Gas tank pressure question

This topic has 9 replies, 3 voices, and was last updated 17 years ago by EARP64.

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May 30, 2009 at 2:15 pm #337843

EARP64
Participant

Last Sunday I was washing the JAG and was at the back cleaning the motor, when I heard a hissing noise like pressure being released. It was coming from the LH tank,on top where you put gas in, I opened the cap and no more noise.. I ran on my other tank all day and it did not make any kind of noise while I was doing my clean up. Ive heard the noise from both sides, but never questioned it, but thought what the heck, Ill ask now. By the way, I started the day with both tanks 3/4s full.

May 30, 2009 at 3:26 pm #337859

Bass Cat Boats
Keymaster

certainly sounds odd since the tank is vented to prevent a vacuum from forming and starving the motor.2008 Pantera Classic2014 Mercury Pro XS 200

May 31, 2009 at 5:55 am #337879

Bass Cat Boats
Keymaster

The vent may still have the plug in it and in the other flip lids, there is a vent that is screwed in the lid (cap) underneath. The hissing is expansion of the fuel in warmer weather. It could be that you had the boat on an angle and the vent was covered with a full tank of fuel. Expansion in fuel is a major concern as things heat up. We have posted a warning the past few seasons as June approaches. BCB

May 31, 2009 at 8:42 pm #337895

EARP64
Participant

OK,What do I do. So far I simply open up the lid and it seems everything is ok, am I assuming correct ? This is a stupid question, do I dare leave the lid open, meaning not completley closed ? Just want to make sure there is no major problem.

May 31, 2009 at 9:07 pm #337914

oldtimer57
Participant

Are you talking about the lid to the battery compartment? That is not doing anything except perhaps making it easier for you to hear whatever it is you are hearing. The tank vent is back in the splashwell. Chrome gadget with a hole on the bottom. When you put fuel in, air needs to get out. When you burn fuel with the motor, air has to replace it or you get a vacuum and run lean. As the weather heats up, so does the fuel and as it expands, some air needs to go out. And when it cools back down, the fuel contracts and you need air in the tank to avoid that pesky vacuum problem again. If that vent gets plugged, it is harder to fill up, and the motor will go dead during a long run when you reach the point where the engine fuel pump cant overcome the vacuum and no longer delivers fuel to the motor. I had a friend that had a boat with dual tanks a few years ago, and you could hear it making “noises” when sitting/sloshing on the water with full tanks. This boat had a single vent, with the two tanks vented such that one tanks vent ran to the next tanks vent fitting which was a T, and the other side of the T went to the splashwell vent. You could get the boat slightly cocked to one side with two big guys fishing on the same side, and the fuel from one tank would cover the vent because of the angle, and then the venting had to force raw fuel thru the rubber hose to the other tank, and sometimes even out the splashwell vent. You could hear that hissing/gurgling when this happened if it was quiet on the water.2008 Pantera Classic2014 Mercury Pro XS 200

May 31, 2009 at 9:24 pm #337929

Bass Cat Boats
Keymaster

OT, Im talking about the the lid where you put gas in. I kept it covered pretty tight, so if I need to keep it loose, its no problem. What Im hearing is where you stand at the back of your boat where you put gas in. I dont think there is any real issue there, just wondering if someone else has encountered this. Ill just keep doing what Im doing, which is nothing. Thanks for the info though.

May 31, 2009 at 10:20 pm #337947

Bass Cat Boats
Keymaster

The only concern would be if your vent ins covered with fuel, which is forcing the expanded fumes out the fill cap. The angle of the boat could be putting fuel on the vent when full. The only solution is to lower the nose or raise it to vent that tank. This is critical on some boats and some models. The vent in the cap may not be venting also. Open the cap and if yours is the flip style look at the vent in there. Remove the cap (flower shaped) and check the vent for a cap inside or blockage.

June 1, 2009 at 12:40 pm #337961

EARP64
Participant

BCB, Ive opened the lid, and I see the flower. Ill take it off next weekend and check the vent to see if it is clogged. Thanks, Wyatt

June 8, 2009 at 12:56 pm #337980

EARP64
Participant

I could not get the flower to unscrew, is there a special way to take it off, I dont want to break it. Also, both tanks were releasing pressure yesterday. Is this normal ? If not, do I need to replace the pressure valves in gas lids.

June 9, 2009 at 12:16 am #337987

EARP64
Participant

Any suggestions or comments ?
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