fuel cap operation

Can someone explain how the fuel cap works? Mine has a one way check valve held closed by a spring. It will let air in but nothing out. Why is it like this when there is already a vent valve to vent the tank? Is it a back-up in case the vent valve sticks closed?

When the computer performs the evap self test what stops air from entering through the fuel cap and causing the test to fail? Is the vacuum in the evap system during the test not strong enough to over power the spring in the cap and therefore no air can enter?

Thanks in advance for any replies...Jack

Can someone explain how the fuel cap works? Mine has a one way check valve held closed by a spring. It will let air in but nothing out. Why is it like this when there is already a vent valve to vent the tank? Is it a back-up in case the vent valve sticks closed?

When the computer performs the evap self test what stops air from entering through the fuel cap and causing the test to fail? Is the vacuum in the evap system during the test not strong enough to over power the spring in the cap and therefore no air can enter?

Thanks in advance for any replies...Jack

you answered your own question...
the fuel tank must have a slight vacuum...very low like about 1-2 inches.
the cap has a relief valve that protects the system from excessive pressures / vacuum..the vent valves operate to test the system and do fail so having the correct cap is important...

with the cap off the PCM testing of the fuel tank, would cause a dtc..this is why, as well as safety concerns, you shut down the engine when re-fueling..

most that have dtc concerning this.. fill the fuel tank to the max then park vehicle...in winter if you do this and have a heated garage you will damage the vapor control system ...

Thanks j cAT, I knew you'd be the one to help...lol.

I got one more I'd like your input on. Two mornings ago I took my fuel cap off and have driven 100 miles with two cold starts and the engine light hasn't come on. The light does work and the pcm has set an evap code a few times before. Now when I purposely try to set a code it won't. Do I have a pcm problem? Thankyou.

Thanks j cAT, I knew you'd be the one to help...lol.

I got one more I'd like your input on. Two mornings ago I took my fuel cap off and have driven 100 miles with two cold starts and the engine light hasn't come on. The light does work and the pcm has set an evap code a few times before. Now when I purposely try to set a code it won't. Do I have a pcm problem? Thankyou.

most owners only care if the dtc is showing..this is a problem when the testing is done...with what occured to you I am not sure there is a problem as some that had the dtc with this fuel cap. they have used the vented caps and then had no dtc's...

it could be you may have a plugged vent line...this can occur when the carbon in the cannister gets saturated with fuel this then creates a sludge that plugs up these lines/valves ..

most owners only care if the dtc is showing..this is a problem when the testing is done...with what occured to you I am not sure there is a problem as some that had the dtc with this fuel cap. they have used the vented caps and then had no dtc's...

it could be you may have a plugged vent line...this can occur when the carbon in the cannister gets saturated with fuel this then creates a sludge that plugs up these lines/valves ..

I don't have any problems getting fuel into the tank so pretty sure it is venting properly. I have learned that many variables need to be met before the pcm will run a test. And even if they are met it doesn't always run a test at that time. Apparently it can go days between tests. Seems odd to me but must be true as I purposely loosened my cap to try and figure out how this system works. I know, I should just be happy that the light is out...lol. Thanks for the help, man.

I don't have any problems getting fuel into the tank so pretty sure it is venting properly. I have learned that many variables need to be met before the pcm will run a test. And even if they are met it doesn't always run a test at that time. Apparently it can go days between tests. Seems odd to me but must be true as I purposely loosened my cap to try and figure out how this system works. I know, I should just be happy that the light is out...lol. Thanks for the help, man.

the cap gasket is most likely still sealing the fuel tank even when loose...remove this rubber seal then try for dtc...I believe at a cold start this test is repeated at some point in the vehicles operation..

the cap gasket is most likely still sealing the fuel tank even when loose...remove this rubber seal then try for dtc...I believe at a cold start this test is repeated at some point in the vehicles operation..
Could have been still sealing but I only had it hanging on there by 1 thread. I though it checked at cold start also.

This is what I found needs to be met:

• No active manifold absolute pressure (MAP) DTCs
• No active intake air temperature (IAT) DTCs
• No active throttle position (TP) DTCs
• No active heated oxygen sensor (HO2S) DTCs
• No active vehicle speed sensor (VSS) DTCs
• No active system voltage DTCs
• The ignition voltage is between 10 volts and 18 volts.
• The barometric pressure is more than 75 kPa.
• The fuel level is between 15 percent and 85 percent of full capacity.
• The engine coolant temperature(ECT)is between 4°C(39°F)and 30°C(86°F).
• The intake air temperature (IAT) is between 4°C(39°F) and 30°C(86°F).
• The start up ECT and IAT are within 9°C (16°F) of each other.