Fuel tank vent lines

[Gerry]

Hi guys
My first post here...
Restoring a 1979 Fiat 2000 Spider.
All the smogs been pulled off and got a intake manifold from an 1800 and a Weber 32/36 on top.
Here’s are my questions
Installing a new fuel tank and I noticed that the 3 vent lines from the top of the tank go into a round plastic 3 into 1 assembly. Well, the one output port doesn’t have a hose on it. It’s just not attached to anything and venting in the trunk. Not good.
In the pictorial I notice 3 lines running thru the front of the trunk body. 2 from the sending unit assembly (get that) but the 3rd port is open.
It seems that the missing hose from the 3 into 1 should be running thru there? If so, does that hose go to the engine compartment and hook up to some now gone smog port or does it just vent to the atmosphere?
Also, the larger vent hose runs up to the filler compartment next to the fill tube as advertised but this seems a bit questionable in my mind. If water gets into that compartment from rain/wash water wouldn’t that run into the tank?
And, anyone know why they apparently have 4 vent lines from the top of the tank?
Thanks for any comments
G

[18Fiatsandcounting]

Welcome, Gerry. This topic has come up before on this forum, so you might see some other posts if you search on "gas tank vent line" or the like. Anyway, here's my understanding for the earlier model years, and it could apply to your '79.

First off, the water "drain" at the bottom of the gas cap well should connect to a hose that drains to the ground. It should not go into the gas tank. There IS a tube (1/2" or so) that connects a fitting near the top of the gas tank fill tube to an inlet to the gas tank. Its purpose (my guess) is to prevent fuel bubbling back out of the fill tube with gas stations nozzles that got a little carried away with the flow rate. I don't know how much it is needed for modern gas station pump nozzles which tend to be slower.

Next, there at two outlets from the fuel tank on that round cap on the top of the tank, the cap with 6 or so nuts holding it onto the tank. One outlet (the larger one I think) goes to the fuel pump and from there up to the carb. The other outlet (the smaller one) goes to the fuel return line from the carb back to the tank. I've never been totally clear on why this setup has a fuel return line, but one theory is that this keeps fuel circulating away from the engine and back towards cooler areas of the car, thus preventing vapor lock. If you didn't have this setup, the fuel pumped up to the engine would be hotter as it spends more time in the hot engine compartment. But, to be honest, this theory never made a whole lot of sense to me...

After that there is the liquid/vapor separator (the 3 to 1 gadget) which collects the fuel vapors that are captured from 3 separate locations in the top of the gas tank and drips any liquid (that is formed by the condensation of these vapors) back down into the gas tank. With liquids now removed, the vapors continue down the center line on that 3 to 1 gadget to a 2-way valve (on my cars it is mounted right in front of the gas tank), and from there to the charcoal canister (vapor recovery). However, the 2-way valve actually has 3 connections: 1) the input, 2) the output, and 3) a connection to the outside atmosphere. The 2-way valve is there to admit air into the system when a vacuum starts building up in the gas tank, like when you are using up the gas as you merrily drive along. Otherwise, your fuel pump would actually be trying to pull gas up to the engine while working against an increasing vacuum in the fuel tank, which doesn't work so well.

It sounds to me like the center line on that 3 to 1 gadget is what is missing on your car. If you no longer have a charcoal vapor recovery canister, then there is nothing to connect this line to and so someone might have just left it open. You could try just capping off that center connection, but the risk there is that you build up a vacuum in the tank as fuel is used up, eventually causing the engine to stall as the fuel pump can't overcome the vacuum. I suppose you could put a one-way valve on that center connection, allowing air to enter but preventing fuel vapors from escaping.

OK, I've gone on long enough. Hope this helps!

-Bryan

[Mattymatt888]

Welcome, Gerry. This topic has come up before on this forum, so you might see some other posts if you search on "gas tank vent line" or the like. Anyway, here's my understanding for the earlier model years, and it could apply to your '79.

First off, the water "drain" at the bottom of the gas cap well should connect to a hose that drains to the ground. It should not go into the gas tank. There IS a tube (1/2" or so) that connects a fitting near the top of the gas tank fill tube to an inlet to the gas tank. Its purpose (my guess) is to prevent fuel bubbling back out of the fill tube with gas stations nozzles that got a little carried away with the flow rate. I don't know how much it is needed for modern gas station pump nozzles which tend to be slower.

Next, there at two outlets from the fuel tank on that round cap on the top of the tank, the cap with 6 or so nuts holding it onto the tank. One outlet (the larger one I think) goes to the fuel pump and from there up to the carb. The other outlet (the smaller one) goes to the fuel return line from the carb back to the tank. I've never been totally clear on why this setup has a fuel return line, but one theory is that this keeps fuel circulating away from the engine and back towards cooler areas of the car, thus preventing vapor lock. If you didn't have this setup, the fuel pumped up to the engine would be hotter as it spends more time in the hot engine compartment. But, to be honest, this theory never made a whole lot of sense to me...

