[xj] Fuel vapourisation

(Frank Andersen) #41

You don’t say which tank was selected, Testie - very important…:slight_smile:

Disconnecting, or clamping, a tank hose prevents fuel reaching the valve from that (disconnected) tank. Which means there can be no return fuel - no fuel feed; no return anywhere. The engine will run for some half minute on residual fuel in the rail without supply from the pump - then stop.

Summing up; if the engine stopped with the ‘left’ hose disconnected - it was fed from the ‘left’ tank. Provided that tank hoses are correctly identified/connected to the changeover valve…

As an aside; you cannot measure resistance with power connected…

xj6 85 Sov Europe (UK/NZ)

(Testie2) #42

Bit hard to misidentify the tank and hoses that are connected to the changeover switch. The pipe is only 8” lone one from the right one from the left. The changeover switch was removed from the car and placed on 12v after checking the resistance. After about one hour the resistance had not change appreciably. The left tank is now low on fuel might a actually be empty and the right tank is over fuel. Now I know where and how to get to the return valves I will pull them out tomorrow and inspect them. At the moment the charcoal canister is removed and the hoses just hanging in the wheelwell.
I will try to drain some fuel from the right tank as well and let it idle with the left tank hose disconnected and a clear hose connected to the changeover switch where the left tank feed hose should be and make sure that the return is only into the right tank(again). Don’t know what else I can do except doing away with the changeover switch or only using one tank.

(Aristides Balanos) #43

Indeed, but not only electrically but mechanically as well.
Mine was switching alright but the seals inside where long gone…

(Carl Hutchins, Jr. ) #44

Left in right out, Imprinted in my mind. needed for Jaguar driving.

Aye, the C/o valve can be eliminated and a “TEE” inserted to join the tanks. I have one. Not used. My C?O works just fine. “knocking on wood”.

Sans adding one way valves, there is a slight risk. Park on a hill side saddle. The up tank will flow in to the down tank. if the down tank can not accept the extra fuel, it will over flow. spill not good…


(David Jauch) #45

Tee for a while: okay.
I have two fuel pumps before a T now. Switching relay for switching.
But I bet it is something like the ℅ relay, with the rubber gone. Or a return valve faulty. Definitely a valve at fault.

(Paul M. Novak) #46

Yes of course, the test of the three valves should verify that they work both electrically and mechamically.

I test them electrically with a 12V DC power pack meant to jump start cars with a dead battery. It is lightweight, portable, and convenient to use. I first test to verify that the solenoid opens and closes with a solid “click” and then use my Mityvac vacuum hand pump to verify that it opens and closes by applying vacuum. I have only tested used valves and there is usually a small amount of vacuum leakage even on valves with a pronounced click. Many of the valves that I have tested from my parts cars did not click at all although some showed evidence of opening or closing during the vacuum test. The ones that I have tested in my running Series III saloons click nicely but a small amount of leakage was evident particularly in the changeover valves.

I believe that lack of use along with fuel system impurities cause the solenoid valves to stick. I try to remember to switch tanks at least once on each trip to exercise the valves hoping this will help me avoid fuel transfer problems. I have no data to prove this, but it seems to make sense.


(Carl Hutchins, Jr. ) #47


Exercise principle !!

Aka "use it or lose it’ !!


(Doug Dwyer) #48


Power windows and locks. The parking brake. The 4-way flasher switch. The seat adjusters. Key locks/cylinders. Door latches. Any mechanism on any car benefits from occasional exercise.


(Paul M. Novak) #49

The attached pictures are provided just in case you were wondering about how to install the additional aftermarket fuel filters between the fuel tanks and the changeover valve to protect the changeover valve and fuel pump from in tank debris. I made this modification to all three of our Series III saloons a few years ago after experiencing some random fuel delivery issues. Since then I have had no fuel delivery issues.
The first picture shows the setup in my former 1987 XJ6 Vanden Plas and the second picture shows the fuel filters that I used.
I got this idea from former list member Bruce Hugo.


(Testie2) #50

The car is in limited use but does not sit around for more than a couple of days. I usually switch tanks when one gets to about half and when that tank gets low I switch back until fill up time. Anyway have tried jump pack on the valve and gentle compressed air and the valve appears to seal. Didn’t try vacuum but I could. Now I know where the return valves are I will test them while I wait for a new changeover switch to come on back order from UK. Was going to order return valves too but you guessed it they are coming from UK too.Don’t want to put used one in as by the sounds of it they are a problem.

(Testie2) #51

Looks like I need more characters try this

(Frank Andersen) #52

Leave them hanging until the present problem is solved, Col - it will also serve as a warning system that a tank is overfilling…

The comprehensive test of the fuel return valves is that fuel is returned only(!) to the selected tank - no further testing is then relevant. However, with excessive fuel in a tank, it may be difficult to be sure. But it is absolutely essential that the fuel return is correct - faulty return will mask any other additional faults…

The changeover valve may fail in two directions; feeding pump from the ‘wrong’ tank - or from both (excluding complete clogging, of course). In the first case; fuel is very quickly transferred - in the second case it takes longer…

Rule of thumb to continue driving; if a return valve fails - switch to the tank ‘filling up’. If a changeover valve fails - switch to the tank ‘emptying’…:slight_smile:

You can the juggle the selection to use both tanks’content while pondering…

xj6 85 Sov Europe (UK/NZ)

(Testie2) #53

I have ordered new changeover switch and two new return solenoids but they are on back order from UK.
In the mean time I will check the return valves. Looks like it will be off the road for a while,

(Frank Andersen) #54

No need to be off the road, Col…

It’s perfectly possible to circumvent crossfeeding or whatever - but the exact symptoms must be identified…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Aristides Balanos) #55

Mind you, the brand new valve that I had ordered was cross-leaking from day one…
Better check them before you install them.


(Testie2) #56

Great, for the cost they should be perfect!

(Frank Andersen) #57

It’s the changeover valve that’s imperfect, Col - and the most expensive bit…:slight_smile:

While you are waiting; bypass the changeover valve with a simple ‘T’ - as you questioned in a previous post. Despite minor drawbacks, it will work; the pump will now suck from both tanks. And which tank fuel is returned to is immaterial - the ‘T’ will equalize tank levels continuously…

The return valves will still be controlled by the changeover switch - and can be checked out at leisure. As long as there is return, by one or both valves, the pressure regulator will work normally…

As an aside; there is no need to run the engine while testing the valves. Remove air filter and prop the AFM flap open - the pump will now run whenever the ign key is ‘on’…

xj6 85 Soc Europe (UK/NZ)

(Testie2) #58

Thanks Frank, I will see if I can find a brass tee at the plumbers and do just that until the valve becomes available. Nice trick with the AFM flap I will try that also

Cheers Col