with a vent, to deal with this expansion, which usually takes the form of a carbon canister.

Fuel vapor that is releases by the fuel expanding in your tank (you will also see this process referred to as 'boiling') is trapped in the activated carbon canister. This vapor is then sucked into your fuel system when the car is running. So there is no air pollution from vented gasoline.

The carbon canisters will become saturated over time and unable to absorb vapors. So they look great but they have stopped working.

This system is really a good thing. I work with a lot of antique cars and the gas vapor smell is quite strong even when they are running as they should.

Somewhere in your fuel system something is amiss. The sad part of all of this is you need to start checking every part of it to see if it is working as it should.

I have seen people drive with their gas cap not tightened down all the way. This is not a solution as every time you turn you are risking losing some fuel out of the filler tube.

Only with a total search of your entire system will you find the problem. This is because it can be one of several places that is refusing to vent.

And such a search is not hard. It is just time consuming.