[Droptop72]

I just replaced my 72' gas tank with a 70' non-EEC tank (no vents) and used a 70' fuel sender with the 1/4' vent line routed through a filter and up over the rear axle. I was running a non-venting locking cap for the 72', but that of course does not fit the 70's tank, so I am wondering what to get that woun't puke fuel or blow up or implode the tank. I would like to stick with a locking cap if possible. I know there has been quite a lot of discussion about this issue on old forum threads, but i haven't found a definitive "will work" answer. Any thoughts?

 

[DUTCH MAX HEADWORK]

I would use a 70 vented cap with your current set up. I'm not sure where the return line dumps back into the tank. When or if it could end into the liquid fuel. A vented cap might keep the pressure in the tank low enough to keep fuel from exiting through your current vent tube set up. I say may because even the vented caps will hold some pressure.

The best vent set up is the EEC system and it's also the will work system, but I guess that's gone. If you have any of the components hang on to them or sell them as they have some value.

Let's see how your system works for you. If you experience negative issues there are somethings we can try to remedy them.

 

[72soft-top]

Interesting? Not sure the benefits of switching to the 70 tank, but that’s neither here nor there.
For the vent filter you installed, you have to make sure it doesn’t vapor lock if it gets saturated with gas. Best bet would be a vented gas cap also to always guarantee venting is accomplished.

I also agree with Al...the factory 72 system works great.

 

[Droptop72]

Well, I made the change because I wanted to replace the tank and fuel line anyway, and the 70' is a much cleaner simpler setup. I feel that it SHOULD work fine with a vented cap, BUT, several people have reported having issues with the replacement vented caps either not letting air in and out properly, or allowing fuel to spill out. There are a lot of pre-71' cars out there that seem to wok right, so I'm just wondering which cap people have had success with.

 

[DUTCH MAX HEADWORK]

Amazon.com: Stant 10492 Locking Fuel Cap: Automotive

Buy Stant 10492 Locking Fuel Cap: Fuel Tank Caps - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

Make sure any cap you get is for a 70 Chevelle and is vented. You don't want one for a 70 California Chevelle as they were non vented.

Free 30 day return to Amazon if it doesn't work. So don't sit on it when you get it.

Most of these vented caps will allow venting in but restrict some venting out. There's a little one-way valve in the cap that controls this. You may build pressure in the tank, but your second vent out of the top will release that pressure. Hopefully, fuel and vapors do not escape through your return line vent system.

Every system and year has its quirks and most have ways to improve issues that arise. By far, the best system was the EEC system, but everyone thinks they are a smog system and also look ugly. I get it.

Don't overfill your tank. When the nozzle clicks off call it good. If you overfill, that return line is likely to be submerged in fuel and you'll be back to 1969 fuel leaking from your vent.

 

[billdini]

Amazon.com: Stant 10492 Locking Fuel Cap: Automotive

Buy Stant 10492 Locking Fuel Cap: Fuel Tank Caps - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

Make sure any cap you get is for a 70 Chevelle and is vented. You don't want one for a 70 California Chevelle as they were non vented.

Free 30 day return to Amazon if it doesn't work. So don't sit on it when you get it.

Are you sure Al? I thought all OEM 68-72 tanks use a non vented cap. My 68 have two vents up high in the axle area. I also know Captain Casey had a repro tank that had a Vented neck and a non vented cap and did not seal at all. No it wasn't leaking from the small vent hole. He bought a 68 Spectra tank and a non vented cap and everything went together as it should. It is my understanding a vented cap will not work on any 68-72 tank, they are all vented in one way or the other and should use a non vented cap..

 

[Johnny Bravo]

I have a 70 with no return line and a vented cap. The cap is aftermarket and after several years of too much pressure building up in the summer, I disassembled the cap and cut 1 coil out of the spring. It works great now and I have zero issues with fuel spillage.

 

[billdini]

I have a 70 with no return line and a vented cap. The cap is aftermarket and after several years of too much pressure building up in the summer, I disassembled the cap and cut 1 coil out of the spring. It works great now and I have zero issues with fuel spillage.

A vented cap is not supposed to allow for backpressure to be released it is to allow tank to breath in as the fuel level drops. I had a 82 Z28 that was building pressure and it had a hole in the muffler blowing hot exhaust gases on the tank and once I welded that hole problem was solved.

