[qwerty123]

hey guys quick question.

is the fuel tank on a sierra supposed to be a sealed pressurized unit.

reason i ask is i have quite a strong fuel smell coming from the fuel cap (which of course stinks up the garage) and i figured out that my fuel tank was able to vent through the locking mechanism on the cap.

fuel cap is aftermarket ebay job that was listed as a sierra cap.

Considering it has an Evap canister i would assume it is supposed to be sealed.

anyone have any solid info?

[bumstein]

Hmm.. Not sure I can add heaps of value to your question. But a fuel cap should be able to let off pressure to a certain degree.

Fuel expands when it's warm so if it was a completely sealed unit the fuel tank would bulge out and ultimately fail if there was nowhere for the pressure to go. Yes it can let some fuel smell out but it shouldn't be a huge amount! If your garage isn't ventilated you'll most likely smell it sure enough though.

Quick question, how full is the tank? If it's over filled there may be fuel venting out through the cap, especially when the day warms up or the sun shines on the car warming the fuel. I'd expect that the emptier the tank the less it should vent vapour out.

Secondly when is it occurring? When it warms up/sun gets on the car or is it happening overnight when it's cooler?

If it's happening overnight I'd start looking to see if there's a fuel leak that you haven't already spotted. From the tank, the fuel lines or filter.

[christover1]

The charcoal canister can be the culprit.
Blocked or missing breather pipe or return lines.
Wrong heights of fuel system parts on modded cars.

[qwerty123]

doesn't matter how full the tank is. and no difference between day/night. it consistently just smells like someone left a Jerry can open in the garage

Also there is never any pressure released when i open the tank

charcoal canister is blocked up. now this adds to my dilemma

there should be excessive pressure in the tank if the evap is clogged but there isn't.

Conclusion: Crappy Aftermarket Cap + Blocked Evap=vapor leaking

Will update when fixed.

[christover1]

I think fuel vapour in top of tank is supposed to be condensed and collected by the charcoal canister, and reused in the carby.

[qwerty123]

Update,

so after lots of mucking around and chatting to the seller Ive figured it out

Ebay caps are not good!!

they have no means of a seal and allow anything and everything to get in and out of them. (Don't drive through water or mud with these)
Basically they are not designed for a Sierra even though they list them as suitable, or any car made after 1985.

the only caps suitable at a reasonable price are CPC 50EC $28.99 from autobarn.
there is a locking version but it will cost you $60
Don't bother with super cheap as they will rip you a new one.

looks pretty ugly but it does the job properly, it has a valve that only allows air in the tank and not out.

Ebay cap on left compared to OEM spec CPC 50 EC on right


I highly recommend getting a good cap as it will stop your tank filling up with dirt and other nasty stuff
It will stop you from loosing fuel via vaporization which leads to better more km per tank. (I read somewhere the average car will loose 113.5 liters of fuel a year just from vaporization)
It will prevent condensation from forming in the fuel tank which prevents your tank from rusting.
You will get better mileage from the EVAP system working properly and easier cold starts.

and my last point is, if your like me and don't daily it. The fuel is going to have the most potential energy as possible when pressurized.
Now we all know sierras lack a bit of power so having fuel with as much potential energy possible is what we all need for a little extra oomph.

[MrRocky]

Blow out your charcoal cannister and tank water seperator lines

[qwerty123]

All fixed already,

although i don't know what you mean by water separator.