Fuel tank vacuum

[longbeach]

Hi, I recently acquired an 86 Chevy K20. It had a crate engine installed by prior owner. Most emissions equipment is gone. Running fine, ( although I thought MPG would be a little better ) but when I open gas cap there is a big whoosh of vacuum. Is this right? Do I need a new gas cap?

 

[AuroraGirl]

Hi, I recently acquired an 86 Chevy K20. It had a crate engine installed by prior owner. Most emissions equipment is gone. Running fine, ( although I thought MPG would be a little better ) but when I open gas cap there is a big whoosh of vacuum. Is this right? Do I need a new gas cap?

you probably have 3 ports on the gas tank.

is your return line on the pump plumbed to the gas tank(s) and is your fuel pump lines not kinked in anyway.

Those emissions components also includes a line that runs to the back for the Tank(s) that has rubber hose probably at the very ends but its meant for vapor . its a small diameter, it doesnt run to the pump but the core support or the areas under headlights is common depending on setups. my truck had no such system but had a plastic vent on the tank sender where the vent came out, but that was a emissions exempt thing yours un likely has. BUt if your vent line on the tank is NOT run to the charcoal canister, NOT have a vent on the tank, or has something plugging it somewhere, you would usually presssurize but depending on fuel usage or cap defects or similar.. I would say check your metal lines and hoses off tank front to back, there are 3 find where they go and make sure not bent, and just to rule out and have a good cap to check against, buy a new nice cap.

If your tether is bad or you want to have one that is a good time for that kinda QOL change

 

[longbeach]

Wow, that was quick. Thanks AuroraGirl. I love this site already. I have been steadily going through this truck to sort it out. There were a lot of things that were done wrong. But, that said, she's on the road running okay and the project continues. Thanks

 

[75gmck25]

If you look up under the passenger side of the truck where the dual tank solenoid is located, there will be three metal lines running from there, up the frame rail to the front of the truck ( fuel feed, fuel return and tank vent). The vent line is the one connected to a T fitting so it can vent both tanks. If someone plugged it, or it’s plugged at the engine compartment end, you need to open it up.

The vent line was originally connected to the charcoal canister in the engine compartment. However, many shade tree mechanics get confused and think it’s part of some emissions device that will make their engine less powerful, so they disconnect, remove or disable it. Bad idea - the tank needs venting, so just leave it in place.

 

[longbeach]

Thanks, 75gmck25. I'll check that out this week.