Hi all,

please advice and help me to stay safe

The thing is : When switching petrol tanks on my '72 XJ6, the engine is taking a lot of time (like 5 seconds) before it is sucking a decent amount of petrol out of the fuel pipes. annoying but still manageble.

It gets even worse when i completely empty let's say one tank 1, then switch over to tank 2, then stop at the station and refill tank 1, and then, while driving again on tank 2, switch back to tank 1. It takes like ages. Even when driving 80 km/h it takes so long the car just almost gets to a halt >> hazards lights on, annoying everyone , and putting my jag and myself in danger.

I assume because i completely emptied tank 1 before, when i later switch back to (a refilled) tank 1, all fuel hoses are empty and it needs tiem to fill up. Or is it air that is obstructing the flow. Or something else that is causing such a long time before things settle again ??
Thank you so much for your generous thoughts on this.
Every decent reply is rewarded with a picture of my belovered Jag :-D

Cheers
Jan

 

I never had a problem with fuel tank changeover when it was all working OK ! You may have a lazy changeover valve.

 

Sorry Frazer, but the OP appears to have a series 1 which do not have a changeover valve. My series 1 with new SU pumps takes about 2 seconds to recover if I run the tank dry. I don't do this often but have done so to check fuel consumption. I would suggest that Jan checks out the performance of each fuel pump by running fuel into a container and having an assistant operate the switch. The scenario where the empty fuel tank is filled and then switched on is quite normal - it will take the pump about three or four seconds to push that fuel to the carbs and fill the bowls.
In conclusion I would not recommend running the tank dry if you can avoid it. Yes the tanks have a mesh strainer inside, but debris will still get through to the fuel fliter and you will end up changing this more often.

 

I try to keep all the changeover bits exercised. Every time I start the car I switch fuel tanks. This keeps the level in the tanks about equal so no matter which fuel guage is reading you know about how much you have in total. Before I started this routine I saw the guage showing low and switched over and I thought I had another 11 gallons but I had forgotten that tank was empty too. Jan, if you do this your changeover will be while you are safely sitting still. Let's see those photos.
Jeff

 

regardless of what Series the XJ-6 is, you should change tanks every 10 miles (or km equivalent) and not empty either tank before switching tanks. Like they do in airplanes? to keep the weight balanced and the switching valves exercised. My two cents.

 

I agree with idea...but every 10 miles ? Wow!

I flip the tanks at 1/4 tank intervals

Cheers
DD

 

nasty habit I know, doesn't have to be 10 miles, maybe more, but just to keep tanks balanced.

 

Sorry, yes you're right ! As I remember the Series 1s had a pump in each tank, and there were no fuel return pipes.

 

There is a 1969 4.2 XJ6 at a junkyard near me and I was looking at it last weekend. Definitely a single SU pump and changeover valves, so your changeover valve is probably sticking. On a Series III you should be able to switch while driving without any hesitation, and on my earlier Daimler with twin tanks and twin SU pumps again I can switch while driving and it's unnoticeable. Something is clearly wrong.

 

Why? Weight and balance is important for airplanes to maintain trim, it's not a factor for cars. If you consider a car with a single driver, the car is inherently unbalanced side to side anyway, the fuel level is irrelevant. Many vehicles like pickups have a large fuel tank on one side only, the exhaust runs on the other side of the frame to keep heat away from the fuel. Ford sees no problem with 140 litres of fuel on the drivers side of the vehicle.

I run one tank empty, then switch to the other and then refuel. Then I know my other tank is always full, I don't run low, go to switch tanks and find it's empty. Only changing when empty also reduces wear on the changeover valves.

 

Are there/should there be check valves in the system, I wonder?

Cheers
DD

 

Fuel pump conversions are common on these cars so we need to know what system Jan's car is running. I was commenting on how the original system will work, and if you run the tanks dry, the pumps do need a few seconds to catch up. Switching tanks with both part full, there should be no engine hesitaion.

 

For me, it was drive til one tank was at 1/4. switch tanks. Drive til that one is at 1/4.
go to gas station and fill or clse to fill. i prepay and jsut estimate. At the timre., My 60 bucks did the fill. Now, not even close.

That is when i drove. have not done much of that for a spell.

Carl

 

My old 1969 Series 1 had two external SU fuel pumps and shared the plumbing, I would keep tanks even. Unfortunately I did have one fail and had to replace that pump, but never run the tanks dry, bad things happen.

 

I would second on how Jan's fuel system is actually set up help make a diagnosis. If the system is stock I would suspect the changeover switch on the dash first. My old XJ6-C would sometimes stumble when changing tanks and rapid cycling of the switch several times would restore order.

Double-check the integrity of the pumps - I've had an S.U. pump suck air without betraying a leak...

 

Yessir! running the tank dry on any ICE is asking for problems.

but, at oe time, i was really bad about it

Finely caught on and have not been stuck!!!

Carl

 

Hello all,
Just wanted to get back to you on the issues i had.
I took your advice and do not empty tanks completely anymore. Cleaned out and replaced the fuel filter once more. Drove another 1000km and since then no more trouble and the tank switches go without any hesitation from the engine.
Thanks
Jan