How do I know if I need a vented or non-vented gas cap for my 1970?

Thanks,
BH

 

This is an original 1970 cas cap.

 

1969 was the last year Corvettes used a vented cap. ALL 1970 Corvettes came with a sealed (non-vented) fuel cap as part of the emission system, to prevent the release of fuel vapors into the atmosphere. The tank is vented through a check valve on the left side of the tank and a vent line that runs down the left frame rail to a charcoal canister mounted below the master cylinder.

 

Thanks for the replies. When I removed the old tank, there was no check valve down on the tank or line on the left side. Just the two lines on the right side and the hoses that connect from fuel tank to the hard lines.

BH

 

I just looked at the 1970 Corvette factory assembly manual, and it does not show a check valve, line down left side or charcoal canister. Unfortunately it doesn’t state vented or non vented gas cap

thanks

 

dennis posted in the C3 Tech section that only California '70s got the charcoal canister and SEALED cap but I don't know that for sure myself (having a '72). The bottom line is that if you do not have the tank vapor separator valve on the top driver side of the tank connected to a steel line running to the front along the frame next to the brake line to the canister in the front behind the wheel well, then go with a VENTED cap.

 

Thank you

 

ALL 1970 Corvettes came from the factory with a sealed cap.
gbvette62 The tank is vented through a check valve on the left side of the tank and a vent line that runs down the left frame rail to a charcoal canister mounted below the master cylinder.
1970s with the LT1 option did Not have a vent line

 

This is what the check valve/fuel separator valve looks like

 

neither of those are shown in the 1970 Corvette factory assembly manual

 

The pic kansas123 uploaded are two views of the same component.

For the '72 AIM it's on UPC 8, sheet B6 (page M214).
The GM number is 7028035 and the parts book indicates it's for '70-'74 with no exceptions noted, however the section below, listing the canisters, the '70 HD 454 (LS-6, I think) is missing.


Corvette Central has a blog page on the system for anyone interested in reading it.

 

My 1970 does not have any vent line or charcoal canister, and it has a "Sealed" gas cap. The gas tank and gas cap are original to the car. My original cap below.

 

Okay, so I just dug out my 1970 AIM and my (very dogeared) 1970 Corvette Parts book.

From what I can see it appears the AIM does not show the vapor line check valve (separator) or the charcoal canister. But....the 1970 parts book does list all of those items for the 70 Corvette, but only when equipped with EEC (Evaporation Emission Control).

The parts book also lists four gas tanks for 1970 (Group 3.001). The first, 3967746, is listed for use with the 350 Special High Performance (the LT-1) and 454 HD (this would be the never made it to production LS-6) except with EEC. The second is part number 3967741 and is listed for the 350 (L-48 & L-46) and (LS-5) 454 except with EEC. The third, 3967749, is listed as with EEC, except with the HD (LS-6) or the Special High Performance (LT-1). The final 70 gas tank is part number 3967755, and is listed for use with Special High Performance (LT-1) and HD (LS-6) with EEC. The L-48, L-46 and LS-5 got a different tank then the LT-1 and (non existent) LS-6, because Q-Jet equipped engines had a right side fuel return line that the Holley equipped LT-1 didn't have.

What I usually call the fuel vapor check valve, but the parts book calls a "Separator", is listed for all 70 Corvettes, but only with Evaporation Emission Control (EEC).

The parts book only lists one gas cap for 1970, 3974743, and only lists it for use with the none EEC tanks. Strangely, there is no cap listed for 1970's with EEC? Even stranger, the vented cap 3952708 is listed for 63-69, but the vented (dealer installed) locking cap, 985919, is listed for 63-70?

One last thing, in May 1970 GM changed all the emission labels by replacing the 2 letter code used on each label pre May, with a new code for cars built from May on. As an example, the LT-1 changed from code "DS" to code "CT" and the LS-5 went from code "DQ" to code "AW".

Based on the information from the Parts Book, and knowing the emission labels changed in May 70, I have to assume that 70 Corvettes built between January and May came without EEC, so they did not get the Separator, vent line or Charcoal Canister, while those built from May on got EEC and the associated parts. This would mean that no 70 Corvette built before May had the Separator or Charcoal Canister, but all of them built from May on, including LT-1's, came with the Separator and Charcoal Canister. If these assumptions are correct, then I think it's possible (but not likely) that cars built between January and April used a vented cap, while those built from May on used an unvented (sealed) cap.

 

That is the very rare, 1970 only "sealed" gas cap. The 70 cap you have has finger notches around the perimeter, like a 63-69 vented cap, but is sealed. Original 71 caps have the notches too, but also had a red plastic tag opposite of where "sealed" is, that said "Caution Open Slowly". For 72-74 the notches and red tag were removed from the caps, leaving them smooth around the perimeter of the cap, and with just the words "sealed" on the cap. Take care of that cap, I've seen them sell for as high as $400-$500.

 

I really appreciate the discussion and hope it helps others out. Since Barry has a 1970 “non ECC” and a sealed/non vented gas cap, that’s direction I have decided to go. Just to throw some more information into all this, mine was born on 5 June 1970. I also believe it to be an L46 based on the picture below. If it is indeed an L46, would that have an impact on having/not having ECC?

 

I went back and edited my post to include the L-46. The parts book calls out Special High Performance (LT-1) and HD (LS-6) separate from the other 1970 engines because they had a Holley carb and didn't get the left side fuel return line. In the end, I forgot that 70 also offered the L-46 350/350, and omitted it, but it would use the same tank as the L-48 and LS-5, because it had a Q-Jet and a return line.

You say your car was built June 5th. I was assuming that because the emission labels changed in May that the EEC was first used in May, but considering lead times for printing (especially in the 70's before computers were involved in printing) it's quite possible that EEC didn't reach production for another month or so?

Who knows, maybe EEC never made it onto any 70 Corvette? It would be strange for the Parts Books to list the separator and EEC gas tanks for 1970, if they never were used. But it also wouldn't be the first time there was misleading information in the Parts Book (the LS-6 is included in a number of places).

 

Just to add to the confusion, mine was built in June, I don't know the exact date. I do not have the Separator or Charcoal Canister. Maybe something to do with it having been shipped to a Canadian dealer.

 

Both my 70's. no eec on either.
June 16 70 L46. Sealed cap
July 6 70 LT-1. Sealed cap.
I believe 70 California cars had eec.

 

So much info for a gas cap!!!

 

My cap on my 74 is sealed and I am switching to unsealed because of gas fumes.