Hey guys my fuel tank is somehow getting over pressurized and the gas cap is blowing off the excess pressure. When I unscrew the cap there's a large rush of air but I have no CE light or codes. Are there any tests I can run to see what might be going on?

I've already replaced the gas cap and blown out the lines and havent found any clogs. I'm thinking maybe the canister purge solinoid isn't opening to release the pressure. Is there a way to test this?

 

The computer has an extensive series of tests it puts the fuel tank system through.
If you have no codes involving that system there is no trouble.
Feel lucky if you don't have a code set.
Good luck.

 

I bet you are overfilling the tank. When it shuts off, do not top it off any more than the next even $1.00.

 

Just a question, because I don't know the answer. Would the tank be "pressurized", or would it be holding a slight vacuum? Seems to me, that with fuel being consumed and with a gas cap that is airtight for smog control, the problem (if it is one) would be a vacuum.

Glc, why would topping off the gas tank cause this? I know it can saturate the evap canister, but why would that cause a pressure difference in the tank?

- Jack

 

If the canister fills up, where are the vapors going to go?

 

Gasoline can expand when it leaves the underground tank and gets pumped to your gas tank, in warmer weather anyhow. And it expands a lot when it warms. That is why you don't top off your gas tank. If the charcoal canister gets flooded, your engine will run rich.

 

Roadie and glc, I understand your points, but I'm making a wildass assumption that he's not opening the gas cap until he's ready to fill up again, and the tank would be maybe half full? That's a lot of space that's suddenly available and I just can't see how the tank is going to be pressurized.

Isn't there some component that allows air to enter the tank to replace the volume available when fuel is consumed? Is it possibly through the purge canister? If that were plugged, wouldn't it cause a vacuum?

What am I missing?

- Jack

 

Well I changed my purge solenoid and had no change, then went under the hood and changed the evap service port one, whatever that's called. That seemed to fix the problem. I'll still have to drive the truck and see if the problem has in fact gone away but it seems to have gotten better. Thank god for spare parts from the old 97

 

The gas cap is vented to allow air INTO the tank, but not escape so you don't get vapor lock like the cars of the 60's with unvented caps and to prevent fumes from escaping and causing California to freak out and Al Gore to sell more trees. It is normal for a tank to build pressure because of the expansion of the gas (not so much the gasoline) and many fuel doors warn of this condition because we live in the USA and if we're not warned, we sue.

In other words, with a properly functioning gas cap and purge system, the tank will never form a vacuum and can therefore only pressurize or maintain static pressure.

 

Thank you sir! That makes sense to me. Sorry for being such a dofus.

- Jack

 

I might be wrong, but isn't there 2 o's in doofus?

Sorry Jack, I couldn't resist.

 

How embarrassing! Guess that makes me a DOUBLE doofus!

And, in doing an online lookup, I've found that "dofus" is actually an online multiplayer role playing game. My "senior moments" seem to come more and more frequently these days.

Thanks for all of you for trying, hopefully successfully, to restart my somewhat undercharged brain. Now, if I could just recall the subject of this thread?

- Jack

 

It's all good Jack. BTW the original subject was excess gas pressure, so I guess us old guys are still somewhat on target!