Air gushing when gas cap removed

[MayberryF3]

So one other thing, temps here have moved from 15 degrees on Monday to 66 today and I'm having an issue that only happpened once in the past and under similar circumstances of temperature change.

Today after driving 40-50 miles I stopped for gas and I had a huge gush of air at the gas cap. The only other time I've had this happen is after coming out of the mountains with snow down into a much warmer temp, stopped for gas and the same thing air gushing out.

I'm pretty sure I have the factory gas cap, or one similar to it, but I do have the aftermarket Mopar FI system on the Jeep. I did a brief search and saw suggestions of gas cap, charcoal canister, fuel pump, etc. I'm curious if it may be the fuel pump or FI related because I have a loud humming noise when driving the Jeep. I assume that could be the fuel pump humming or dying? It it is loud.

(Hence my other thread about motor swap questions)

 

[Peanut Butter]

My first guess is no vent line any more and maybe no return fuel line.

 

[Torxhead]

Somewhere along the fuel tank vent system something isn't working right. As fuel is removed from the tank a vacuum is created in that space and the vent system lets air in. Removing the gas cap lets the air in. A CJ repair manual should have a diagram of how the vent with rollover valves work. The gas cap is vented but does not work all that great. An electric fuel pump will not work correctly with vacuum in the tank. There could be an issue with the charcoal canister filter clogging up and not letting air into the fuel tank.

 

[Posi]

Replace your fuel cap.

 

[Hedgehog]

Fuel caps certainly don't last for ever and should be replaced occasionally. But your year CJ shouldn't have a vented cap. so fuel tank pressure would sort of rule out a faulty cap, the one you have certainly is sealing.

Do you have an electric fuel pump? If you do the system isn't stock, so it would be difficult to know what changes were made to the system. No matter what, your tank is not venting properly. ....... does the CJ run well otherwise?

Edit: I see in the other thread that you have a highly modified fuel delivery system, your tank is not venting, why, I don't know.

 

[MayberryF3]

Thanks guys, I searched and saw some other threads with similar issues (often the gas cap was suggested) but I didn't see any where someone was using an aftermarket FI system.

I have the Haynes and Chilton manuals and will look for the vent lines and check the charcoal canister. I have a download of the factory manual somewhere and will look there as well.

I didn't know if it was a common issue or not.

Thanks again

 

[Posi]

Fuel caps certainly don't last for ever and should be replaced occasionally. But your year CJ shouldn't have a vented cap. so fuel tank pressure would sort of rule out a faulty cap, the one you have certainly is sealing.

Not sure where you got your info but it's not correct.

His 83 CJ came from the factory with a 2 way check valve, fact is it vents both ways.

 

[Hedgehog]

Sorry to interrupt.....

No matter what the tank should be venting in other ways, I know that in Arizona they check my gas cap every time I go in for emissions. Also my jeep mechanic spoke on the subject with me He only has 40 years working on CJ's but he might be wrong.

 

[Posi]

Not a problem, your input is welcomed.

Your mechanic is wrong if he says they don't have a 2 way check valve from the factory, and if he says they came with non-venting caps

Edit: did a search and found specs. The valve will open (vent) when the pressure in the tank reaches .75 - 1.5 psi, or a vacuum of 1.1 to 1.8".

 

[Hedgehog]

Occasionally that is true.

 

[Lumpy Grits]

The tank is set up to retain up to 5 psi. Part of the SMOG tests here is a positive pressure test for this, along with a gas cap test.
LG