Good day all,

After a very long time, my 14k original mile '76 XJ12 airport queen has finally moved under its own power from one side of the airport to the other in one of my climate-controlled hangars.

I've done quite a bit in the past month to get this car moved. 2 new fuel changeover valves came in, installed, and seem to work wonderfully. AAV has been overhauled with replacement parts from Jag Improver, along with his throttle body arm bushings. Timing set and idle now within controllable parameters. I may have pressed the top of the AAV down too much as cold idle is a bit too low. I have discovered one of the fuel pressure regulators is WAY off at 60psi and the other was adjusted down to 29~, then the high PSI regulator flat out failed and fuel is being totally bypassed, so I've just clamped the return line so I can still run the engine. The replacement regulator is on its way as well. Many more things have been completed, but those have been the most fun.

I have run into a really odd issue that I can't get my head around. Currently, the left-hand petrol tank has about 3 to 3.5 gallons of fuel in it and the right-hand has about 2.5. I've been running off of the left hand for most of this time and just split a 5-gallon container between the two, so the left hand is a total estimation from the initial 3 gallons I put in there in early 2020. The fuel pump works perfectly and both changeover valves are actuating. The curious thing is that when I switch tanks to the right, the car will shut off as fuel pressure reduces steadily, so 7-10 seconds. I have the fuel presser tester still connected, so I am actively watching it drop. I switch back to the left and pressure holds. Now, if I switch to the right, start the engine, the pressure dies down, the engine shuts off, I turn the key off and turn it back on. When I do this, the pump does its normal startup, and fuel pressure builds again. Start it off the right tank, fuel pressure dies down, car shuts off. Repeat. So it's still getting fuel, but not maintaining it somehow.

I have tested flow in primitive ways. I have used a brake bleed vacuum tool to verify that fuel is in fact still coming from the right-hand side after the filter, so the valve is open. I connect a hose from the filter, activate the pump off the relay, and fuel comes out. I have checked the voltage to the pump when the engine is running when the switch is on the left or the right, so there is no pump disconnect when I switch. What I haven't done is measure flow from the left side versus the right side, but the only way I can see doing that is with a timer and a measuring cup of sorts. I suspect a few things. There may be an obstruction in the right-hand tank going to the valve, or the valve isn't opening all the way. I suppose writing this out is helping me further diagnose, but I'm hoping some may have insight as to other possibilities?

 

With the pump maintaining activity when you switch, pretty much rules out relay, switch etc.

Gravity supply to the pump from the RH tank would also be high on my list.

Valves sticking, maybe, give them a whack with a small hammer, be gentle but firm.