 

[Johnny Bravo]

A vented cap is not supposed to allow for backpressure to be released it is to allow tank to breath in as the fuel level drops. I had a 82 Z28 that was building pressure and it had a hole in the muffler blowing hot exhaust gases on the tank and once I welded that hole problem was solved.

You're correct, I misspoke. What was happening was my tank was not breathing properly and the filler neck would drop about an inch. When I opened it to fuel up it would suck in air and go back to it's original position. It still only happened in the middle of summer, but it's fine now.

 

[DUTCH MAX HEADWORK]

Are you sure Al? I thought all 68-72 tanks use a non vented cap. My 68 have two vents up high in the axle area. I also know Captain Casey had a repro tank that had a Vented neck and a non vented cap and did not seal at all. No it wasn't leaking from the small vent hole. He bought a 68 Spectra tank and a non vented cap and everything went together as it should. It is my understanding a vented cap will not work on any 68-72 tank, they are all vented in one way or the other and should use a non vented cap..

68-69 non vented cap. Tank had provisions for venting in front right. 2 tubes. One of which was always open to the atmosphere unless the tank was overfilled. They had that plastic vent that connected the 2 vent lines. The theory was that fuel in either line could return to the tank instead of the ground. Some earlier cars in that period did not come with that plastic vent connector. I heard but don't have proof that earlier cars were warrantied and the dealer would install the vent combiner after complaints of fuel leakage. The upgrade was better than nothing. Installing a lengthen tube to the single side of the combiner and running it up as high a possible is an improvement to this system.

70 non CA final destination used a vented cap. Tank has no provisions for venting. A vented cap was needed.

70 CA destination cars had a non vented cap and EEC system. Tank has additional vent and returns installed on tank.

71-72 went to the EEC SYSTEM and a non vented cap. This is by far the best system to control and separate vapors and return fuel to tank. It had the fuel vapor separator mounted on the body of the car behind the rear seat. They also had a spaghetti factory of hoses and a carbon canister mounted under the hood that collected and separated then burned vapors.

 

[DUTCH MAX HEADWORK]

A vented cap is not supposed to allow for backpressure to be released it is to allow tank to breath in as the fuel level drops. I had a 82 Z28 that was building pressure and it had a hole in the muffler blowing hot exhaust gases on the tank and once I welded that hole problem was solved.

That's the reason on the 70 non eec tank to build pressure. It's a common complaint. The vented cap only lets air in. But the tank heats and builds pressure. So we drill a tiny hole in the cap to release this pressure. The hole is drilled through only the first layer of the cap from the inside and doesn't extend through the outside of the cap. You can see the hole at 9:00 on this 70 cap. The hole in the center of the cap is the stock vent that contains the one-way valve.

 

[billdini]

68-69 non vented cap. Tank had provisions for venting in front right. 2 tubes. One of which was always open to the atmosphere unless the tank was overfilled. They had that plastic vent that connected the 2 vent lines. The theory was that fuel in either line could return to the tank instead of the ground. Some earlier cars in that period did not come with that plastic vent connector. I heard but don't have proof that earlier cars were warrantied and the dealer would install the vent combiner after complaints of fuel leakage. The upgrade was better than nothing. Installing a lengthen tube to the single side of the combiner and running it up as high a possible is an improvement to this system.

70 non CA final destination used a vented cap. Tank has no provisions for venting. A vented cap was needed.

70 CA destination cars had a non vented cap and EEC system. Tank has additional vent and returns installed on tank.

71-72 went to the EEC SYSTEM and a non vented cap. This is by far the best system to control and separate vapors and return fuel to tank. It had the fuel vapor separator mounted on the body of the car behind the rear seat. They also had a spaghetti factory of hoses and a carbon canister mounted under the hood that collected and separated then burned vapors.

Al I know for a fact some vented caps for Chevelle (and my 69 Camaro) would leak fuel like crazy. The cap just didn't fit right and like I said Captain Casey had the same issue. The caps do not seal if wrong. Totally different neck. A vented cap will not seal on a 68 for sure if you have the correct tank with two vents that go into the plastic housing fastened in the axle area against the tank support.

