Fuel smell and pressurized tanks

[Dutch Rutter]

Hey all,

Finally starting to tackle some of the other on going issues I've been having with my 84, this is probably going to take me a while to fix. After driving the truck (a bit more noticeable on hot days) after shutting her down I hear a "hisss" sound and get a huge gas smell from between the bed and cab. I will also experience the same thing when driving on a warmer day with the back glass open. I can open the tank I am running and it will depressurize which stops the hiss, and smell.

A quick search and I was reading about the lines leaking between the tanks and the pump would this be accurate for my problem or would the fuel pickup in the top of the tanks needing resealed be more so what I'm looking at?

Both tanks work, the passenger side does it more then the driver's for some reason. And to my knowledge there is no emission stuff on this truck, had to of been removed by a PO.

Thanks for the help.

 

[1987 GMC Jimmy]

How are the fuel tanks venting?

 

[Dutch Rutter]

How are the fuel tanks venting?

Honestly, I have no idea. I don't really have any experience with the fuel system from fuel pump back. How can I check this out and where should they be venting from?

Currently I want to say the hisss sound is coming from the pickups in the top of the tanks, but without pulling a tank I cant see a way to verify this.

 

[1987 GMC Jimmy]

From the factory, there’d be a vapor line going to the charcoal canister. If the canister’s been taken out, the line should still be there and hooked up to the vapor ports on the sending units. Ideally, you’d place an inline filter at the end of the line so crud and mud daubers don’t get in there. If the line is open and obstructed, this is a problem. It needs to be clear unless you switch to using vented fuel caps and properly plug the line. If it still has the charcoal canister, it’d be prudent to change the little mesh filter at the bottom of the canister and replace your fuel caps. I think if your pump was sucking enough air that you could hear it, the truck would either be running very badly or not at all. If this isn’t the case, I’d check the venting aspect first.

 

[Dutch Rutter]

Oh, gotcha so I do have a line on the driver's side, behind the headlight. I'm thinking this would be it? if so last I looked it appeared to have a one way type of cap on it. would only allow flow out and not in. If the canister would be under the hood then it has definitely been removed as there is nothing of that sort there. And nope, the truck otherwise has been running great actually.

Is there any reason why someone would not want to just run vented fuel caps? if that and plugging that hose is all I need to do I'll be super happy. I do believe you are correct and it is a venting issue, like you said other problems should occur if there was a leaking line, and since it only happens after its driven and on hotter days it make sense.

 

[Dutch Rutter]

https://www.oreillyauto.com/detail/...p/10819/5699385/1984/chevrolet/k20-pickup-4wd

Would this cap be the correct one to get? its what I can grab locally, if not then I'll need to order some. It lists the venting type as "emission control" I'm not really sure what that is supposed to mean.

 

[hatzie]

A basic three hose vapor can is easy to hook up and costs you nothing on performance. It keeps the garage/barn from smelling like fuel.
If you aren't in need of emissions compliance I'd steer clear of the overly complex 1980s hose farms and get a 17113148 three hose can from a GM V8 A, B, or X-Body or pickup from the early to mid 70's or Corvette from 1970-82.

Three hoses and all done. No need to find vented fuel caps either.
Use SAE J30R7 fuel emissions hose. It can be used to replace the 40 year old PCV hose that's turned to jello too.

The cans are marked but basic rule of thumb is...

1. Big diameter hose barb is the Vapor purge out to manifold vacuum. I usually Tee into the PCV hose. Usually stacked under the purge valve ported vacuum hose.
2. Medium size hose barb is the Vapor hose in from tank.
3. Smallest barb on the purge valve is run to a ported vacuum on the carb.

 

[1987 GMC Jimmy]

I agree. I'd argue that the evap stuff should be the number one emissions component you keep. It sucks to stand or walk by your vehicle and have it smell like a bucket of gas. The vented cap and the sealed vapor line is a functional alternative, but I don't think it's going to trap the smell of vapors. If you're correct on the way it's currently setup, I'd lose the one way check valve and put a little filter on there just to keep it simple. Walmart carries Stant caps on the shelf at TLE for cheaper than the parts stores. They should have a catalog there telling you which one to get, but if not, Stant also has an online catalog.

 

[Dutch Rutter]

Adding one of those 3 hose cans like hatzie mentioned does seems pretty simple, it is a shame the PO pulled it all. I highly doubt there would be any performance loss to speak of, In fact I might see some very slight improvement to MPGs as it would be burning the gas vapors instead of venting them (not much of course as its a big rolling box)

For the time being I will grab some of the vented caps and cap the line, for a test and to temporarily remedy the issue while I shop around for a canister and wait for the shipping time, then when that all shows up I'll get it installed. There isn't any emissions testing in my area, but it is definitely not pleasant to smell after and sometimes while driving, plus wouldn't want to put a canopy on it with that venting between the bed and cab, might also make one think they might have a fuel leak that needs addressed ASAP.

Thank you both for the help. I'm glad I don't need to pull those tanks down (was not looking forward to that)

 

[Dutch Rutter]

Back to this, yesterday after work I picked up those fuel caps. Went home and double checked the breather line, it looks to have the same kind of end I would find on a differential breather line. Kinda figured since my tanks are venting fumes from (I can only guess) around the sending units that my vent line is clogged or blocked somewhere. So I put on the caps (have not drilled a hole in them, read that some people have done this) and it seems to be a little better but I do still get a hiss from between the cab and bed.

I read somewhere that the tanks need to build up to 2 PSI before valve to the charcoal canister line will open. Is it possible that the seal between my sending units and fuel tank is old enough that it'll leak at <2PSI which would make it impossible for the vent line/coal can to do anything without re-sealing the tanks first?

I don't really like the idea of drilling a small hole in my fuel caps, I believe they open at >3 PSI or they only let air into the tank and not out.. I've read a lot of probably bad internet information on this.