I have a 1989 CRX that on hot days with a full tank as the day gets warmer fuel will begin to seep out of the filler cap. If i remove the cap fuel will spray out. When the tank is not full then there will be a constant hiss of fuel vapor leaking out of the cap and a huge rush of vapor when i remove the cap. My guess is that the two way valve on top of the tanks that controls vapor pressure is bad. Does anyone know the part for this. Or is there anything else that might be causing this issue that I can test out.

 

That tells me your gas cap is bad.. it should never allow vapor to leak. When it cools down, the vaporized gas will condense back to a liquid. a leaking gas cap, even vapor, means you are getting worse gas mileage and technically the car is polluting MORE because of the unburned gas going into the atmosphere.

That is why at emissions places they test your gas cap to make sure it seals properly.

it is normal for newer cars to release vapor in the manor you described when you fill them up when it is warm out.. it means your gas cap is functioning properly.