In cab gas tank question
 

I don't know jack about the tanks in our trucks. I do see threads where folks are talking about vented cap or no vented cap. My guess is there must be some difference in the tanks themselves.

Are the tanks all the same but the piece where the sending unit goes in different? Or is the vent a part of the fill tube?

Anyway Im just curious about stock tanks and how you tell the differences in years, and what years have a vented cap vs. pressurized (right terminology?)

Re: In cab gas tank question
 

I just did a in cab replacement on my 72. From what I learned (or think I learned lol) during the process, there are three types of original tanks.

67-69 non-EEC
70-71 with EEC and
72 with EEC. The 72 had a smaller ID fill neck and cap.

EEC systems require the non vented cap and non EEC systems require the vented style.

Appears most all replacement tanks with EEC are based on the 71 design. Which has the larger ID fill neck and cap. I did not find an exact replacement 72. I used the 70-71 style EEC tank.

As far as the sending units and small blocks go.. I read that in 72 the return line was introduced on automatic trucks to help with vapor lock. These are the sending units that have two lines on them along with the three line fuel pump. 67-71 have the single line 5/16 sending units; 72 manuals had single line 3/8 and 72 auto had dual line with 3/8 supply.

This pic shows a tank with EEC. Note the vent lines.
http://67-72chevytrucks.com/vboard/a...7&d=1273332712

Re: In cab gas tank question
 

Lots of good info for me to chew on, thank you!

Re: In cab gas tank question
 

72, at least mine, has a EVAP canister so venting is to that. The cap appears to be non vented. Mine is a 3 OTT

Re: In cab gas tank question
 

To complicate matters, 3/4 and 1 ton trucks do not seem to have made the change to EEC. Possibly because whatever laws that GM were abiding by for the half tons they were able to sidestep for those "heavy duty" vehicles: I don't think that evaporative cannisters were standard on them.

http://67-72chevytrucks.com/vboard/s...d.php?t=645283

Re: In cab gas tank question
 

My 70 with 250/3OTT had an evap can. It is a Fremont built truck

Re: In cab gas tank question
 

Yep. My K20 is a Fremont-built California truck with non-EEC tank and no evap (charcoal?) canister.

Re: In cab gas tank question
 

Buying a gas cap for my 72 GMC 2500 was a really confusing experience until I found this out.

Re: In cab gas tank question
 

my 72 big block C-20,Fremont built truck has no evap canister or a EEC vented tank.there are 3 ports on the fuel pump,3/8's fuel in,3/8's fuel to the carb and a small 1/4 i believe return hose to the sender.

Re: In cab gas tank question
 

And to muddy it further my 72 K20 has only one line on the sending unit, and just an in and out on the pump-no return, and no evap. 1/2 tons were the only ones that had the evap stuff as far as I know.

Re: In cab gas tank question
 

Yessir, same with my '72 K20.

Re: In cab gas tank question
 

Observations on the 3/4 and higher trucks are correct. They are considered and licensed as a commercial vehicle in California which tends to drive most emissions rules for cars this age. In the seventies California exempted commercial vehicles from emission devices and the feds followed. I still have to license my 1972 3/4 with a commercial plate costing a few bucks more than a 1/2 ton truck. You can get an exemption from the commercial status if you have a permanent camper attached. You will get a ticket if a cop notices the camper is removed but most do not know this rule on the older trucks and would miss the citation.

Re: In cab gas tank question
 

Yet my 1972 K20 Suburban, with California emissions, has evaporative emission system with a charcoal canister.

Re: In cab gas tank question
 

For same reason you can register a c20 non commercial if you have a camper on it. The burb is a passenger car. It's the weight and intended use that classifies if a vehicle is commercial or not.

Re: In cab gas tank question
 

70 3/4 ton non etc, no evap canister vented cap. Also no air pump

Re: In cab gas tank question
 

Also, as was briefly mentioned by jnewt, the neck size of the two tanks were different as can be seen below. So there are two different grommets, and you have to use the one that matches the tank:

67-70
Attachment 2274783

71-72
Attachment 2274784

Re: In cab gas tank question
 

My Fremont 71 C/20 has no EEC items .