Impossible but it happened

Ok so let me explain because I'm stumped.

Today I noticed that my fuel gauge was reading full despite me not having filled up for a week or two. Yesterday the gauge was reading properly so I knew there was only about a 1/4 tank of fuel. I naturally assumed that either the float was stuck or the wiring had shorted giving the full reading or someone had p*ssed in my tank.

Using my handy fuel pump hatch in the rear floor I unplugged the electrical connector to the pump and float and the gauge immediately returned to zero. Replugged and the gauge went immediately to full thus proving the wiring was OK but possibly the float was stuck or the float arm bent (no idea how this could happen) giving a false reading or the sender was shot. Just to confirm the wiring I unplugged and shorted the gauge connectors at the tank and the gauge once again read full. So far so good and all as expected. All the time I was doing this the engine was running.

I pulled the pump and gave the float level tracks a wipe with thinners, cleaned the filter and on reconnecting the pump the fuel gauge now read as normal with full range when the float was manually operated out of the tank. Problem sorted and refitted the pump. Reconnected the electrics and started the engine and all was well except that the fuel light is on all the time even when the gauge reads full.

However I thought back to when the engine was running whilst the pump was unplugged and decided to experiment. I started the engine and disconnected the fuel pump/sender plug. The engine continued to run and I expected it to stop after a short while due to lack of fuel. 30 minutes later the engine was still running at idle so I decided to drive around the block and see how much longer it would continue to run. After about three miles of normal driving the engine spluttered and eventually died. Reconnected the pump and cycled the ignition until the pump stopped. Disconnected it and drove home with no issues. It then ran for a further 20 minutes until it eventually gave up.

HOW CAN THIS BE? The fuel lines dont seem big enough to hold that much fuel and the fuel needs to be pressurised to fill the injectors.

I'm going to try it again tomorrow and see if I can replicate the results. The only explanation can be that fuel is being syphoned from the tank into the fuel lines by the running engine but as there is no other fuel pump on a TD5 I dont see how this can be happening.

Spooky or what? Any ideas?

none what so ever except whilst offroading i ripped fuel lines from filter head and it ran for about 40 secs before dying, it can only have been on what was in the pipes but the engine must of dragged it there itself as pump was spraying it all over the field i was in

My disco td5 ran without my in tank fuel pump when it packed up. it wasnt working at all, i got from york back home to hull. and drove around about 15miles on the next day because it was a sunday and couldnt get a pump until monday. Fired up fine even when stone cold, idled fine. just couldnt get it over 50mph or full throttle otherwise it would misfire because of starvation.

Having left the motor overnight I went out and started it straight up from cold WITHOUT the fuel pump connected! Started as normal. Drove about 5 miles before it started spluttering so reconnected the pump and all is back as normal.

I have no idea how the engine can be getting fuel without the pump running. Its not running on sump oil as my levels are spot on perfect. It must be sucking fuel up the lines as they dont hold enough to keep the motor running for so long and I'm beginning to wonder why there is a need for a fuel pressure regulator as the engine obviously runs without any pressure from the pump?

Weird.

Be glad m8, it means that you have a perfecly sealed fuel delivery system

The theory is: the FPR is there to keep the pressure in the rail and to not let the it grow above 4 bar(it's on the return circuit),... the engine will run untill the pressure drops under 2 bar... when you stop the car the pressure in the system is 4 bar and as yours is perfect it will stay at 4 bar for days unstarted(it's conceived so)...so in the morning even with the pump disconnected the pressure is very close to 4 bar(it will drop a bit as the temperature of the system drops and P/T=constant. As the circuit is sealed and the volume(V) and temperatre (T) is the same along the circuit by universal thermodynamics law (P1xV)/T =P2xV)/T, the pressure in the main component of the circuit(engine) and the secondary(pump) tends to equalise and when the engine is running the whole streamlined circuit acts like a kind or hydrostatic perpetual motion cos there is an input 4 bar in the system which will start the motion but will fade in time(it works with liquids which have pressure see this Robert Boyle' Flask - YouTube ... dig the net about Boyle's self flowing flask if you want to know more.

it's not simple but it's quite hard for me to explain in english... anyway, what happens is pure physics so nothing strange

the fact as i said is that you have a well sealed system and the pressure drops slowly due to this fact... believe me, an engine with worn injector seals wouldnt act like yours cos it would'nt keep the 4 bar till the morning... that's a way to check the seals, to put a gauge instead of the fuel temp sensor and leave it overnigt... if the pressure will drop hard it means the seals are gone

i hope it make sense to you

Sort of but I dont see how it could maintain the 2-4 bar needed without a pump? Especially as I have been working the motor hard so its not a constant level of fuel being demanded. Given the fuel circuit works on a continuous tank return I would have expected the fuel pressure to bleed away almost immediately once the pump stops running.

BTW the guy who posted the video is famous for his hoaxes. Check out some of his other vids - they are quite impressive at first glance but if what he was claiming was possible he would be the richest man in the world.

If what he was showing in that vid was truly possible without a hidden pump (check out the power cord towards the end of the vid) it could be used in a hydro power station to return the water to the reservoir without pumps and then we would all have free or very cheap electricity for ever. Perpetual motion does not exist in any form. Interesting though.

... the video was just for a kind of enlightment... and it's real ... forget it, try to understand the rest of the explanation.... there's a sealed circuit which begins with 4 bar pressure untill it drops... believe it or not...but the fact that your engine is running with the pump disconnected it's enough...isn't it? if not ask the aliens

i know that perpetual motion doesnt exist...believe me

Today there was snow on the motor so I tried one last time before buttoning everything back up.

Disconnected the pump. Started as usual. Idled for 15 mins while I had a brew and let the front screen try and defrost. Still had to scrape the poxy thing as my heater only gets warm after 5 miles or so. Bored now so its all back together. All connected as suppposed to be and even the fuel gauge is behaving itself. Still none the wiser but its been interesting.

Cheers for all the input.

Next time you need to prime the system, turn on ignition and pump throttle 5 times, engine light will flash and self prime takes about 5 mins

your injectors are pulling fuel through the system,it will run like this for a while untill the revs drop low enough. theary that the system is under pressure is correct, it will start normally and then draw fuel mechanically,it wont perform well but will ceep goin.

Mine does this too, as I discovered when fault finding a while ago. It surprised me at first, because of all the trouble people say they have had when their pumps are not delivering at full pressure. Disconnecting and reconnecting changes the engine note, but doesn't stop it. The longest it's run without the pump connected is half an hour. I can also make the engine play tunes by waggling the terminals on the fuel pump relay under the driver's seat.