The Dodge Challenger Message Board

Hello - new to this board, looks like a great one & should have joined years ago... I have a 1970 Challenger R/T 383 4-speed originally from Oregon & I've owned in Seattle, WA area since 1998. Black on Black (originally stripe delete but I added the white bumblebee stripe), build sheet, about 80K orig. Miles and restored over the years.

Needed a little help on fuel system. Car runs great but noticed when I replaced the sending unit years ago I just capped the return line (1/4"). The sending unit has two outlets - one for fuel to carb (upgraded to 3/8"), and the return one (1/4") which I capped. After 11 years of driving it this way I noticed that the cap had cracked and might have been leaking fuel as my mileage went south. I have the flip top gas option and have a replacement Stant non vented cap. Is this right to have it non vented for 1970? Also have the small return line that goes through the trunk that I'm guessing should have a hose to the sending unit return line. Right now I have the short hard line that exits just below the passender trunk floor through the fitting and goes to the fuel filler tube. So my question is do I hook up a return hose (1/4") from sending unit to filler tube? And should my cap be non vented or vented? I suppose with it capped like I have it maybe should have vented cap for expansion. It's run great for all these years so not sure.

Thanks in advance for any info.

Brad

1970 you should have a vent running from the filler tube inside the trunk with a hard line going through the trunk floor. As long as this tube is not blocked you do not need a vented cap and you were right to plug the return line at the sender.

Oh and welcome.

Thanks. I went ahead and ran a new 1/4" fuel line to connect the return hard line that comes out of the trunk area to the nipple on the sending unit. Still running the non-vented cap for now but filled the tank and seems fine......Still not sure if the cap should be a vented or non-vented type with this application (not CA emissions). It is basically a sealed system at this point and just not sure if it needs the vented cap to allow expansion to escape. The downside to a vented cap would be the minimal release and evaporation of fuel from the tank. I'd appreciate any input from other owners with simlalr setups. My system has a mechanical fuel pump as per original as well.

Gorgeous Challenger! As Dave stated, the line that comes from the fill tube should be there. There HAS to be a vent or else the fuel will expand when hot/warm and the tank will collapse if there is no where for the pressure to escape, actually it will be 'negative pressure when cool. I would use either the vent tube from the fill or a vented gas cap. With the type of cap you have,("Fuel" Flip top cap), it might be easier to run the fill tube vent. Dont believe me? fill a small 1 gallon gas tank 1/2 full and leave it sealed in the garage. When it gets hot it will 'balloon out. When it cools off it will collapse. You could also install a vapor seperator as used on Big block R/Ts 440's and Hemi's,, helps prevent vapor lock. BTW, the factory service manual shows the different setups in pretty good detail, ECS,(Evaporative Control System), you can either get a tank vent line kit for your's with or without the vent line attached to the fill tube thru year one. Black IS beautiful!!

You don't want a vented cap.
What you're looking for is here (look at the second and third pics):
topic-1999

or there (last pic of the trunk area) : http://www.moparaction.com/Article/PoP/PoP.html

This is much better than having a vented cap which will let fuel out on the quarter when you're cornering a bit hard with a full tank.

1970RT383 wrote:Thanks. I went ahead and ran a new 1/4" fuel line to connect the return hard line that comes out of the trunk area to the nipple on the sending unit.

Pretty sure that will not work. To use a non vented cap you MUST vent the filler neck to the open air. Otherwise your fuel tank will collapse and the fuel pump will struggle to suck fuel out of the tank.

dave-r wrote:

1970RT383 wrote:Thanks. I went ahead and ran a new 1/4" fuel line to connect the return hard line that comes out of the trunk area to the nipple on the sending unit.

Pretty sure that will not work. To use a non vented cap you MUST vent the filler neck to the open air. Otherwise your fuel tank will collapse and the fuel pump will struggle to suck fuel out of the tank.

Dead right

Thanks for the input. My trunk looks just like this from the article you gave the link to:


I totally get the fact that with a closed system the tank can't expand and that issue. I still don't know what was correct when these were new. It makes sense to me to have a vented cap, with the vent lines hooked up from the sending unit to the trunk line which runs to the filler neck. Otherwise it can't breathe. Can anyone tell/show me what was correct for this application when they were new?

Just checked Year One catalog and still confused....
Non-vented-style fuel tank cap for 1970-1971 Challenger models with Flip-Top fuel cap. #3194N $9.99
Locking vented fuel cap for 1970-1974 Challenger models with Flip-Top fuel cap. #LFC74 $17.99
I think I'm just going to run a non vented cap and call it good.

Let me try again then.

That hard line in the photo from the filler neck MUST NOT be connected to the fuel sender or anything else for that matter. It should just exit under the car.

It's job is to breath air in and out. If you drive the car with that line not open to the air you WILL damage the tank and possibly the fuel pump.

I can't put it more plain than that can I?

dave-r wrote:Let me try again then.

That hard line in the photo from the filler neck MUST NOT be connected to the fuel sender or anything else for that matter. It should just exit under the car.

