My 89RS (2.8MPFI and th700r4) is leaking gas out around the cap whenever I have more than 1/2 tank of fuel. I have tried swapping caps, and venting the cap by drilling holes. Nothing seems to work. Really need help with this, because leaking gasoline out the side of the car is gonna catch up with me one day. Don't have any codes popping up either.

 

think i'd start by replacing the cap you tried to "fix". have you tried getting under the car and looking for a trail of gas from the site of the leak? that might halp you locate it.

 

I know where the leak is from, it leaks out the gas cap. I have tried other caps, doesn't make a bit of difference.

Pressure is building up in the tank and pushing the gas out the overflow hole. I just don't know how to fix it.

 

Have the evap can checked... Other than that, get a new FACTORY gas cap and see what happens...

 

Tried another factory gas cap. No dice. No codes for the EVAP deal.

 

Evap can doesn't usually set a code. You have to go out and buy yourself a Haynes manual and vacuum pump so you can test the solenoid and purge valve. I have written the test process in another thread in the V6 forum. There are 3-4 things that go wrong with the can:
1. Purge valve diaphragm leaks or breaks, rendering valve inoperative.
2. Can gets cracked or broken.
3. Solenoid mounted on can gets broken either electrically or physically, either permanently blocking vacuum from the can or permanently allowing vacuum through the can and creating a constant purge condition.
4. The filter in the bottom of the can gets clogged. Not quite sure what this does.
5. You may have a blockage in the vapor line leading from the tank to the can...
Something else can be wrong in any of the lines. Either way, there is positive pressure in the tank that causes the gas to be pushed out of the vent holes in the cap. That's all I know...

 

I got a purge can off my 91 RS that I might just use. See if that fixes it, if it doesn't I don't know what to do.