Should Fuel Cap vent in or out or both?

[Jim Cobler]

When I remove my fuel cap air rushes in. It appears to me that the air and fuel in the tank being heated by the engine develops a vaccuum when it cools. I really don't know, of course.

The fuel pump is pulling fuel from the tank so if air does not enter the engine would stall but after my truck sets awhile and I remove the cap quite a bit of air rushes in.

Anyone know Why? Do I have a faulty cap?

Thanks,

[jbplock]

Jim,

The OE cap will vent in, but not out. I tested the OE cap when I first got my truck because I wanted to use a locking cap that I already had. I ended up buying a locking AC Delco cap designed for our truck that included tether strap and DIESEL label. If you search the forum you should find some threads on this subject. smile.gif

[Dmaxin]

When i open my cap i have no air rushing in. Should I? And if so what does it mean to not have any? Thanks

[jbplock]

Dmaxin,

I also don't get any "hissing" air sound when I open my cap. If the cap is venting "in" the only time there should be a "hissing" sound is if some pressure has built up in the tank from an increase in temperature (from sitting outside on a hot day). smile.gif

[Gbenzx01]

JimCobler,

I usually leave my dmax running when I fuel up & began to notice the suction when removing the fuel cap. This is a real nono with our suction fuel system, and at the dealster they replaced it & said the new one might not be no better & keep an eye on it. Said they had replaced 5 or 6 bad ones. So chunk it as soon as you can & if not the next thing you will see is your power bogging down under heavy loads.
Now if I can just get someone to help me insert my stanadyne pump/filter around here for my post cat filter.........

Gben

[cmadmaxman]

The cap has a two way vent. For venting in the spring for the seal is vary weak so to let air in easy. The spring for venting out is much stronger to allow high presure in the tank to escape to keep from tank or line rupture. I had mine apart and this is what I found.

[jbplock]

Chris,

Thanks for pointing out the presence of the high pressure vent in the fuel cap. When I tested mine (don't laugh :D ) i cleaned it with dish detergent, then used my mouth to see if it would vent in or out. :eek:

[ 06-05-2003, 08:35 AM: Message edited by: jbplock ]