vented gas cap

[down4thakrown]

Would I be able to put a vented gas cap on my k5 and plug the vent on that sending unit? But didn't find much on results on running a vented cap and plugging vent on tank. And if this has worked for you what cap did you use? I found a manual push button cap but was unsure if it would vent on its own

 

[dyeager535]

Yes it works. You don't need a vented cap. Flip yours over (or a wrecking yard donor to play with) and on all I've ever seen, you can see the valves. Use a small screwdriver to hold one open, then try and blow through the cap. If you can blow through it, you've vented the cap. Went through this years ago, the caps being sold at parts store for vented cap (pre-/no EVAP) applications were the same as non-vented, which is of course wrong. IIRC the GM spec for "blow off" on the non-vented caps is 16PSI. So they are technically vented, just at a relatively high pressure.

Personally if you have the EVAP canister and can do so, run it. Can't say I've had any problems with venting the tank through the cap, but in the past have seen rigs where the vapor has caused problems with the paint, and venting fuel vapor externally isn't necessarily a good thing, if it can be avoided.

 

[down4thakrown]

Ok sweet, I noticed the vents on a cap I found in the garage lyeing around. My frame is a 79. So there's no factory vent line going to the front.
Edit: I'm thinking the line I used for the return line might be the vent? Not sure there was not much left on the frame when I got it

 

[dyeager535]

Might have been, no idea when they went to a return system, but IIRC, EVAP was around before the return line setups. There were trucks that didn't get EVAP, but they were pretty few and far between, especially later on.

 

[down4thakrown]

Yea I can't even remember if it had a return line near the mechanical pump. But I was able to easily blow through my gas cap. So I think it will allow the tank to vent enough with out worry.

 

[dyeager535]

Needs to be both ways. Usually not an issue, but my "vented" cap eventually stopped venting properly. I just wedged the valve open.

 

[down4thakrown]

What did u do to wedge it open

 

[dyeager535]

On the one I have I was able to just get a jewelers screwdriver in there and pull the valve out of its seat and it pretty much wedged itself. I have no idea what this cap is, it's actually metal capped, so it's probably fairly old. The newer Stants I recall playing with you may have to actually disassemble them to get at the valves.

If nothing else, remove the valve, or if they are set up with springs, perhaps you could reduce the spring rate to get them to vent at a much lower pressure. Only concern I'd have about leaving them open is dirt/dust getting in there, along with insects.