Fuel starvation
SPEAKING OF Lightweights, I have just sent one back to its owner after a rather bothersome few days. It came to me fresh from having various structural and cosmetic works attended to, smelling strongly of NATO green paint and with a box of bits to be attached and a few mechanical issues to attend to. Among these was the fuel gauge. Lightweights had twin underseat fuel tanks with a changeover tap attached to the seat box behind the driver’s calves, but only a single fuel gauge. Two plunger-type switches were incorporated into the fuel tank to connect the gauge to the sender unit of whichever tank was selected. On this particular vehicle the gauge sat resolutely at zero regardless of which tank was selected.
Both tanks were new and had been fitted with new sender units. I ran a wire direct from the back of the fuel gauge to earth which gave me a full reading as it should, and then traced the wiring with a multimeter through to the two plunger switches. These are of the ‘normally on’ type: depressing the plunger cuts the circuit. Neither switch was conducting current. New switches are available at a price, but I decided to see whether these could be saved. They have a steel body crimped over a plastic inner: I carefully prised the two apart and found I could clean up the corroded contacts and get them working. Success! With a little adjustment both switches now worked perfectly.
Having sorted out a few other electrical issues (mostly caused by ill-advised bodging and complete ignorance of the way the lighting system on a Lightweight is supposed to operate) I ran it down to the MOT station where it soon gathered a small crowd of admirers. There is something about Lightweights which gets them noticed. Needless to say it passed with flying colours. I rang the owner to let him know, and ended up agreeing to run it back to his house about ten miles from the workshop. It was a nice afternoon and I just fancied the drive. I locked up the workshop, walked over to the vehicle, and… what’s that dark spot underneath the right hand fuel tank? Smells like petrol… It was petrol, leaking from the bottom of the brand-new tank.
I thought it might be a loose drain plug, but no such luck: fuel was seeping through a small crack where the threaded boss for the drain plug was brazed to the underside of the tank. I rang the owner again and we agreed I would remove the defective tank and sling it in the back so that he could send it back to the supplier for a warranty claim. I drained the fuel out of the right-hand tank (which was the one the vehicle had been running on while I had it), tipped it into the left-hand tank and moved the changeover tap across. The defective tank came out in about two minutes (no rusted bolts here) and I set off. I had covered about two miles and was bowling along at a noisy, bouncy 50 mph or so when the engine started to falter and rapidly lost power. I checked that I had selected the correct fuel tank (I had) and after a bit of cranking the engine fired up. I suspected either a fuel system blockage or a defective fuel pump, so I headed slowly back to the workshop using as little throttle as possible.
Most readers will know the feeling. When you are trying to nurse an ailing vehicle back to base it is astonishing how the roads suddenly fill with traffic, everyone in a tearing hurry and desperate to get past you and your struggling old vehicle. I managed to get almost all the way back to the workshop but all the time the fuel starvation was getting worse, and a couple of hundred yards from home the Lightweight coughed its last. Luckily my good friend Lee was around to help, and the Lightweight completed its journey on the end of a towrope behind a Discovery Td5.
Bonnet up, and the first thing I noticed was that the fuel pump was of the cheap pressed steel type more commonly found on 200Tdi engines. The original Series pump has a glass sediment bowl and gauze filter to strain out any rubbish floating around in the tank. Although the tank on this vehicle was new, the owner had filled it from an elderly jerrycan. I disconnected the fuel line at the carburettor, waggled the pump priming lever and fuel came out, so I turned my attention to the carburettor. This was the original Zenith 36IV, a delightfully simple device well-suited to being stripped and cleaned out in the middle of the desert with only a Swiss Army knife and a couple of pipe cleaners. I had it to bits in no time, blew out all the jets, reassembled it and the Lightweight fired up and seemed to run fine.
At this stage I reckon I was justified in thinking that dirty fuel had brought the vehicle to a halt. The question was, how much muck was in the tank? I switched off the engine, unscrewed the filler cap and there was a sharp hiss of air being drawn in. A-ha! I had finally, and quite by accident, found the cause of the problem. The filler cap on a Lightweight has a one-way valve built into it, allowing air into the tank as fuel flows out. If the valve is stuck or blocked (as in this case) a vacuum gradually builds up in the tank until fuel can no longer flow. Only the left-hand tank had a defective cap: if the right-hand tank had not started leaking and been removed I would have delivered the vehicle running on that tank and would not have come across the problem.
I fitted a plastic in-line fuel filter between the pump and carburettor as it seemed like a good idea, and on the second attempt the Lightweight completed the journey with no problems at all. The drive took me straight back to my early Land Rover days when I had a Lightweight as student transport: the deafening noise, the jolting ride, that distinctive smell of hot EP90 gear oil overlaid with a faint whiff of petrol from the underseat tanks, and the nagging worry that the thing might breakdown at any minute, just as my first Land Rover used to do. There is a lot of happiness to be found in driving a Lightweight, I think.