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  #1  
Old 06-10-2001, 08:49 PM
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Default Gas tank pressure?

I'm in the final stages of getting my 3+ year CJ buildup complete, and have a question about pressure building up in the gas tank. I put about 11 gallons in the tank (4wdh extra capacity 22 gallon tank) to check the fuel gauge reading and to have plenty to get it up and running. I am using the stock CJ filler with the vent hose, but the gas cap is a lockable one and is not vented. I also plugged the two holes on the driver side of the tank that originally had lines for emissions.

During the day, I hear a noise that I swear is coming from the gas tank. The best way to compare it is to thumping an empty barrel with your hand. Almost a "doing" sound. When I take the gas cap off, there is a whoosh of pressure, but nothing extraordinary.

My question is if I should somehow vent the tank to prevent what sounds like expansion and contraction with temp changes or just leave it alone. I figured I could look for a vented cap or take one of the plugs out and run some type of vent line. Thanks for any help!

Greg J.

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  #2  
Old 06-10-2001, 08:51 PM
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Default Re: Gas tank pressure?

yes
it has to be vented...are you sure the vent hose in the sending unit is open?

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Old 06-10-2001, 09:19 PM
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Default Re: Gas tank pressure?

I've got two lines going into the sending unit. One is the supply line going to the fuel pump, and the other is connected to the return line from the fuel filter. I just looked at my factory service manual diagram, and it shows the two outlets on the driver side are supposed to run to a liquid check valve that's connected to a rollover check valve and the charcoal cannister (all of which I took off during the frame off process). I guess I'll run some hoses from the tank to some type of vent (maybe one of the small K&N filters I'm using for the diff, transfer, and tranny vents). Thanks.

Greg J.

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Old 06-10-2001, 09:24 PM
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Default Re: Gas tank pressure?

Yea..that's the ticket...you will want to loop it so fuel won't pour out if you roll

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Old 06-10-2001, 09:45 PM
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Default Re: Gas tank pressure?

1. Is the tank actually being pressurized? If all extra vents are blocked then the tank is isolated, with the gas outlet/return being the only exits for pressure.
2. As the fuel is siphoned out the tank is not pressureized, it's reacting by producing a vacuum.
3. The noises you may be hearing is the outside atmospheric pressure acting on the now depressurized tank. The metal is actually being flexed by the atmospheric pressure much like the hull of a Submarine when it deep dives.
4. Why not run one vent line to a charcoal canister and then from the charcol canister to the pvc output line?

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Old 06-11-2001, 08:41 AM
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Default Re: Gas tank pressure?

I just handled this exact problem a week ago. I removed the check valves, ran the fuel return line to one of the driver's side openings, and capped the other opening. Call your local Jeep dealer and order the (as an example) "vent cap for a '94 YJ Dana 35 rear axle." ($3) It's the plastic cap that goes on top of the breather lines of the axles. It fits a 5/16" rubber hose - the same as your fuel line. Run some 5/16" hose up from that extra nipple off the tank. Put the cap on the end of the hose and run the hose up behind the wheel well where the check valves used to be. Use one of those rubber-coated screw-in hose clamps like H8 is using to hold his extended breaklines. This setup vents my tank while keeping water and dust out.

canis
'82 CJ-7/360/727/D300/cobra, M.O.R.E. shackles, 1/2" steel bumper, 1" body lift and lots of ideas!

'96 ZJ - 4.0L/42re/np242/optima/cobra and other goodies
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Old 06-11-2001, 09:38 PM
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Default Re: Gas tank pressure?

The easiest way is to get a vented cap. The correct way is run both the vent lines into the special liquid vapor seperator, which in turn goes to the canister. This stops the liquid fuel from slugging the canister, which will ruin it. Go to your local Mopar dealer, and have them look up the seperator number for about a '94 YJ with the small 15 gallon steel tank. Then look at the picture, and you can easily see how it goes. Probably about $12 with the special hose.

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Old 06-12-2001, 11:35 AM
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Default Re: Gas tank pressure?

You do need to vent the tank. If you drive and the gas in the tank can't vaporize fast enough to replace the gas you use running the engine, you can crush the tank from the vacume.

The oposite is also true: I had a 87 Dodge Caravan with the wonderful (POS) Mitsubishi 2.6L motor with that "interesting" Mikuni carb that had a bad vent line. The heat from the hot exhaust and the hot summer sun caused the gas to expand which filled the intake and a cylinder with raw gas. There is nothing better on a hot afternoon than to relize you just hydrolocked the motor while you where in the mall shopping! Gas will also leak past the rings and dilute your motor oil too!

Karl
80 CJ/YJ-7 FrankenJeep
94 Grand Cherokee
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