| Rochester Valve |
The Rochester valve (Part CBC 7714) is a cylindrical disk (approx 2 inch diameter) with an inlet from the fuel tank vent pipe and outlet for the vent to connect to the carbon canister. A third outlet at the top is to connect to the manifold vacuum.
This top vacuum pipe opens the Rochester valve when the engine is running. Hence you should not have pressure in the fuel tank when the engine is running (If you do, check the Rochester valve).
The location of the Rochester valve is next to the carbon canister under the car, near the differential and left wheel arch. The valve stops the fuel tank from venting below 2 psi, (i.e. the valve only opens above 2 psi on the fuel tank side), and the valve opens due to a small vacuum on the tank side. That is why you get a whoosh when the car is standing on a hot day. A simple check is to open the tank cap after standing for some time on a hot day. A huge whoosh and an oil canning sound means the valve is not working -- the vent is connected wrong, or the vent is blocked. If you have no whoosh then the vent pipe or fuel tank is leaking, and the car will either have a fuel leak or a fuel vapor smell. It is important to disconnect the Rochester valve and check it opens at 2 psi and under vacuum. Also note the Rochester valve must be connected in the correct direction. It all sounds complicated, but the system is simple.
The Rochester valve opens at 1.0 to 1.50 psi.
The fuel pump in the XJ-S (in the trunk) can put out nearly 200 psi. This is way too much for ordinary fuel hose -- do not use it anywhere in this fuel system. Most auto parts stores now carry "EFI hose." It is much more expensive, but a fuel fire is no fun.
http://www.jaguarforums.com/forum/xj-x300-xjr-26/pressure-control-valve-rochester-discontinued-81248/
The two vent pipes were wrongly connected, i.e. the fuel tank vent was connected to the valves for PVC pipe (two valves operated by vacuum from the LHS manifold), and the PCV vent pipe was connected to the Rochester valve that should be connected to the fuel tank vent.
"So what's the big deal? Well, the valves that would normally operate the PCV valve only open under vacuum when the engine is running. When parked and sat in the sun the tank cannot vent! I tested these valves with 10 psi and still the valves would not open. The tank is now a sealed container with fuel vapor inside!
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