I read somewhere that replacement tanks for 70's have problems with the stock vented cap.

Well I sure did with mine.

With an original GM vented cap on it, my tank would build up so much pressure it would shoot the cap right off in my hand when I'd turn it to take it off. The pressure build up would actually cause the car problems with running, so I put a real loose fitting cap on for a while so that it would run right, but then of course it would leak gas whenever I would take off fast and gas sloshed up againts it.

Can't have that. I mean not being able to take off fast :D

_________________________________________________


What to do?

Searches I did on here showed me that numerous people have problems with this same issue.

I realize that some other years have vent tubes and such and that not everyone's problem will have the same solution. However, since I have come up with a great working solution for my problem I thought I'd share it with all of you who are still looking for a fix.

I bought a Murray plastic locking gas cap and drilled three small pressure release holes in a very precise location.

_________________________________________________


When I put the cap on without any modification it behaved just like my GM cap. In that it fit so snuggly the tank would again build up pressure like before.

Here's how I located the ideal spot to drill:

The cap comes with two solid guides, that line the cap up with the slots in the tank filler neck, and two spring loaded "catches" that snap inside the neck to hold the cap on.

The springs for the two catches are housed inside of two pockets in the cap that extended all the way back under the rubber seal. So drilling holes
through the outer cap and into these pockets allows the pressure to weep out past the catches. While at the same time the tight fitting rubber seal keeps any gas from leaking out!

In this picture I'm showing the two solid guides (on the sides) and one of the spring loaded catches (in the center).

http://img40.picoodle.com/img/img40/9/7/26/f_cap1m_616a4a1.jpg

In this picture I've pulled the rubber seal forward (using a couple of flat head screw drivers) to locate exactly where one of the pockets is.

http://img36.picoodle.com/img/img36/9/7/26/f_cap2m_6391f2b.jpg

In this picture you can see the location where the three holes were drilled through the outer cap in line with the pocket.

http://img32.picoodle.com/img/img32/9/7/26/f_cap3m_634354e.jpg

I bought the cap off eBay and have noticed that at least one person seems to sell them regularly.

http://img35.picoodle.com/img/img35/9/7/26/f_cap4m_903fe73.jpg

The part number for the cap is 5092.

Hope this helps at least one person, as it did me!

Thank's for the info. i have the same problem, i thought it was the fuel being heated on the return line from the hot fuel rail's.:thumbsup:

Anyone know where these caps are sold (NAPA, Pep, Zone, etc.) other than E-bay?

PM sent Rich.

damn that's one ugly cap.... i guess if it prevents the usual leakage though, might be worth it.

We're not talking about a hood ornament here :D

This is definately a function over fashion issue.

Well, all that I can say is IT WORKS!

After trying both vented and vented/anti surge gas caps on the replacement non-vented gas tank on my '70 Elky, pressure build-up in the new tank was definitely a problem. The build-up was sufficient enough to be a real concern. I rigged the check valve in my vented cap so that it would be "open" all of the time. It solved the pressure build-up problem but created a gas fume issue in the passenger compartment when making turns or under hard acceleration.

Aside from the cap's non-stock appearance, Don's solution is definitely the answer to the pressure build-up problem. The three holes are unnecessary. I used a single 5/32" hole in just the right spot and it works great. All that's needed is a vent hole to work in concert with the cap's rubber gasket. The gasket seals the filler neck but allows excessive pressure past and out the vent hole. I've driven the car with both a full tank of gas and a half-tank for various distances just to see if there would be a difference based upon gas/air volume in the tank and there's no difference in the performance of the cap.

I don't recall exactly what the cap cost, but it was right around $20.00 including shipping and handling. Well worth the expense. I must admit however, the cap had me baffled for a minute when I first put it on. It doesn't twist on like a stock cap. Instead, it merely pushes straight on and snaps into place.

If you have a pressure build-up problem, I highly recommend that you give Don's remedy a try.

Anyone know where these caps are sold (NAPA, Pep, Zone, etc.) other than E-bay?

I haven't seen them anywhere else yet but through this guy on eBay ( lyerlybill ). But he usually seems to have at least one listed all the time.

http://stores.ebay.com/BG-Collectibles-Plus_Automotive-Parts_Chevy-other-General-Motors_W0QQcolZ4QQdirZ1QQfsubZ14079327QQftidZ2QQtZ km

I'd like to hear other success stories if someone else uses this solution too :thumbsup:

Glad you're happy Rich!

No need to drill. Pep Boys listed two different caps for my '70 Elco, w/ or w/o California emissions. I got the w/emissions, only the prior owner had ripped out the vapor canister and plugged the lines, and I still had tank pressure. So I got the other one and it appears to have solved the issue. I checked and the cap labeled for California emissions had a one-way vent only -- it would let outside air get sucked into the tank, but not back out. By contrast, the cap for non-emission cars has a two-way vent and will pop open to let excess pressure back out of the tank. Naturally, I tested this by blowing/sucking on the two caps and lung pressure was enough to pop the valve on the non-emission cap. But the valve is spring-loaded, so I'm not anticipating any sloshing problem. By the way, these are standard Stant caps. The part numbers are 11806 (California emissions) and 11646 (non-California emissions). Naturally, if you've got tank pressure the one to get is 11646 since it has the two-way valve.