It's job is to breath air in and out. If you drive the car with that line not open to the air you WILL damage the tank and possibly the fuel pump.

I can't put it more plain than that can I?

No Daveman, you can't .....................if I remember right that metal vent is poked into a slot or gap in the frame rail

I think you are right mate.

I get it - thanks. Why the 1/4" elbow at the sender then?

I have two options:

1 - Go back to the way I've had it with the elbow at the sender capped and the line from filler open to vent and run the non vented cap

2- Hook up a line to both the sender and fill tube vent and run a vented cap

I just wondered how it was originally. Doesn't make much sense to have a vent at the sender and just cap it?

Thanks again for the input.

Ain't that for the return line from the pump or charcoal canister then? On my 72 there is a four pipe vent that runs right up to the underside of the rear qtr panel, but for that you need the four pipe tank.
You could do worse than have a look at Barrys excellent Challenger T/A registry and ahve a look thru the service bulletins, I guarantee he'll have something on there

1970RT383 wrote:I get it - thanks. Why the 1/4" elbow at the sender then?

I have two options:

1 - Go back to the way I've had it with the elbow at the sender capped and the line from filler open to vent and run the non vented cap

2- Hook up a line to both the sender and fill tube vent and run a vented cap

I just wondered how it was originally. Doesn't make much sense to have a vent at the sender and just cap it?

Thanks again for the input.

My take is to go with your second option.

One thing to remember is both the tank and filler tube need to vent. When the pressure inside the tank builds, the filler tube vent (or vented gas cap for that matter) can pass fuel which is not good in an enclosed garage.

Sorry Jon I don't agree.

Fuel expension is very minor and there is more than enough room in the filler neck. Just don't fill right to the edge of the cap.

The vent is to allow air INTO the tank as fuel level drops. That is its purpose. Otherwise your fuel pump becomes a vacuum pump.

The return on the sender is for a return line from the carb in high performance and electric fuel pump set-ups where high pressure and flow rate fuels sytems bleed off spare fuel not needed at low rpms and light loads. Electric pumps in particular need a return otherwise they tend to fail.

You could just fit a vented cap and leave what you have done alone. But just be aware that the tiny hole in the cap is borderline too small for a performace big block and you will be sorry if it gets any dirt blocking it.

I would re-cap that return at the sender and just leave what Chrysler intended alone and forget about it.

I don't want to point the obvious here, but connecting the vent tube on the filler neck to the return nipple on the fuel sender really means you've connected the inside of the tank to the inside of the tank.
Not much venting to expect from that.
The OEM setup in 1970 was a non-vented cap, a sealed tank (*), and a single vent line from the filler neck to the right rear frame rail.

(*) post-1970 and California tanks had a row of vent tubes at the front of the tank (a little bit worse each year), but that was rather a fuel vapor recycling thing.

Agreed the OEM setup is the safest way. Mine being a 70 ca emission I have no personal reference to the 49 state models though. A vented cap is currently installed however it was a Chrysler replacement from the early eighties so can't say for sure.

As for venting the filler neck and tank I reference a gas can with the pullout spout which is similar to the fuel tank. If you have ever opened one of these while pressurized , fuel is forced up the spout and leaks uncontrolled until equalized. When i forget to Open the vent on the can first and that is the usual result.

My thought is to vent the tank (vapor area only) to the filler neck(vapor area) then to the atmosphere with a vented cap. That way any pressure build will be in the zone above the liquid and released as vapor only. Option #2

Jon wrote: My thought is to vent the tank (vapor area only) to the filler neck(vapor area) then to the atmosphere with a vented cap. That way any pressure build will be in the zone above the liquid and released as vapor only. Option #2

Yes but if the filler neck vent is left alone as stock you never get pressure in the tank to start with and a vented cap is redundant.

dave-r wrote:

Jon wrote: My thought is to vent the tank (vapor area only) to the filler neck(vapor area) then to the atmosphere with a vented cap. That way any pressure build will be in the zone above the liquid and released as vapor only. Option #2

Yes but if the filler neck vent is left alone as stock you never get pressure in the tank to start with and a vented cap is redundant.

Exactly right, vented twice and now with the added ability to piss some fuel out of the cap during overenthusiastic cornering...................just do the filler neck Jon and stick the line inside a frame slot or something

Adrian Worman wrote:

dave-r wrote:

Jon wrote: My thought is to vent the tank (vapor area only) to the filler neck(vapor area) then to the atmosphere with a vented cap. That way any pressure build will be in the zone above the liquid and released as vapor only. Option #2

Yes but if the filler neck vent is left alone as stock you never get pressure in the tank to start with and a vented cap is redundant.

Exactly right, vented twice and now with the added ability to piss some fuel out of the cap during overenthusiastic cornering...................just do the filler neck Jon and stick the line inside a frame slot or something

Thanks, but i'll stick with my 70 ca emission setup. Only problem in 36 years is burping from the gas pump at top off. Maybe a vent on the filler neck would help that.