1980 gmc vented gas cap? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**1972BackInBlackC10** · 07-23-2012, 06:11 PM

would a 1980 gmc have a vented gas cap or would it have a vent line on the sending unit? i inquired about a sending unit at the parts store, they phoned their tech line and the guy told them since im running one tank that i need the sening unit with 2 outlets, my tanks has a sending unit with 3 outlets

2 lines from the fuel pump run to the sending unit and the 3rd line has a volt shoved in the end of it, i was assuming one line was for fuel from the tank to the fuel pump and the second was a return line from the pump and the third is my vent liine does this sound right? my truck does not have a charcoal canister, it did have dual tank but im only putting one 20 gallon tank in, what sending unit do i need the 3 or 2 outlet? please help i need to get my truck on the road iv been working n this thing for a year now i need to get it driving

ray_mcavoy · 07-23-2012, 09:58 PM

The 3-line sending unit is correct for the return-line type fuel pump you're using. Doesn't make any difference whether you're running one tank or two.

The line plugged off with the bolt is what is supposed to be your vent. On most trucks, it was connected to a charcoal vapor canister. But on trucks that didn't come with a vapor canister from the factory, they simply put a short 2" or 3" section of rubber line on the sending unit and stuck a plastic vent cap in the end to keep out dirt & bugs. Same type of vent cap as what GM commonly uses on transmissions, transfer cases, and axles.

I know the gas caps used on vehicles with the charcoal vapor canisters have a spring loaded vent/vacuum mechanism built into them. It normally stays closed and only opens up if there's too much pressure or vacuum in the tank due to a plugged up vapor canister system. I have a feeling the same caps were also used on trucks w/o the vapor canisters as well.

**1972BackInBlackC10** · 07-23-2012, 11:11 PM

thanks ray thats what i thought, i had originally thought the line went towards the back of the truck but you saying its just a 2 or 3 inch line confirms it doesnt as mine has a 2-3 inche rubber line with the bolt shoved in it, im assuming this is why my truck dies a short time after you start driving it on a hot day thanks again

07-23-2012, 06:11 PM	#1
1972BackInBlackC10 Senior Member Join Date: Jan 2012 Location: Estevan, Saskatchewan Posts: 1,025	1980 gmc vented gas cap? would a 1980 gmc have a vented gas cap or would it have a vent line on the sending unit? i inquired about a sending unit at the parts store, they phoned their tech line and the guy told them since im running one tank that i need the sening unit with 2 outlets, my tanks has a sending unit with 3 outlets 2 lines from the fuel pump run to the sending unit and the 3rd line has a volt shoved in the end of it, i was assuming one line was for fuel from the tank to the fuel pump and the second was a return line from the pump and the third is my vent liine does this sound right? my truck does not have a charcoal canister, it did have dual tank but im only putting one 20 gallon tank in, what sending unit do i need the 3 or 2 outlet? please help i need to get my truck on the road iv been working n this thing for a year now i need to get it driving

07-23-2012, 09:58 PM	#2
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,366	Re: 1980 gmc vented gas cap? The 3-line sending unit is correct for the return-line type fuel pump you're using. Doesn't make any difference whether you're running one tank or two. The line plugged off with the bolt is what is supposed to be your vent. On most trucks, it was connected to a charcoal vapor canister. But on trucks that didn't come with a vapor canister from the factory, they simply put a short 2" or 3" section of rubber line on the sending unit and stuck a plastic vent cap in the end to keep out dirt & bugs. Same type of vent cap as what GM commonly uses on transmissions, transfer cases, and axles. I know the gas caps used on vehicles with the charcoal vapor canisters have a spring loaded vent/vacuum mechanism built into them. It normally stays closed and only opens up if there's too much pressure or vacuum in the tank due to a plugged up vapor canister system. I have a feeling the same caps were also used on trucks w/o the vapor canisters as well.

07-23-2012, 11:11 PM	#3
1972BackInBlackC10 Senior Member Join Date: Jan 2012 Location: Estevan, Saskatchewan Posts: 1,025	Re: 1980 gmc vented gas cap? thanks ray thats what i thought, i had originally thought the line went towards the back of the truck but you saying its just a 2 or 3 inch line confirms it doesnt as mine has a 2-3 inche rubber line with the bolt shoved in it, im assuming this is why my truck dies a short time after you start driving it on a hot day thanks again



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