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I am looking at replacing my current fuel setup (cell in trunk and electric pump) and going back to a stock gas tank and am not sure if I should get the sending unit that has the vent in it, or get the gas cap that vents.

I was getting this sending unit:
http://cgi.ebay.com/ebaymotors/SEND...ewItemQQcategoryZ34204QQitemZ4616319639QQrdZ1

but if I get that one, I need a vented gas cap, right?
What are the pros and cons of each type?
 

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1974 Nova, 1977 Nova
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Hi Bill,

The extra lines on some sending units are return rather than vent lines. It is my understanding that GM used return lines on some carbureted vehicles in order to help prevent vapor lock. They used a special fuel filter or mechanical fuel pump with a return line outlet. That outlet has a small orifice that allows vapor to escape from the fuel line. And any liquid fuel that goes through the orifice gets dumped back into the tank through a return line that connects to the tank through the special sending unit with the return line fitting.

There are 2 types of gas tanks ... with and without EEC (evaporative emissions control). The EEC tanks have built-in vent lines that connect to a liquid/vapor separator mounted behind the rear seat (on 72 & earlier Novas). The 73-up tanks have a built-in liquid/vapor separator and no longer have the part behind the seat. A vapor line runs from the separator up to a charcoal vapor recovery canister in the engine compartment. The canister stores any gas that evaporates from the tank. A purge line connects the canister to the engine so the stored gas vapors can be drawn out. In theory, this system should save a bit of gas but may be difficult to re-install in your car if many of the original parts are missing.

There are also 2 basic types of gas caps. Emissions and non-emissions. The non-emissions caps freely vent the tank to the atmosphere (which can waste gas through evaporation). Emissions gas caps have a built-in pressure relief valve that will only vent to the atmosphere if the pressure gets too high. Both styles of caps provide vacuum relief. The EEC systems require an emissions type gas cap in order to be functional. But you should be able to use an emissions type cap on a non-EEC tank as well. In fact, I think this was done originally on some cars prior to the introduction of the EEC systems. This will basically keep evaporated gas contained in the tank unless the pressure gets too high. It's probably not quite as good as EEC but definitely better than a non-emissions gas cap in terms of reducing gas lost to evaporation.
 

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thats the same sending unit i put in my new gas tank. i bought the vented tank that way down the road if i put an electric fuel pump back in it i have a place to run return line to the tank already there. on my 70 nova the old tank i took out had the vented tank in it also, not sure if it is the factory tank but sure looked old enough to be. but the vent holes were pluged and not used for anything.

what i would like to know is on the sending unit, the wire comes for the positive conection and the ground goes on the other connector. how do you attatch the grond to the sending unit? its not threaded on the connector for a nut, or at least mine isnt..
 

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The ground wire should be attached as shown in this pic. The other "hot" lead terminal is the path wire to the gauge. IF you don't have an existing place to ground the sender, you'll have to find a spot on it to attach the ground.

You don't want to ground the tank in lieu of a proper sending unit ground due to the big "O" ring seal will tend to act as an insulator.
 

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my mix up, ok then what should i use to attach the hot wire to the terminal?
 

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Hi Bill,

There are two trunk floor pan braces that also serve to hold the gas tank straps. The factory ground point is on the passenger side brace just rearward of the tank. At least that's where it is on 73-79 ... 68-72 are likely in the same general location. It's basically just a self tapping sheetmetal screw driven up through the bottom of the brace.

You're right, you do need a rust and paint free spot to make a good ground connection. However, you can always paint over it after you've screwed the terminal in place to prevent rust.
 
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