Weird gas smell in cab '72 C20

[TheCougarine]

So recently my truck as developed a fairly strong gas smell in the cab but heres the strange thing, it only happens when the drivers side window is down and the trucks moving. At idle theres nothing and if you're outside the truck theres nothing. This ones got me stumped. Any ideas what might be the cause?

[truckster]

I would check the fuel vapor lines. When your window is open, it will create a slight vacuum, drawing air from the cab. That might be just enough to pull any vapor that's escaping into the cab.

[cdowns]

fuel vapor lines are in the passenger side rear door jamb area

[Sklptrljay]

I had the smell when I left the truck in the sun with the windows up. But, it's probably the same system causing the problem.

See if you still need to finish running the vapor lines off the tank. Also, My vented gas gap was not venting. Others reported the same problem with their caps. I drilled a small hole on the internals of the cap to let it breathe.

This link has a photo of the vent that runs up the passenger side pillar to show you what you're looking for. http://67-72chevytrucks.com/vboard/s...d.php?t=631734

[TheCougarine]

I'll check that out guys, thanks.

[TheCougarine]

I just went and looked them over but they appear solid. If they are leaking you would smell it when you're right in front of them correct? Would the gas cap not venting cause this same problem and if so if there a way to check whether its venting or not?

[tommys72]

I'll be watching for the answer as mine just started doing this, in the garage nothing, sitting outside nothing, but drive with the window down...might as well be huffing gas from a can.

[davepl]

I don't know what they did in 1972 but in prior years, like 1970, medium duty trucks were exempt from tank vents and so on. He said he had a C20, so if it were a year or two older, he wouldn't have a tank vent or vapor line or any of it. But I don't know about '72

The filler neck is your only source (short of a metal failure, of course). Other than that there's a short rubber flex section under the truck. I would imagine it sort of has to be one or the other (absent vents, as I postulated above).

[TheCougarine]

It appears to have something that looks like those vapor lines but they don't run up the passenger B pillar truck they run straight down through the cab floor. Is this different than those vapor lines I'm guessing?

[blkseetin]

I had a bad gas smell and figured out that my gas cap was leaking gas. I replaced the cap, but now I'm having a problem that seems like vapor lock, so who knows.

[El Dorado Jim]

Originally Posted by TheCougarine

So recently my truck as developed a fairly strong gas smell in the cab but heres the strange thing, it only happens when the drivers side window is down and the trucks moving. At idle theres nothing and if you're outside the truck theres nothing. This ones got me stumped. Any ideas what might be the cause?

my advice, get rid of the in cab tank,what was gm thinking? put on a set of saddletanks underneath.....

[TheCougarine]

Update: I picked up a new gas cap to see if that helped and well it didn't. Not that it didn't need one anyway but I guess I'll have to continue to search for the source.

[Sklptrljay]

Originally Posted by TheCougarine

It appears to have something that looks like those vapor lines but they don't run up the passenger B pillar truck they run straight down through the cab floor. Is this different than those vapor lines I'm guessing?

Those should be your fuel lines.

[TheCougarine]

That would make sense. So it seems the truck is lacking vapor lines.

[88Ironduke]

The line running up B pillar is your vent line. In 72 the emissions folks ran the vent line parallel to the fuel line up to the charcoal canister on the front passenger side fender corner. It was then tied in via vacume to keep a constant draw on the tank to prevent vapors from exiting the tank. Your gas tank will have two lines coming off it, one at the corner and that feeds to your vent. The other is obviously the fuel line. The vent goes in the pillar about 10" or so to prevent fuel from being able to slosh up and thru to charcoal canister.

Hope this helps.
Paul

[mechanicalman]

Originally Posted by TheCougarine

That would make sense. So it seems the truck is lacking vapor lines.

So check out this pic. My 72 had a cracked line, and it was cracked exactly backside so it was hard to see, use an inspection mirror. Or just replace all your fuel hoses all at once, if one is leaking then they are all too old.

[Savidio44]

I had the same problem on my '71. It was a broken connection at the top of the gas tank. Check the top of the tank by moving the tubes going into the tank. A broken seal there is a problem. A new sending unit fixed mine.

[TheCougarine]

Another update. I might have spoken too soon on the gas cap. Drove the truck around today and did not smell in gas so that just might have been the problem and it took it a bit to air out. Also this truck for sure does not have vapor lines. Theres nothing running up the B pillar at all.

[slikside]

Originally Posted by mechanicalman

So check out this pic. My 72 had a cracked line, and it was cracked exactly backside so it was hard to see, use an inspection mirror. Or just replace all your fuel hoses all at once, if one is leaking then they are all too old.

This is what I did. The fuel smell in the cab was powerful. I did some looking around and found that a couple of the small rubber hose sections connecting the lines to the tank at several spots were deteriorated, and raw fuel was seeping through them when the tank was full or nearly so. I replaced all these hose sections, and haven't had a whiff of it since.