[xj] Fuel vapourisation

(Frank Andersen) #21

Replacing the charcoal filter in toto is a regular service schedule, Testie…

However, while it is not supposed to be ‘repairable’; it is possible to take it apart and renew the charcoal inside - which is the same as used in aquarium filters.

Before that; you should bypass canister the relief valve as described - to see if either the filter and valve is actually involved. As David says; the valve is as likely to cause vent clogging as the hoses and canister…

But if fuel is ‘migrating’ from one tank to another; it’s independent of the tank venting system. ‘Migration’ has two fault causes; either return valves or the changeover valve function improperly. As fuel is tested to only returning to the selected tank - the cause is the changeover valve.

The fuel feed (tank/pump/rail), the fuel return (pressure regulator/return valves/tanks) and the vent system (tanks/separators/canister) are three completely independent circuits…

xj6 85 Sov Europe (UK/NZ)

(Testie2) #22

Thanks Frank, this is getting stranger by the day. Today I was surprised to see the l/h tank is now half empty and the r/h nearly full. (I have been running on the r/h tank to reduce the level, but have now changed to l/h ) Both tanks were filled to the top at the same time.! Looking in the filler neck return flow is to whichever tank is in use and no flow in the other. So change over valve?

(Paul M. Novak) #23

It sounds like your changeover valve might be stock in an intermediate position, or is inoperative. Here is a test that you can quickly do to verify whether or not your changeover valve is working.

Open the trunk, remove the carpet over the spare tire and locate the changeover valve at the rear of the spare tire well. Then have an assistant sit in the driver’s seat, turn the ignition ON (no need to actually start the engine) and have them push the fuel tank selector switch changing tanks every few seconds while you stand at the rear of the car listening for a pronounced click as the changeover valve switches from one tank to the other. This strong click from the solenoid valve should be heard clearly every time the fuel tank selector switch is selected. You may also hear the return valves in the wheel well click as well. If you hear the changeover valve clicking, then your fuel tank selector switch and changeover valve are working electrically. If you don’t hear the click then you either have a failed changeover valve or an electrical problem.

There is another test to see if the changeover valve is opening and closing fully in each position, but for now try this test to see if the changeover valve is at least working.


(David Jauch) #24

I think it is just stuck or inoperative, not halfway as the tanks would probably even out instead of developing pressure. For information, the RHD tank means all valves have power. No power on all valves selects the left side tank. One 3-way and two 2-way valves for the returns.


(Frank Andersen) #25

Most likely, Testie…

In which case; fuel is drawn from ‘wrong’ tank (like ‘left’ with ‘right’ selected) - but returns correctly to the ‘right’ (in this case) correct tank. So the ‘right’ fills up…

But it is very(!) important to keep track of which tank is filling up with which selection. ‘Left’ is the default for all valves - spring loaded, but debris may overcome spring force…

With a faulty return valve; driving on the ‘emptying’ tank works fine. With a faulty changeover valve; drive on the ‘filling up’ tank - while pondering actions…

The original problem does not relate to return/changeover valves - unless a tank is run empty. Nor would it normally relate to tank venting (which is not involved in tank ‘crossfeed’) - but fault with either should be rectified. And a tank venting problem require some urgency…:slight_smile:

xj6 85 Sov Europe (UK/NZ)


(Testie2) #26

Ok so the charcoal canister has some (cupful) of petrol which emptied out when I inverted the canister. Out from the large pipe which normally vents to atmosphere. Blowing in the hose that goes to the tanks I can hear fuel bubbling ? The valves appear to be working, that is the valves at the front of the car. All pipes appear to be intact. Change over switch operates as it is suppose to. Power on right, power off left.
When listening to the change over switch there is another clicking towards the back seat. Is that the changeover for the return line? As these seem to be working I have not chased them any further.
Next remove the changeover valve and check for any crud.
Any further comments?

(Frank Andersen) #27

The canister should not have any liquid petrol, Testie - it ruins it…

The vent line is supposed to be above the fuel level in the tank; vapour then goes up to the fuel separators in the C-pillars - and liquid fuel is returned to the tanks. However, overflowing tanks may indeed overwhelm the separators - allowing liquid fuel to enter the canister.

