Does anyone, please, know a reasonable reason why this XJS was equipped with two simultaneously operating pumps ?
Since these are two connected tanks, they ultimately appear to be one.
And so (in my opinion) one "bottom, lower, interior" pump would suffice.
I am considering this replacement of the original system and I would like to know the opinion of others.

Thanks Roman

 

It was because the totally disappearing hood meant that the OEM fuel tank had to be smaller. In order to keep the range reasonable, an extra tank was added below the OEM one. If you are familiar with the pre facelift XJS, you will know that the sump tank is gravity fed from the OEM tank and that the sump tank is located under the battery. Therefore the fuel HAS to come from above the sump tank. However the H&E extra tank was low down, and thus could not gravity feed the sump tank. So the H&E solution was:

1. Smaller main tank feeds the sump tank
2. Low down extra tank has a low pressure pump to transfer fuel from it to the smaller main tank
3. The high pressure fuel injection pump, down stream from the sump tank, feeds the fuel rail as OEM.

If you eliminate the low pressure pump to the higher tank, you have to find a way to get fuel from that lower tank to the sump tank without using a pump to do it. It would be possible, but you would sacrifice a lot of boot space, as the lower tank would have to be mounted much higher. Alternatively, you could find a way to mount an integral fuel pump/sump tank combo under the car somewhere, feeding it by gravity having linked both tanks. You could use something like this, if you could find room for it:
https://www.formacars.com/store/80008

That still leaves you the problem of making the fuel gauge sender work!

 

Thanks for the answer.

The reasons are obvious why there are TWO tanks. Hood low profile.
Grrrr. YES sump tank is problem. I fully understand.
There is probably no simple solution, thanks again.

 

The possible simple solution is to eliminate the smaller of the two tanks and go to a single tank system. Downside is lower total range, upside is more reliable system.

 

If you do not want to use the solution I mentioned, what you could do is somehow convert the lower tank into an in-tank-pump tank. I do not know the dimensions of the lower tank, so I do not know whether it would double as a sump tank. But, in effect, you could use the lower tank as a gravity fed tank from the upper tank, and somehow rig up an in-tank injection pump system. Or, if you are not bothered about boot space, maybe use something like this in the boot (or the link in my first post) and see if you can get it low enough to gravity feed it from the H&E lower tank and get rid of the OEM sump tank:
https://www.ebay.co.uk/itm/Bosch-044...MAAOSwRFNcz1hD

 

"Upper" tank is about 14 US gallons (50 liters) and (for me) it is about 200 miles. (320 km).
Hm, yes, there is possible way.
Anyway, at present I only drive to the upper tank, because the fuel pump in the lower tank is weak and cannot supply the upper tank during strong acceleration or driving down a steep hill.
And it was these problems that I wanted to solve with a solution, where the fuel enters the system using a pump from the bottom tank. And gravity from the top tank.
I can think of another solution. Sufficiently powerful pump in the lower tank, which pumps fuel into the sump tank. ???
Or directly into the system and omitting the sump tank. ???
However, in both cases, I do not know, if there is a risk of clogging the system / injectors.

 

The lower tank is abou 11 US gallons.
There is no room for another sump tank, the bottom tank is lower than the boot floor.
The idea of connecting the bottom tank directly to the system (as You wrote) seems to me, probably, the best.

 

OK, you have to keep the bottom tank or the range will be dreadful.
Could you post a photo of the boot and tank setup? It might prompt a few ideas?
The key is to somehow get a tank/pump combo under the car, but low enough to be gravity fed from the lower tank. If the bottom tank is below the boot floor, this may not be possible, then you might as well stay with the H&E setup...

 

Yes cca 300 km is not good for GT.
Below is the chart. The lowest part of the lower tank lies on the floor of the car. Sump tank lies on the floor of the boot. And floor of the boot is higher then car floor.


And upper tank (there is missing sump tank, but its place is evidently - right lower corner of picture)




And lower tank:

 

So the lower tank is, in effect, under the back seat?
What a horrible lashup! And that diagram too.
The problem you have is the fuel sloshing about in the lower tank. In theory you could install a high pressure injection pump into the lower tank, but the problem would be it's running dry on long corners/low fuel, and failing as a result. Realistically your options are, assuming no budget constraints:
1) buy and install a much better quality and higher rated lift pump to the bottom tank. Again, even though I am sure the bottom tank is baffled, this pump will inevitably risk running dry on long corners/steep hills at low fuel levels.
2) pull the bottom tank and take it to a tank welding/modifying place, ask them to (in effect) take about 4 inches off most of the bottom of the tank, so it is like the attached diagram, ie with a built in sump tank in the bottom tank. This is a good solution as it eliminates the lift pump and is inherently more reliable. The space on the LHS of the diagram allows you to change the filter and pump from above. then you just connect the high pressure pump outlet to the feed line to the engine. NO sump tank in the boot. You would have to sort out the fuel gauge though. The easiest thing is to put a sender into the second tank, disconnect the top tank sender, so your gauge would read full until the top tank is empty, and set the warning light to come on with (say) 5 gallons left in the bottom tank. There are loads of aftermarket fuel gauge senders, if the OEM one cannot be used, which I think it could be. You would probably lose about 5 gallons of fuel capacity , maybe less; but have a nice simple system. Your only cost would be the tank mods, not much, even a custom tank is not that expensive to have made.

It would be fun to do too!