Replacement tank or not, I experienced excessive tank pressure with my '70, LS-6 Chevelle, the week after it was delivered to the dealership. When I would remove the cap whenever I would fill up with Sunoco 260 (leaded), I would always hear a "Whoooooosh," exiting the fuel tank. The fuel gauge would also indicate more fuel than I thought I had before I yanked the cap off. This would mostly occur only if the car had been running and was warmed up and I don't recall it happening if the engine was cooler. I also never experienced any fumes in the cabin and never replaced the cap.

I surmise all those cars with those engines were finicky. :cool:


Respectfully,
John R.

Replacement tank or not, I experienced excessive tank pressure with my '70, LS-6 Chevelle, the week after it was delivered to the dealership. When I would remove the cap whenever I would fill up with Sunoco 260 (leaded), I would always hear a "Whoooooosh," exiting the fuel tank. The fuel gauge would also indicate more fuel than I thought I had before I yanked the cap off. This would mostly occur only if the car had been running and was warmed up and I don't recall it happening if the engine was cooler. I also never experienced any fumes in the cabin and never replaced the cap.

I surmise all those cars with those engines were finicky. :cool:


Respectfully,
John R.

Interesting. Thanks for sharing that John!

I take it from your comments that you bought it new? Do you still have it?
.

(Off Topic)

Interesting. Thanks for sharing that John!

I take it from your comments that you bought it new? Do you still have it?
.


Don,

Yes, I ordered it new upon arriving home from overseas in January 1970, and it was delivered about six or seven weeks later. I was still recovering from combat wounds, so I was able to take delivery of it about three or four days after it was delivered.

Yes, I still have it with 14,460 original miles. Original window sticker, PoP, Dealer contract, Dealer invoice and build sheet. All Red with Black stripes, black interior, buckets, console, Special instrumentation, AM-FM radio w/rear speaker, tint, LS-6 engine, Cowl Induction hood, H-D battery, M-22 transmission, 4.10 posi-rear. NO P/S...No A/C. ALL Numbers Match. Assembled in Baltimore, MD.

Thank you for your interest in it, Don. :thumbsup:

Respectfully,
John R.

(Off Topic)
Don,

Yes, I ordered it new upon arriving home from overseas in January 1970, and it was delivered about six or seven weeks later. I was still recovering from combat wounds, so I was able to take delivery of it about three or four days after it was delivered.

Yes, I still have it with 14,460 original miles. Original window sticker, PoP, Dealer contract, Dealer invoice and build sheet. All Red with Black stripes, black interior, buckets, console, Special instrumentation, AM-FM radio w/rear speaker, tint, LS-6 engine, Cowl Induction hood, H-D battery, M-22 transmission, 4.10 posi-rear. NO P/S...No A/C. ALL Numbers Match. Assembled in Baltimore, MD.

Thank you for your interest in it, Don. :thumbsup:

Respectfully,
John R.

Awesome story John! :thumbsup: That car must be priceless to you!

My dad still has his 73 Nova SS that he ordered while serving as an MP. I thought his was low mileage with 21,000 on the odometer! Yours sounds like a time capsule! :cool:

O'Reilly auto parts has that cap listed under the Master Pro name with the 5092 part number for $10.99 each. However,they list the proper gas cap for my '72 Elky as 5771 for $10.99 each also. I'll stop in there at lunch and see if it is similar and maybe get one to try.

Rick

I had that problem in 1971 with my 70 LS-5. El Camino I drilled a .060 hole in the cap under one of the ears, And the hole can not be seen. I had to do something, I just couldnt take the pressure--LOL . Been working fine since 71.
Bob ACES--AACA:thumbsup:

my 70 chevelle built up so much pressure it pushed the filler neck down and would flood the engine. this was 15 years ago. at the time i couldnt find the 2 way vent cap and ended up putting a vent on top of the sending unit and ran it to a filter

Thanks Don for the tip. By the way Auto Zone carrys this vented locking gas cap in stock they have the same part number 5092 cost 10.99 Cst Inc makes it and its not made in china. I did as Rich said one hoe in cap. Now there is no pressure and the tank dosent look like its getting pulled in. I run a holley electric fuel pump. And they have auto zones parts stores in all states I believe. So dont bother with ebay and shipping. If there was a tip of the year award this one gets it. Dave

Thanks Don for the tip. By the way... / ...If there was a tip of the year award this one gets it. Dave

There is! They awarded it to me at last year's annual banquet.

http://farm4.static.flickr.com/3079/2537355997_a0a670d984.jpg

What? You thought I would document all that out of the kindness of my heart? :cool:

There is! They awarded it to me at last year's annual banquet.

http://farm4.static.flickr.com/3079/2537355997_a0a670d984.jpg

What? You thought I would document all that out of the kindness of my heart? :cool:

Too bad Don. It used to be a $1,000.00 check instead of the loving cup. Guess you'll just have to keep wearing that tacky blue flannel suit.

Too bad Don. It used to be a $1,000.00 check instead of the loving cup. Guess you'll just have to keep wearing that tacky blue flannel suit.
I need to read my '07-'08 TC handbook better.
I thought they WERE still giving out the cash
awards when I wrote this thing, DANG IT!

http://bestsmileys.com/reading/2.gif