As the engine is turned off, eventual fuel vapour from the tanks is ‘adsorbed’ by the charcoal (fuel settles on the surface of the charcoal) - and the ‘clean’ air is vented out into the atmosphere. When the engine is started; (restricted) manifold vacuum draws air through the canister - drawing off the fuel from the carbon to be burnt in the engine…

With the charcoal drenched by fuel, it won’t prevent fuel venting to the atmosphere. But secondary; the soggy charcoal may prevent outside air from passing through the canister - and manifold vacuum is applied to the tanks…

Basically, being connected to the open air (through the charcoal) - functioning properly there is never any vacuum in the canister…

In theory; a flooding tank may cause fuel to enter the engine, causing it to stall - the original problem. However, the xk engine is tolerant to running fat - and there is a tight restrictor in vacuum line to the engine to deliberately restrict air flow. However…:slight_smile:

In any case; with ‘crossfeeding’ of tanks verified; it should be remedied as a matter of course. In the meantime; driving on the ‘right’ tank should prevent crossfeeding. And tank venting should be verified; the fuel contaminated canister may cause it - and should be bypassed for testing.

The ‘bubbling’ may be residual fuel in the fuel separators, the vent line should be well above tank fuel level. However, that you can blow through indicates that the vent line is open…

xj6 85 Sov Europe (UK/NZ)

(Testie2) #28

OK next, just went down town only a couple of kms and it stalled again. not start the same as in the beginning. The right hand fuel tank is filling to overflowing while the left hand tank is inuse. Disconnected all hoses from the charcaol canister and fuel is pouring out of the beather line from the tank.
So now what do i do. There must be a breather

(Testie2) #29

Sorry now the computer has stalled too!!! there must be a connection between the two tanks besides the changeover switch? Nearly ready to drop a match in it as there is plenty of petrol everwhere but in the injectors!!!

(Frank Andersen) #30

First, Testie; switch to, and drive on, the ‘right’ tank - that should stop ‘right’ overflowing while you ponder further actions.

The reason for stalling is likely that the overflowing tank feeds the engine through the ventline - drowning it. But tank venting itself is not a factor…

As said before; the cause of the overflow is failure either with the ‘right’ return valve not closing (unpowered with ‘left’ selected)…

…or with that eliminated (return verified correct as per yourself!); the changeover valve is feeding from the ‘left’ tank with ‘right’ selected. With the return valves working correctly; with ‘right’ selected, the feed, the return and the venting should work normally…

Actions; select ‘right’, keep the canister vent line disconnected - and verify that no fuel is ejected ith the pump running…

As the engine is likely flooded it may take some time to clear it - try flooring the pedal while cranking to dry out the engine quicker…

With the engine then running normally; you can verify changover valve function - either by removing the valve for test.

Or alternatively; with the engine running (on ‘right’ tank) clamp the ‘right’ tank fuel hose at the valve. After a while the engine will die from lack of fuel - confirming that the engine is being fed from the ‘right’ tank.

In principle; the next step is to unclamp hose (to restart the engine), select ‘left’ - then again clamp the ‘right’ tank hose. Again the engine should stall - confirming that it is indeed fed from the ‘right’ tank with ‘left’ selected…

Pitfalls; combinations of faults may lead to confusion. It’s therefor absolutely(!) essential that fuel return is checked first. It is either correct or incorrect - and if the latter it must(!) be rectified before further actions are taken.

xj6 85 Sov Europe (UK/NZ

(Testie2) #31

Fuel is returned to the correct tank either left or right depending on the switch position. Varified by looking in filler and holding the flap open.
The right tank if full and rising while drawing from that tank and therefore pushing fuel out the breather and into the charcoal canister.
This is getting ridiculous I must be missing something.
I will clamp the hoses off tomorrow and see what happens. I pulled the changeover switch and it seemed to be working and with the power off, soleniod in relaxed position, there was no flow from the left input to the right input only to the supply to the pump.
Where are the return line valves?

(David Jauch) #32

The return line valves are in the back of the rear wheel wells. The return line makes a loop coming out of the valves, and there is a protective piece over the valve. But they seem to work if the return is always correct. I assume you checked the return on the not selected side as well.

