Fuel tank question

Dohn

Dohn

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When I bought my coupe, the PPI told me the fuel tank was dented, but not seriously enough to worry about. After four years of not worrying, it developed a leak when I was in Greenville for the Foundation Fest. Close inspection showed a leak at the edge of the dent. When I got back home, a new tank was ordered and installed. I've now noticed a couple of times that the tank is talking to me: it gives out a low thump sound, like hitting an oil drum. Noticed it especially after driving on the highway and stopping at the bottom of the ramp. It seems like a problem of excessive suction in the system, and if so, could it be a clogged vent or bad gas cap? This is a '74, with a 3.5 B35 and Motronic.
 
Would it not simply be fuel sloshing around?

Fuel tank ventilation comes in 2 variants as far as I know.
EU (and possibly also early US?) have an open tube that splits from the fuel filler neck and it vents out through the floor, near the bumper mounting bracket.
(Later) US versions run a line to the engine I believe, but our US owners will know better.
 
Sounds like you have a venting issue. If your coupe was carbed it would eventually stop running due to the vacuum build up. Being that yours is injected you’re probably overworking your pump.

As Chris asked, after driving does the fuel cap make a hissing sound when you remove it?
 
The "cowboy" way of testing this would be to remove the filler cap, stuff a rag in the hole to prevent fuel from sloshing out and seeing if the thump goes away. If "yes" then the previous posters' theory about tank venting is correct (which I agree is the most likely cause).

And yes, this is a terrible idea. But as long as your tank isn't completely full and you only do it as a short-term test, I predict you and your coupe will survive.
 
HB Chris said:
When you remove gas cap is there a swoosh? If so a vent is plugged. You still have evap tank in trunk?
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Yes, there is a swoosh, sometimes fairly pronounced. Where is the vent on a US tank, and how is it routed? The car goes back to the shop that installed the tank on Monday, so I'd like to have some idea of where to look. Not sure what an evap tank looks like, so I'm fairly useless. Still.
 
There’s no need for a vented cap. All tanks have a vent on the filler neck.
Erik has noted in Europe the tank vents to the back of the car.
 

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looks like something i need to get. i may put the tank back in but i just assume run the hose out the bottom as the hose that ran to the front of the car is long gone.
 
Stevehose said:
What does your gas cap look like?
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Here are shots of the cap. After looking at the art Don sent, I just wonder if the vent pipe is disconnected or blocked. Won't be going into the shop on Monday, as snow is forecasted, and I'm sure the roads will be brined. Might poke around myself and see if I can find anything.

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Belmetric has the hose
Walloth has the rubber fitting
your car has 10’ of the blue pipe
5CE7F50E-39FF-4F07-B741-AAE1808E2879.jpeg
BB3C15FA-FD15-491B-AA6E-B220C3E88313.jpeg
 
It starts at the tank and goes through the car, through the firewall and around to below the battery tray.
It's the evap hose on a USA car.
 
Look above your passenger fender, they grey evap container was there with blue pipe going thru trunk bulkhead connected by fuel hose
 
I believe your vent hose goes into the tank on the passenger side of the trunk wheel well. But if it is vented to the trunk bottom and you need the bell-shaped rubber fitting,let me know. I bought it from W&N thinking I needed it only to find otherwise. I still have a slight gas smell in my trunk when it's opened and I have checked everything.
 
There are at least 15 possible sources of a gas smell in the trunk.
clamps, seals, hoses, fittings ....
 
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