My car has all the evap system, canister, and solenoid valve but as I am seeing on internet, people have [a] solenoid valve on their outlet vent on canister too which open and close by ECU to let the excessive vapor escape.

My car doesn't have that and I am having problem with my car [building] air pressure in the fuel tank.

I am really confused, so that means my gas tank itself has a vent line on it and it is connected to the fuel inlet hose and it evaporates the excessive vapor from there? It doesn't make any sense.

this is the picture from what i understand

From Comments

In this question I am trying to find out why some cars have vent solenoid valve with their canister and what happens when some don't have that, how do they vent the vapor out then.

  • Essentially a duplicate of : mechanics.stackexchange.com/q/53919/10976
    – Solar Mike
    Commented May 2, 2018 at 18:55
  • 1
    No sir, in this question i am trying to find out why some cars have vent solenoid valve with their canister and what happens when some don't have that, how do they vent the vapor out then.
    – ZBT248
    Commented May 2, 2018 at 18:57
  • Show us a detailed diagram of the evaporator system as fitted to your car then we can see what components are where and which may be causing the problem.
    – Solar Mike
    Commented May 2, 2018 at 19:42
  • 1
    @SolarMike - He's not asking about "his car". This is a generalized question revolving around the EVAP system. Commented May 2, 2018 at 20:17
  • @Pᴀᴜʟsᴛᴇʀ2 the second paragraph specifically mentions his car and its problem so....
    – Solar Mike
    Commented May 2, 2018 at 20:19

1 Answer 1


Normally, pressure that develops during filling the gas tank is relieved by venting to the atmosphere via the filler tube (some locales require filler nozzles to be equipped with vapor recovery, in that case the fill nozzle seals to the filler neck and vapors are sucked/pumped back into the filling station tank).

The evap system is designed to scavenge vapors that might otherwise occasionally be vented to atmosphere through the filler tube when you open the gas cap. Filling up is usually preceded by a period of driving, when the system could operate and burn the vapor in the engine via the purge valve, usually solenoid operated and controlled by the ECU. The system is not designed to relieve pressure to prevent pressure-related problems, because the vapor pressure of the fuel is minimal and not a problem.

Think about it, if malfunction of the evap purge/vent could cause pressure problems in the fuel system what happens for days on end when your car sits and fuel vapor pressure develops in the system? The only "vent" is the purge vent to the engine, any other vent would defeat the purpose of an evap recovery system.

Note- not an expert, just using the little I know combined with common sense.

  • Key off engine off the vent solenoid is always open. the only reason there’s a solenoid is to check the system integrity. That doesn’t mean that the vent line doesn’t allow excess vapor to escape out the vent. When the vent solenoid is stuck closed or the line is obstructed there are literally problems with excess pressure in the system.
    – Ben
    Commented May 3, 2018 at 16:21
  • @Ben ...KOEO purge open makes sense. I will downvote my own answer, thanks. Curious though, where would fuel vapor go? Out the exhaust valves to atmosphere? That would seem contradictory to the purpose. Commented May 4, 2018 at 16:08
  • The canister is supposed to capture fuel vapor In normal operation. Excess vapor or a canister that isn’t doing it’s job will vent to atmosphere. Barring a restriction in the canister or vent line
    – Ben
    Commented May 4, 2018 at 16:26

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