After that there is the liquid/vapor separator (the 3 to 1 gadget) which collects the fuel vapors that are captured from 3 separate locations in the top of the gas tank and drips any liquid (that is formed by the condensation of these vapors) back down into the gas tank. With liquids now removed, the vapors continue down the center line on that 3 to 1 gadget to a 2-way valve (on my cars it is mounted right in front of the gas tank), and from there to the charcoal canister (vapor recovery). However, the 2-way valve actually has 3 connections: 1) the input, 2) the output, and 3) a connection to the outside atmosphere. The 2-way valve is there to admit air into the system when a vacuum starts building up in the gas tank, like when you are using up the gas as you merrily drive along. Otherwise, your fuel pump would actually be trying to pull gas up to the engine while working against an increasing vacuum in the fuel tank, which doesn't work so well.

It sounds to me like the center line on that 3 to 1 gadget is what is missing on your car. If you no longer have a charcoal vapor recovery canister, then there is nothing to connect this line to and so someone might have just left it open. You could try just capping off that center connection, but the risk there is that you build up a vacuum in the tank as fuel is used up, eventually causing the engine to stall as the fuel pump can't overcome the vacuum. I suppose you could put a one-way valve on that center connection, allowing air to enter but preventing fuel vapors from escaping.

OK, I've gone on long enough. Hope this helps!

-Bryan

I am planning to replace the fuel sending unit in hopes that it gets my gas gauge to work, but one of the fuel lines that connects to the sending unit doesn’t want to come off of one of the metal spouts.

It's not a rubber hose, more like a braided material and it seems old, brittle and saturated. I’m worried if I get it off I won’t be able to get it back on, or it could leak after.

Any advice on how to remove it safely without damaging it, or if it does get damaged, how would I go about replacing the hose?

[18Fiatsandcounting]

It's not a rubber hose, more like a braided material and it seems old, brittle and saturated. I’m worried if I get it off I won’t be able to get it back on, or it could leak after.

Any advice on how to remove it safely without damaging it, or if it does get damaged, how would I go about replacing the hose?

This braided hose was Fiat's version of a rubber hose back in the 1970s. It's rubber inside, but covered with a cloth braiding of sorts.

Given the age, this hose is almost certainly failing or about to fail. Almost a necessity to replace these. They usually are stuck to the metal lines, but simply wrestle off any clamps and cut off the old hose with a razor blade if you need to. Normally you would take care not to scratch the inlet tubes to the gas gauge sending unit, but since you're replacing this, using care isn't as necessary.

Then go to your local auto parts store and get yourself some fuel hose, and I'd suggest getting hose that is rated for ethanol use as well, as a lot of gasoline is blended with ethanol these days. [RANT=/ON] A result of massive subsidies to farmers to grow way more corn than is needed, so they use it to make ethanol or high fructose corn syrup and force it on consumers. [RANT=/OFF]

There are two fuel hose sizes, 5/16" for the supply line and 1/4" for the return line (inside diameters). The stock hose was in millimeters, but 5/16" and 1/4" are close enough. Get yourself enough hose to replace the lines from the tank sending unit up to where they meet the metal line under the car, and you probably should think about replacing these same hoses in the engine compartment as well.

-Bryan

[Mattymatt888]

It's not a rubber hose, more like a braided material and it seems old, brittle and saturated. I’m worried if I get it off I won’t be able to get it back on, or it could leak after.

Any advice on how to remove it safely without damaging it, or if it does get damaged, how would I go about replacing the hose?

This braided hose was Fiat's version of a rubber hose back in the 1970s. It's rubber inside, but covered with a cloth braiding of sorts.

Given the age, this hose is almost certainly failing or about to fail. Almost a necessity to replace these. They usually are stuck to the metal lines, but simply wrestle off any clamps and cut off the old hose with a razor blade if you need to. Normally you would take care not to scratch the inlet tubes to the gas gauge sending unit, but since you're replacing this, using care isn't as necessary.

Then go to your local auto parts store and get yourself some fuel hose, and I'd suggest getting hose that is rated for ethanol use as well, as a lot of gasoline is blended with ethanol these days. [RANT=/ON] A result of massive subsidies to farmers to grow way more corn than is needed, so they use it to make ethanol or high fructose corn syrup and force it on consumers. [RANT=/OFF]

There are two fuel hose sizes, 5/16" for the supply line and 1/4" for the return line (inside diameters). The stock hose was in millimeters, but 5/16" and 1/4" are close enough. Get yourself enough hose to replace the lines from the tank sending unit up to where they meet the metal line under the car, and you probably should think about replacing these same hoses in the engine compartment as well.

-Bryan

Thank you very much for the detailed response, this was a huge help! I will absolutely plan to replace the hoses when I do the fuel sending unit swap.

In regards to finding that “metal line” under the car you referenced, I will check my workshop manual to make sure it is a job I am able to perform. Thanks again sir.