I’m just in the middle of fuel hose replacement operation all-over the fuel tank. I’ve got 1989 XJS with 3.6 UK version pre-facelift (non-carbon canister).
While approaching area of the fuel vapour separator I’ve tested rochester valve located on the picture. It can’t be that wrong…
One side of the valve is connected to vapour separator, the other one to the metal line going behind the tank (and out under wheel arch) which makes sense.
The tiny pipe on top is actually connected to nothing.
If I’m getting it right - rochester valve is venting the vapour overpressure exactly through the tiny pipe… Straight to the trunk - just checked, pressure going out through the small tube.
Am I missing something or this makes absolutely no sense. Where the tiny pipe is normally connected?! Also there is no fuel smell in the boot…
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
2
Good pic of a Rochester valve. We should keep that here on j-l for reference.
That’s actually viable. The idea behind a fuel tank vent system is that the tank should vent only through a carbon canister, but just letting it vent freely would overwhelm the carbon. So the 2 psi valve was invented, allowing flow in either direction only if the pressure or vacuum exceeded 2 psi. But then some cars, including Jaguars, had issues with the stresses on fuel tanks while the car was running, so the Rochester valve was invented. When the car was parked, it’s a 2 psi valve. When the car is running, intake manifold vacuum is plumbed to that little connection on top, and the vacuum holds the valve wide open, no more 2 psi needed for flow, it becomes a straight-through passage.
Now, if you need a 2 psi valve and can’t find one, one solution would be to use a Rochester valve and simply don’t connect any vacuum to that small port.
I dunno why you’d use a 2 psi valve on a car without a carbon canister. Maybe it minimizes odors. Or maybe it was an early feeble attempt at some emission control. Whatever, I’d kinda suggest you connect manifold vacuum to that small port, even if it didn’t originally come that way.
Heck, I might even suggest you add a carbon canister. I presume you could pick one up in a junkyard.
For some time, for US-spec cars with carbon canisters, I have recommended adding a bypass around the Rochester valve with a solenoid valve that opens when ignition is on. This duplicates the function of the vacuum, opening up the passage, just in case the Rochester valve fails – which it seems to do only too often. I also recommend installing inline fuel filters in the hoses on either side of the Rochester valve to keep contaminants out of the valve itself, as they often seem to fail due to crud in the ports inside. That crud sometimes appears to be carbon granules that have leaked out of the carbon canister, so you might not have that problem.
Thanks Kirbert, that make sense. What doesn’t make sense is the fact - this Rochester valve is there since the car left factory… Obviously there is a solenoid as well… Connecting vacuum line would make the same job as solenoid is making right now (purges pressure with ignition key on). Do you think is actually necessary? Just checked with spare valve and it seems the one installed is faulty/leaking (somehow). That won’t explain lack of fuel odour in the trunk… (?) Thinking of British cars design… Yes, probably it’s an ancient function reserved for US spec, left there just in case or due to the fact someone forgot to change assembly drawing revision.
Do you think removing it would put much strress on the tank itself?
@XjsBanger would know, I don’t understand the function of it. But if it purges with the ignition on, replacing engine vacuum, it should be a normally closed version that is powered by the ignition. No guarantee though!
Trivia; Jaguar also used them as an extra air valve so the engine isn’t losing much speed when the AC cycles.
I Made that by-pass… It stopped the tank from tin canning… boom noise… I found the electric valve in a box of odd Jag parts and don’t know what it was originally for. Found an ign. switched hot wire and a few tees and hose… Easy to do and cheap insurance against future tank problems…however it didn’t fix gas smell in trunk.
No, not if it was normally closed. I suspect any gas smells creeping into the trunk or cabin would wait until the car is off and the air around is still.
These valves were installed in non-US cars, probably due to relaxed emission laws and some fuel smell in the boot on brand-new cars as factory bonus (carbon canisters in US).
I was having both: rochester valve and solenoid installed. Solenoid in normally closed and opens when you start to crank the engine / with ignition on (not sure/don’t remember).
Rochester valve is connected to a tiny metal tube that goes down to the bottom and exits righ next to rear sway bar’s body mount (works all the time)
As per Kirbert’s stress theory - I’ve added additional rochester valve before solenoid to disable it. Thst gives uniform pressure in the tank all the time.
Now thinking of Kirbert’s stress theory and guys from Coventry… Pressurised tank takes more stress during the ride really…
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
14
That ain’t per my theory! The whole point of the Rochester and the solenoid is to ensure there is no pressure/vacuum in the tank when the engine is running. Zero, zip, nada. Sticking another Rochester in there is just introducing another possible failure point.
If that line is the only vent for the tank it should definitely not be vented by the NC valve alone. Fuel will find a way when parked in the sun. The tank will be fine with a little overpressure like 2 psi, I don’t get the thing but it’s not my problem - can’t both valve and solenoid be removed? It will release like a gallon of fuel to the atmosphere over the rest of the cars life.
The point of this is to add a redundancy in the system Janusz.
The Rochester Valve and the Solenoid should be connected in parallel.
If one fails the other will be there to save your tank from imploding.
Remember that there is no other vent to the atmosphere but the Rochester Valve, and when fuel gets consumed air must replace it.
Just to experiment… I will temporarily remove the electric valve and put the hoses together giving vent to canister all the time. I’ll see what happens after a few days.
Only risk is canister saturation; full fuel tank and high ambient temperature may result in canister overflowing liquid (condensed) gasoline dripping from left front…
