Pressure build up in 87 accord gas tank

I have a 1987 honda accord (carburetor).

Pressure is building up in the gas tank. I replaced the gas cap but it
still happens.

Any suggestions as to what else I could try?

 

How do you know pressure is building up? What tells you this?

 

leave it alone. it's the evaporative control system working normally.

 

I had this on my 86 accord. I was used to a small release of pressure and a
whoosh when removing the gas cap, but it got more and more pronounced until
it was like opening a carbonated beverage.

Finally pressure built up so high that it forced gas past the o-rings on the
carburetor float and flooded the engine. I traced it to a blockage in the
vent line of the evaporative control system. There is some sort of valve in
the tank that connects to the cannister in the engine compartment. I used a
vacuum pump to suck on the end of the hose and it opened up. Since then
everything is back to normal, small release of pressure when removing the
cap.

 

Thanks for the info. I did some further checks on a honda manual and
became aware of the "2 way check valve" which allows vapors to escape
to the charcoal canister.

Where did you attach the pump? To the line coming from the fuel tank
where it connects to the charcoal canister or close to the tank itself?
Where exactly is this valve, do you have to drop the tank to get at it?

As for the flooding problem, isn't the fuel tank cap supposed to
release the pressure if it get beyond a certain threshold to avoid such
problems?

 

Thanks for the info. I did some further checks on a honda manual and
became aware of the "2 way check valve" which allows vapors to escape
to the charcoal canister.

Where did you attach the pump? To the line coming from the fuel tank
where it connects to the charcoal canister or close to the tank itself?
Where exactly is this valve, do you have to drop the tank to get at it?

As for the flooding problem, isn't the fuel tank cap supposed to
release the pressure if it get beyond a certain threshold to avoid such
problems?

 

Yes, it was the 2 way check valve. Either it was stuck, or something was
stuck in the line between it and the cannister in the engine compartment. I
disconnected the line in the engine compartment, then connected a
'Mighty-Mac' hand vacuum pump to the line. I forget how much pressure that
2-way valve is supposed to hold, but I was able to pull a considerable
vacuum on the line before something gave way. Then it worked normally, a
small amount of vacuum would open the valve and pressure was relieved.

I believe that the factory cap is supposed to open and relieve pressure, but
I lost that some time ago and have an after market. I could not see any
pressure relief feature on it.

The pressure definitely built to the point that it blew right past the
o-rings on the float valve. I could see raw gas pouring down the throat of
the carburetor. I wound up getting a carburetor gasket kit just to get the
o-rings. I replaced those and things have been fine since.

 

Upon a little further thought, perhaps applying pressure to the line would
be more effective since debris from the tank that might plug the valve would
be forced back into the tank. I may have done vacuum and pressure on the
line, it's been a while so I'm not that sure. The Mighty-Mac can do either.