1976 C20 Burb: 3/4 ton fuel system and EVAP

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TheC5Surgeon

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1976
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C20
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Starting to plan the fuel system and am trying to find out if my truck ever came with the EVAP canister/system. I have read one thread on another forum that states up to 1983 3/4 ton vehicles were considered heavy duty and did not come with the charcoal canister. Those vehicles reportedly came with a one way plastic valve attached to a small fuel hose attached to the 3rd line on the sending unit. If my burb ever had one the previous owner deleted it. Can anyone attest to any validity to this? If so it would save me money in not having to make/ buy a long vapor line and canister setup. Thanks

Matthew
 

bucket

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That's how my '78 C20 Suburban is set up.
 

75gmck25

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My ‘75 3/4 ton has HD emissions, and it includes a charcoal canister. The telltale sign is that there are three metal lines running from the gas tank switching valve up to the engine compartment.

Two lines end on the frame rail right next to the fuel pump (supply and return), and the third line runs across under the radiator support and ends near the PS gearbox. My charcoal canister is mounted on the firewall support, on the driver’s side. I have seen the canister mounted in other locations, but all would have the metal line.
 

TheC5Surgeon

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Interesting that some 3/4 ton came with it and some did not. I think in this case I will try running a short fuel hose from the third metal line on the sending unit ending it somewhere near the filler neck or rear of the vehicle. Will top it with a rollover vent valve such as https://www.summitracing.com/parts/tnk-vvr.
 

bucket

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My ‘75 3/4 ton has HD emissions, and it includes a charcoal canister. The telltale sign is that there are three metal lines running from the gas tank switching valve up to the engine compartment.

Two lines end on the frame rail right next to the fuel pump (supply and return), and the third line runs across under the radiator support and ends near the PS gearbox. My charcoal canister is mounted on the firewall support, on the driver’s side. I have seen the canister mounted in other locations, but all would have the metal line.

What to you mean by "HD emissions"? Do you mean your truck has a specific emissions package as an option? Or do you mean your truck has the higher GVW rating and and has the associated emissions equipment?

I ask because a lighter GVW truck could have received a charcoal canister by default. Or a heavier duty truck could have been optioned (just my speculation) with a specific emissions package to meet the regulations of a certain state.
 

75gmck25

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My truck has 8400 lb GVW, and therefore for 1975 it did not have catalytic converters. I am not sure what other controls were standard, but it’s a lot simpler than the lower GVW trucks.

When I run the numbers on my original Quadrajet the book shows it as a California version. I don’t think the truck came from California, but if it did that may explain the extra evaporative controls. I think my original ‘75 emissions sticker was on the air cleaner, and it’s long gone.

At that time I believe the emissions split was at 6050 lbs GVW. Trucks below that GVW required emissions controls similar to cars, and trucks with a higher GVW (which I recall being referred to as HD) used less restrictive emissions devices.

That split in emissions also meant leaded vs. unleaded gas, and it brought about the F150 in 1975, and similar in-between models from other manufacturers. As I recall, you could not buy an F100 with a high enough GVW to use leaded gas (because of the required converter), but if you bought an F150 or E150 you could buy a GVW above 6050 lbs and still use leased gas.
 

bucket

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My truck has 8400 lb GVW, and therefore for 1975 it did not have catalytic converters. I am not sure what other controls were standard, but it’s a lot simpler than the lower GVW trucks.

When I run the numbers on my original Quadrajet the book shows it as a California version. I don’t think the truck came from California, but if it did that may explain the extra evaporative controls. I think my original ‘75 emissions sticker was on the air cleaner, and it’s long gone.

At that time I believe the emissions split was at 6050 lbs GVW. Trucks below that GVW required emissions controls similar to cars, and trucks with a higher GVW (which I recall being referred to as HD) used less restrictive emissions devices.

That split in emissions also meant leaded vs. unleaded gas, and it brought about the F150 in 1975, and similar in-between models from other manufacturers. As I recall, you could not buy an F100 with a high enough GVW to use leaded gas (because of the required converter), but if you bought an F150 or E150 you could buy a GVW above 6050 lbs and still use leased gas.

California emissions would make sense on that one. Do you also have an EGR valve? I'm just curious. My '78 doesn't have one, or any other emissions device. The only thing it has is the vacuum advance is run through a temp switch. Oh, and a PCV valve of course.
 

fast 99

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Emissions depends on GVW and or California compliant, not by 3/4-ton designation. Way to figure out what was originally on it is find a vacuum diagram or emissions decal for the truck. 8600 GVW and over had a different requirement in this state, WA.

Here's an example of why a vehicle needs to be researched. Have 2 SB in my garage. Both are similar, 350V8 automatic 3/4 ton, neither has a floater axle so neither are 8600 GVW. One is an 81 originally came with a cat, second is an 85 that never had a cat. Other than that, the emissions equipment is similar. Both have air pumps, egr and vapor recovery, go figure.
 

75gmck25

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I have the GM repair manuals for my truck, but they are not very specific about emissions equipment and what was included with each powertrain combination.

I got my truck in about 2002 and it looked like the 350 engine was relatively unmolested; however, it had been in my wife’s family for many years, and know the engine had been replaced. I’ve always assumed they moved all the original parts over to the replacement engine, but you never know.
 

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