Please clarify: C20/30, K20 Pickup tanks - send unit wout a vent?

[DeadheadNM]

Trying to clarify if 3/4t pickups had a vent tube in the sending unit or not. I can't find a factory diagram or a definite answer for a 1972 model year example with auto trans and a 350 sbc.

The repro sending units have a vent - I'm unsure if this actually gets capped or not for a 3/4t and I don't have my factory manual available to consult.

Did the gas tank send units for 72 3/4t V-8 auto trucks have a vent tube?

[Willys_MB]

There are some much smarter folks on here, but I believe only 72 (and maybe 71) tanks were vented.

[Ol Blue K20]

My 72 K20 did not have a second tube on it. My girlfriend found a new one for me. It only has one tube too.

[72 Super]

Pete, to my knowledge the 3/4 ton trucks were not vented, only the the half tons. I know the 72 1/2 tons had the EEC tanks and the sending units are vented. I’m not up on 3/4 stuff that much but I don’t think the sending units would be vented.

Ray

[DeadheadNM]

My recollection was that no 3/4 t sending units were vented. Parts vendors seem to list but a single sending unit for 72 tanks that varies according to transmission. Here’s one I have on the shelf as a spare that I’ve had for a few years and it’s just like that on mine now with the vent tube.

That small tube in the sending unit is not a vent. It is a return line, 1/4", from the fuel pump with three ports. The sending unit with the return was only used on a 72 C10 with a 350 and an automatic trans. The vent in the EEC system comes out of the passenger side of the tank, loops up into the B pillar and then runs up the frame to the evaporative canister under the hood. All others did not use this system and did not have the return on the sending unit. The sending units without the return come with either a 5/16" or a 3/8" feed line to the carb. Make sure you check which one you have before ordering. I think it was 5/16 for the 6 cylinder, and 3/8 for the v8s.

Don't forget, make sure you are using a vented gas cap when you don't have the EEC system

[57taskforce]

Originally Posted by DeadheadNM

My recollection was that no 3/4 t sending units were vented. Parts vendors seem to list but a single sending unit for 72 tanks that varies according to transmission. Here’s one I have on the shelf as a spare that I’ve had for a few years and it’s just like that on mine now with the vent tube.

Pete I believe I read on here that the 3/4ton and up trucks weren't required to have the eec/return system that the 1/2 tons were mandated in 71-72. Maybe California emissions got the heavier trucks eec and or a return fuel system? I’m not sure honestly. I can’t speak for C/20 or C/30 but I know I’ve noticed several 71-72 K20s on here that seem to be like mine. My K20 is a 4 speed, and only has one outlet on the sending unit and one inlet/outlet on the fuel pump. No vent tube that goes up into the passenger side cab corner or anything like that and no evidence it ever was there. It still has the factory plug in the floor where the return line would have gone thru the floor up to the sending unit. I’ve got a 72 c/10 350/350 tank and the relief for the lines looks wider than my K20 tank to me. I’ve never measured it though. You can clearly see only one relief in the tank where the supply line runs and the tank has what appears to be an original 1972 print on it. I can take picture of that if anyone would like it. I believe the tank to be original.

[randy500]

I have 2, 72 k20’s here and they both have single 3/8 line sending units without return lines nor vents on the tanks. Both take the larger diameter gas cap which i guess is vented.
My tanks are the same sending unit as 57taskforce which is different than earlier tanks which have an inverted flare fitting instead of the hose nipple in the sending unit as shwn in the picture he posted.

[Richard]

My 72 3/4 ton had no smog equipment, no charcoal canister etc. My 72 1/2 ton while missing the smog pump (lived in an area where no smog required) when purchased had a charcoal canister and obviously had smog equipment. Note that both trucks were originally sold in California. My Dad's 72 1/2 ton had smog equipment as well. Will try to take a pic of the sending unit on my original tank tomorrow. Sitting in the side yard as it will not be used anymore.

[DeadheadNM]

Guys thanks a lot for the replies. I’m going to keep that extra tube capped and forget about it. I’m running my original tank with a correct GM cap. It’s never been a problem.

I’m going with the understanding that the repro sending units for 1972 applications are designed to accommodate either 1/2t (emissions) or 3/4 and 1t use when installed on 3/4t and 1t trucks the extra line can be capped (assuming all else kept as per oem specs).

As for emissions, my understanding is that these regulations applied to light duty trucks (1/2t) as well as Suburbans including 3/4 t subs. Something about the Suburbans tied them to light duty pickups as regards emissions.

[Sheepdip]

These trucks came along right after "Smog" was being applied to autos/trucks and was a ever evolving issue with many variables and hurdles to clear.
I believe 1967 was the first year of the smog pump being required on any vehicle and I understand also that there were different requirements or exemptions for GVW's and engine displacements.

Rumor has it that one of the reasons we got the 402 big block in 1970 which was called the 400 and the 396 in trucks and cars was a numbers game with the EPA on compression and other smog criteria with 400 cubic inches being the cutoff point.

Many variables over the years with including which state the car or truck was originally sold even.