# HB fuel tank pressure issue(its mooing..)



## clbarrett (Jan 17, 2010)

94 HB 3.0 4x4 se king: Pressure builds up in the fuel tank and every 30 minutes or so the pressure releases and sounds like a cow "mooing" for about 30 seconds, unless I unscrew the fuel cap - then the pressure in the tank will release at once, but will build again slowly when I reseal the cap. I know that there is an evaporator hose in the system, could there be a restriction? I also wonder if the big black cartridge by the air box under the hood should be replaced. Anyone ever have similar problems in this area? Thanks.


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## TheRepairMan (Jun 30, 2009)

It sounds like a symptom of a malfunction in the "evaporative emissions system". It normally captures the fuel vapors as they expand, and filters them through a charcoal filter. The gas cap helps retain these pressures up to a point. I'd try a new gas cap first, just to make sure it's holding the correct vapor pressure, and if it continues to release that way then you will have to check the purge control valve and charcoal canister for proper operation. 

-R


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## clbarrett (Jan 17, 2010)

Where is this "purge control valve" and is the charcoal filter the black canister under the hood, behind the air box driver side? thanks.


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## SPEEDO (Jun 9, 2003)

the carbon canister is on the lt side, round cylinder with a couple of vacuum hose's going to it, the purge valve should be mounted by the egr on a brkt, 3 vac hose's going to it..


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## skramer360 (Aug 14, 2005)

I've had two V-6 Nissan pickups. One was a '91 and the one I have now is an '95. Both of them do the exact same thing. I never thought too much about it, doesn't seem to effect performance.


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