Hey guys, I am a “new kid on the blog” and have a quick question about the fuel tank switch on the XJ6 S3 (4.2 manual, 1983): It seems that I am only running on fuel from my left tank, regardless of which tank I choose to run on. The switch button seems to be working just fine (I hear different noises at the car trunk near the fuel switch valve as I switch over the tanks) and I check that fuel is returning fine on both tanks (I used the pencil trick to push on the little metal flap at the tank filler openings) and according to whatever tank I choose to be running on. So, it seems that the fuel returning valves are working fine, so I am starting to suspect that something is not quite right with the fuel switch valve. This valve was replaced about 4 years ago by the previous owner (and it does look like new), so it is a bit odd that it might be faulty already. I noticed that when I switch to the right tank, there is a stronger buzz noise at the fuel switch valve (as if something is stuck). Any ideas?
Many thanks in advance!
I’m inclined to agree
Not entirely odd, given the lower quality of replacement parts these days.
Anyhow, remove the valve for bench testing and inspection. Sometimes a simple cleaning will do the trick. Drain the tanks first, or have to plan to quickly plug-off the fuel hoses, or you’ll have a flood of gasoline.
I agree with Doug that your changeover valve is suspect and should be removed and tested to verify if it is working properly. It is a common failure item in these cars. If you search the Jag-Lovers XJ archives for “fuel changeover valve” you will find hundreds of posts from others who experienced similar problems, including some from me. I experienced several random fuel delivery problems in my XJ saloons that I attribute to debris in the fuel tanks that made its way to the fuel changeover valve, fuel pump, and fuel filfer.
Many years ago a former member named Bruce Hugo and Doug Dwyer came up with the idea of installing inexpensive aftermarket inline fuel filters between the fuel tanks and the changeover valve to protect that valve and the fuel pump from debris in the fuel tanks. I made that modification to my two former XJ6s and my current 1990 V12 Vanden Plas and I haven’t had a fuel delivery problem since then.
Attached is a picture of the the fuel spare tire well in my car showing the fuel changeover valve circled in red and the two aftermarket fuel filters circled in blue.
The fact that the prior owner removed and replaced the fuel changeover valve before makes me suspect that you might have some debris in one or both of your fuel tanks. Inspection of your fuel tanks is recommended before you purchase and replace that valve again. I always found some debris in the bottom of my fuel tanks when I inspected them, some more than others.
Dear Doug and Paul,
many thanks for the prompt response, much appreciated! I would not be surprised of some debris in the tanks, these actually caused cloths, some of which I manage to expel by pushing hard on the accelerator. That brings me to a new question: how do I empty the fuel tanks (sorry if this is too basic a question…). Many thanks once again!
There are caps under the side rear valance and underneath you will find the drains.
With the tanks less than 1/3 ful, you can inspect them without draining them from the fuel sender hole behind the rear lights.
If you only want to change the switch valve you can just clamp the fuel hoses.
Adding filters before the pump as Paul suggested is a very wise idea indeed.
They have to be high flow though, if they are too restrictive and the pump will start cavitating.
In this case, it’s also a very good idea to remove the in-tank filter sockets, they are just above the drain plug.
Drain plugs at the front bottom of the tanks - small inner for emptying, big outer for cleaning out debris and in-tank filter change…
You need an adequate receptacle and, depending on the amount of fuel in the tanks - it will gush. You may use the pump to empty the tanks first - beware of high - or siphon out by connecting extension hose to the tank hose.
Remember to clamp all hoses at the changeover valve before disconnecting any hose…
xj6 85 Sov Europe (UK/NZ)
Dear Aristides and Frank,
many thanks for the tips on how to empty/clean the fuel tanks.
Attached is a picture of the finisher panel beneath the left fuel tank of my former 1984 XJ6 Vanden Plas with the location of the fuel tank drain circled in red. As previously mentioned there were plastic covers installed at the factory that cover the drain plugs. In order to access the drain plugs you will have to remove the plastic covers. I was able to do this by removing the screws circled in blue and then using a small pry bar to open up a space at the bottom of the finisher panel. If your car does not have the plastic drain plug covers that means someone has accessed the drain plugs before and chose not to reinstall the plastic cover.
If you are planning to work on your car I highly recommend that you get a copy of the Jaguar Series III Service Manual. These can be found on eBay or at any of the Jaguar parts suppliers. These cars are sufficiently complicated that having a good shop manual will save you a lot of time and money over the long run.
Many thanks Paul, I do have a copy of the Service Manual for the series III, but the photos do help a lot! Cheers!
I managed to replace the changeover valve without too much spilling. I did everything by the book, including depressurizing the system and reconnecting all the wires as they used to be with the previous valve (by the way, the right side of the valve was completely blocked by some black clots, so that seems to explain why I could not have an effective tank switch). However, the engine now doesn’t start, it seems that no fuel is being injected into the system. Any idea what may be the reason? Fuel pump was working just fine…so I have no clue now! Many thanks!
Is your fuel pump working?
It should run for 2sec when you turn the ignition on and restart when you hit the starter.
Is the fuel pump still running in ‘crank’. Maiato - or with the relay jumped?
Does the injector ‘click’ while cranking - listen or using an injector test lamp? Does the valve click over when switching? The valve defaults ‘left’ without power, but is of course open either to the ‘left’ or ‘right’ tank - do you have fuel in both tanks? Do you have fuel return to either fuel tanks - indicating that rail is pressurized?
If ‘yes’ to all of the above; check if you have ignition…
Incidentally; a clogged changeover valve can usually be cleaned to restore function…
xj6 85 Sov Europe (UK/NZ)
Not on a 6 cylinder car, Aristides
Given the sequence of events it might be that you disturbed something while replacing the valve. Trace all your previous steps and look for a loose, broken, or disconnected wire
Just FTR, it was neither Bruce nor I who came up with the idea. I’m sure one of us read about it right here on J-L, though.
A note on the drain plugs and covers. My replacement left tank has a larger permanent internal filter and only the small drain plug.
Many thanks, I will double check everything. One note: the engine did start after changing the valve and was kept on for 2-3 sec before dying…that suggests that ignition and all the wires are OK. I have the feeling that whatever fuel was left in the hose feeding the fuel pump kept it live for few secs, but then no fuel was reaching it anymore. Is it possible that some air got into the system?
I guess so, since the engine did start and was kept on for 2-3 sec before dying…and yes, I have two full tanks with fuel! With the keys turn in the position before cranking, I do listen a click noise at the changeover valve. I did depressurize the system before changing the valve by removing the fuel pump relay and cranking the engine for few seconds. Then put the relay back before changing the valve.
That would be a good area…fuel pump relay… to start looking. Sometimes disturbing elderly connectors and wiring can cause a problem.
Will re-check the relay. Everything looked fine, as it only fits in one position…what can be done about it? Can we still buy this part?