Dangerous exercise switching gasoline tanks

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 :smiley:


Jan, this is not normal. I would check that both tanks are vented well (not sure about NL but in the US there was a complicated system involving a charcoal canister under a front wing and condensing pipes inside the C-pillars. Also check the fuel filter screens at the bottom or each tank. The SU pumps only produce about 3 PSI pressure difference between suction and output, which means that they generate very little suction before they reach zero head pressure.

When operating properly there should be no hesitation at all–fuel in the carb bowls is more than enough to last through the transition from one pump to the other. Not good to run out of petrol in either tank though–that’s one advantage of having two tanks.IMHO.

Try with the engine off, ignition on - listen to what the pumps do. Do they click fast, slow, do they click faster or do they slow down - with the line under pressure do they still click?

You can also try driving with a toothpick holding the filler seal open. That takes care of possible tank vacuum.
If the pump clicks fast there’s obstruction or leaks ahead of the pump, if it’s slow pressure is reached or line is clogged. From rest it should click a few times and then slow down.

1 Like

It’s probably more than that, Jan - draining a tank completely likely drew a lot of debris into the system, now likely clogging that tank’s system. Do you have submerged (in-tank) pumps or externals?

With petrol in the tanks; there is always fuel at the pumps - gravity fed from the tanks. As Robert says; the delay during switching is completely wrong, the changeover should be instant and seamless. Without pump delivery, the running engine will first use up petrol in the carb bowls, so if the carbs are kept filled there should be no instant hesitation at changeover, as he also says - it will take some time, A check on float level, state of needle valves and general cleanness of carb bowls is relevant. It is a matter of interest if changeover symptoms differ when changing from 1 to 2 or the other way. And, I think, using ‘left’ or ‘right’ tank rather than ‘1’ and ‘2’…:slight_smile:

While checking pump action as David says is certainly relevant - I think you need to disconnect the fuel line at the carbs to get an idea of the fuel flow. Apart from faulty pumps, the fuel lines may be clogged up - with tanks about equal; gravity should make fuel flow equally at the disconnected carb connection with either tank selected. if not; it may indicate clogging, lines or pumps.

To test pump function; as the fuel safety relay prevents the pumps from running with ign ‘on’ without the engine turning - you need to bypass the fuel safety relay to run the pumps at will. You can either put gear lever in ‘D’ (to prevent actual cranking) and turn the ign to crank - this will run the pumps as selected. Or, at the fuel safety relay, disconnect the green/yellow wire from the oil pressure switch and ground the wire - the pumps will now run with ign ‘on’. Or simply jumpwire power to the pumps. All checking is of course easier with external pumps…:slight_smile:

Whatever the reason for the changeover problem; it is necessary to remedy it - but it may take some time and effort to solve it…

xj6 85 Sov Europe (UK/NZ)