 

[Revere]

I did the same, put a 70 tank in my 71 without all the vent line stuff on top. I bought this cap from this write up and drilled one small 5/32 hole and it works perfect. The rubber seal seals to the filler neck perfectly and the the small vent hole lets out any pressure build up. It worked so well I bought three caps just to always have one if theyre ever discontinued. Write up link - Gas tank pressure problem solved

The cap is Motorad MGC92 on ebay

 

[Prong]

I did the same, put a 70 tank in my 71 without all the vent line stuff on top. I bought this cap from this write up and drilled one small 5/32 hole and it works perfect. The rubber seal seals to the filler neck perfectly and the the small vent hole lets out any pressure build up. It worked so well I bought three caps just to always have one if theyre ever discontinued. Write up link - Gas tank pressure problem solved

The cap is Motorad MGC92 on ebay

I was able to pick this cap up at the local Napa store. Part number 703-1836 for $12.99 + tax.

 

[hydro462]

On my 69 I used the 72 tank. It has three vent fittings across the top front of the tank. I ran hose to all three and tied them all together to a fitting that also had a 3/8" fuel filter. This took all my fume and surging issues away...

 

[Dean]

When I got the new gas tank for my 70 down out of the attic and took it out, it was a 71 tank with 3 vents and since it had been so long since I bought it, I couldn't return it so I capped off the 3 vents and
made a vented cap out of two caps, a 70 cap and a 71 cap.

The only problem is now I have a new 70 fuel gauge sender in a 71 tank and it shows E with 10 gallons in the tank.

 

[billdini]

I don’t get why so many people are using the wrong tank for their cars. They aren’t expensive. I think I paid like $139 delivered. Use the right tank with the correct vent with the right cap for the right car. This excludes the mods to a cap like in the 70 because of a design issue. Why are we making this such a big issue.

 

[Droptop72]

I don’t get why so many people are using the wrong tank for their cars. They aren’t expensive. I think I paid like $139 delivered. Use the right tank with the correct vent with the right cap for the right car. This excludes the mods to a cap like in the 70 because of a design issue. Why are we making this such a big issue.

Well, if your car has been stripped of all of the evaporative equipment, and you are NOT going to find it all and put it back on, what becomes the correct tank? I guess the mistake I made is ASSUMING that when Chevy built the 1970 cars, that they had designed it correctly and they would ALL work properly. It appears that may not have been the case. I would have also ASSUMED that the fuel cap manufacturers, KNOWING there was an issue, would have engineered a fix, the kind of fix that we must make on caps now. I have now purchased two of the MGC92 caps on Rock Auto for less than $6 a piece. I will modify one as instructed, and see how it goes. Unfortunately, it will probably be June before I know whether it works or not.

 

[billdini]

Well, if your car has been stripped of all of the evaporative equipment, and you are NOT going to find it all and put it back on, what becomes the correct tank? I guess the mistake I made is ASSUMING that when Chevy built the 1970 cars, that they had designed it correctly and they would ALL work properly. It appears that may not have been the case. I would have also ASSUMED that the fuel cap manufacturers, KNOWING there was an issue, would have engineered a fix, the kind of fix that we must make on caps now. I have now purchased two of the MGC92 caps on Rock Auto for less than $6 a piece. I will modify one as instructed, and see how it goes. Unfortunately, it will probably be June before I know whether it works or not.

I am not saying to install carbon canisters and other emission stuff but I am saying to buy a tank and cap that work together. Obviously the the 70 tank with cap modified to release back pressure would be the easiest for all 68 to 72 Chevelles if originality is taken out of the equation otherwise 68/69 setup would be next. I can't tell you how many cars that I see leave a light leaving gas as they take off.Many just think it is normal, well it isn't.

 

[Dean]

I don’t get why so many people are using the wrong tank for their cars. They aren’t expensive. I think I paid like $139 delivered. Use the right tank with the correct vent with the right cap for the right car. This excludes the mods to a cap like in the 70 because of a design issue. Why are we making this such a big issue.

What I probably should have done was order a new 70 tank and try to sell the new 71 tank.
Now (if I could get under there and change the tank) I would have to sell a slightly used new 71 tank with less than 100 miles on it .