Clamping ahead of the changeover valve sounds like a plan.

(Testie2) #33

Yes. I checked in the rear wheel ell took a cover of where the wires go in but could not see any valves.

(Aristides Balanos) #34

If this is so then your change over valve in not changing over…
You can verify this by pinching the fuel line from the corresponding tank, before the pump, if the pump starts to make cavitating noises then you know it’s drawing from that tank.
This off course doesn’t include the possibility that it’s drawing fuel from both tanks… a bench test would be more conclusive.
I had a hard time with this valve and after several attempts to make it work I gave up and went with a different solution.

(David Jauch) #35

As we all agreed numerous times :slightly_smiling_face: maybe pinch of the LH feed as Aristides suggested. I was thrown off before too.

(Paul M. Novak) #36

Maybe the two attached pictures will help you find your return valves. I took these pictures today of the fuel return valve in my crash damaged 1987 XJ6 parts car that I am disassembling. The first picture shows the left rear wheel well with the word “LEFT” written in chalk on the panel covering the left return valve. The second picture shows the left return valve circled in red after I removed the cover panel but before I removed it from the car. It would be a big surprise if this valve, and the one on the right side, were not present in your car. A big surprise, but not impossible.
Prior Owners and their shops know no bounds when it comes to what they will do in the name of maintenance and “upgrades”. But hopefully with these pictures you will be able to find your return valves. If not, you should really get some and get your fuel system working the way it was designed to.


(Testie2) #37

Ah Ha. Looks like I didn’t delve deep enough. Thanks for those pictures it realy helps.
The circular cover were the wires go in was what i was looking at.Having taken advice of others on the board I removed the L/H hose from the tank,as it is impossible to clamp the pipes,blocked it to prevent any more spillage and ran the car on the L/H tank until it ran out of fuel. It didn’t! 5 minutes later.Pulled the changeover valve and applied volts and again it seems to work as designed left is de-energized and right is energized and there is nothing right when left is selected and nothing left when right is selected. How can the car be still running without a fuel flow? it is not that good on fuel!!
Has anyone just put a tee in where the changeover valve is? What are the ramifications of doing that. will the fuel in each tank just equalize?

(Testie2) #38

Just did the hose off and the motor stopped after 33 sec. even with the left hose disconnected the return was flowing in the correct tank until it stalled. Could the changeover switch been faulty and now cleaned be working correctly? I had had it out of the car and on 12v to see if the resistance changed but the resistance stayed nearly the same at 23 ohms.
I have just had the handbrake pads changes and as it is nearly impossible to do had it at a mechanics. Now i am wondering if a hose has been kinked or something stupid like that.
I now have acces to a hoist so next week if i can keep it running i will check.

(Frank Andersen) #39

The ‘missing something’ is somewhere in previous posts, Testie…:slight_smile:

My impression was that the ‘right’ tank overflowed with ‘left’ selected? Which, if return is correct, nominally means that fuel is wrongly drawn from the ‘left’ and returned to the ‘right’.

You say ‘…right tank is full and rising while drawing from that tank…’ - which is technically impossible if return valves are working correctly.

One possibility is that the fuel lines from the tanks to the changeover tank is crossed. Another is that switch is misread - which will cause confusion. ‘In’ is ‘left’ and ‘out’ is ‘right’…:slight_smile:

As faults may combine, mechanically, electrically or simply blockages, symptoms may become very confusing…

xj6 85 Sov Europe (UK/NZ)

(Paul M. Novak) #40

If I were you I would drain all the fuel from both tanks, remove the changeover valve and the two return valves, and test them independently to verify that they all open and closed properly electrically. I suspect that one or more of them are not opening or closing fully as they should. They can get clogged with debris and sediment especially if they haven’t been exercised regularly or if the in tank strainers are missing or torn and debris made its way to the changeover valve.

Some of us have followed former list member Bruce Hugo’s advice and added aftermarket fuel filters between the fuel tanks and the changeover valve to to protect the changeover valve and fuel pump from in tank debris… Lots of info about this can be found in the archives.