hzj79l dual fuel tank problem advice?

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Jan 29, 2015
Hi, I have a hzj79l pick-up with dual fuel tanks.
The problem is that the truck loses power intermittently on the main tank. When I switch over to the sub-tank the truck goes like a train!
Most obvious things have been checked including things that would probably affect both tanks, for example fuel filter checked/changed, screen on injector pump checked/cleaned, fuel filter primed in case of air.
The truck uses 2 solenoid valves on the chassis just to the right of one of the tanks. I've checked the pipes and vent going from the problem tank to the solenoid valves and they appear to be free of blockages. The tank has also been drained twice.
None of this has improved the problem in the slightest and I still am not sure how the damn fuel systems works!!!
Does these tanks have an internal electrical pump or just a level meter (if so could be pump fault)? Or, do the solenoid "valves" also have a small pump integrated? How do I check if the solenoid valves are working correctly (these are actually the only things I haven't had time to take out and clean)?
Is there anything else I am missing?

Cheers, Mike
I had the exact same symptoms about a month ago. The valve / solenoid that allows you to switch from tank to tank was only partially opening and while on the front tank, the engine was starving for fuel. There were no issues when the rear tank was selected. I replaced the solenoid with an aftermarket unit and all is well.
OK, I'll try and clean the solenoid (I'm out in the country and have no hope of getting a solenoid anytime soon). I will clean both, but just out of curiosity which one is the solenoid that switches, the larger one lower on the chassis or the smaller one higher up? From what I've read on is the return solenoid and the other is the supply/switch solenoid, but I could be mistaken...
I'm away from home until Sunday and won't have access to the truck until then. I will gladly forward you that information on Monday. If I recall correctly we used a G.M. valve to replace the bad one.
OK, cheers...
Ok, I've had my coffee and I'm thinking clearly now. In my application, (1985 HJ75 Ute) there is only one switching solenoid which is located inside the frame near the back of the cab. There are 3 fuel lines, one for the rear tank, one for the forward tank and one that of course feeds the mighty 2H. Hope this helps.
There are no pumps in the fuel system except for the feed pump inside the injection pump which is obviously working ok.
Have you checked inside the tank, there is a piece of gauze around the fuel pick up, they can become clogged?

Yes , one solenoid is for supply and the other is for return .
presumably this means dropping tank and taking out fuel pick up from above? Or can it be done from the drain plug?
I hope so, will save a lot of effing about!!! I just pulled the hoses off the supply solenoid and tested it with the switch in either position and it appears unblocked and works in both modes main open-sub shut and vice versa. So the only thing that I haven't checked is the pick up gauze. Will probably do that tomorrow after I've washed the taste of diesel out of my mouth with lots of fresh beer!!! :D
I had the problem in my 80 with the solenoids and just replaced them and all was good
thanks all. I checked the solenoids and all was good. Blew through the pipes and everything unblocked. I tried accessing through the cab, but the inspection hatch only gives you access to the fuel level float.
So... had to rip the tank out, took out the pick-up pipe and the gauze covering it was absolutely filthy (see pic). Gave it a good clean and engine purring nicely now. Thanks everyone for their help!!!

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