Quote:
Originally posted by ensteele
It was positive pressure (which was quite negative!) After I messed with the check valve and a couple of other things that I am sure really didn't help, I didn't have any more problems. Untill I can find a problem that happens all of the time, it will be hard to figure it out. I will keep you posted. Thanks all for your suggestions. 
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Earl had a good think about it, if it is positive pressure as you say, the problem would have to be either the line from the tank to the carbon canister is blocked or the hose that runs out of the bottom of the carbon canister and plugs into the frame rail under the air box is blocked.
To check them you can remove the pipe from the center of the canister, remove the tank cap and blow in the pipe, it should be clear. (my 95 model has a small one way valve fitted to the pipe at the canister, I don't if all models have this valve, if it has you will have to remove it to blow). Then blow into the pipe in the center of the canister. This should be clear right through to the frame rail.
The way the system works, which may help you or somebody searching the subject, is.
The system is to prevent hydra carbons from escaping to the atmosphere. This is done by fitting a canister filled with carbon to the tank breather pipe. As the tank contents expand and contract, the fumes pass through the carbon which removes the hydra carbon from the air, so that the fumes don't escape, while the car is standing.
When you start the car, the ECU waits till the engine is up to temp and is cruising, to open the purge valve solenoid to apply manifold vacuum to the canister. This pulls air through the tank cap, through the tank to carry the fumes up to the inlet manifold to be burnt. Also at the same time the manifold vacuum pulls air up the hose to the bottom of the canister, through the carbon, to remove the suspended hydra carbons,which are burnt in the engine. At this time it consumes the fumes from the tank and cleans the carbon of the hydra carbons held there.
It is normal to find a small negative pressure when removing the petrol cap, the volume of air rushing in depends on the amount of fuel in the tank at the time. It should never be a positive pressure.
Harvey.
