V35 Fuel system

[KWESHAZ]

Hallo guys. I have a Nissan Skyline V35 250GT. I hav seen the car has a fuel tank split into two, possibly the main tank and the reserve tank. In the main tank is the pump. My problem is fuel is getting finished in the main tank whilst there will be fuel in the reserve tank.Are they not supposed to decrease in level at the same time? How is fuel added to the main tank from the reserve? I see there is a fuel pipe from the bottom of the reserve tank to the pump filter.How does it work and what could be the problem.

 

 

[V35_Paul]

So what's your problem?

Are you running out of fuel but there is fuel still in the tank?

 

 

[sonicii]

it is one big tank, not a main and reserve, but due to the shape of the vehicle it is high in the centre, there is what is called a 'jet pump', which is responsible for moving fuel from the side without the pump to the side with the pump.

see this page for a description.

https://www.sard.co.jp/after_eng/products/engine/jp_killer/jp_killer.html

This isn't quite the same as the V35, I believe the 250GT is the same as the 350GT and has the pressure regulator built into the fuel pump assy, not external like the image in the link.

[GTSBoy]

Yuh, they are referred to as a "saddle tank" because they go up and over the horse in the middle. As @sonicii above says, there is usually a venturi type pump that runs from the return flow or similar to draw fuel from the dead side to the live side (live side being where the main pump draws from).

[KWESHAZ]

Sorry for the late response guys but would like to thank you for your responses.V35_Paul --- Yes I am running out of fuel yet there will be fuel on the other side of the tank. The 250GT has a single line from the pump to the engine and another line from the other side tank (dead side)onto the pressure regulator as shown in pics below. Any idea as to why we have 2 level sensors one on the dead side and the other on the live side attached to the pump.

  

[KWESHAZ]

So guys what could be the problem with my fuel pump system or what sort of challenges that are usually encountered on these type of fuel systems

[sonicii]

That orange component you can see in your pictures is the jet pump, it is responsible for transferring fuel from the 'dead' side of the tank if the fuel level in that side is higher.  If this isn't happening then I would be looking for issues with the hose or the jet pump.

Keep in mind the 'jet pump' isn't an active pump, it is venturi device that depends on the fuel leaving the pressure regulator and returning to the tank to draw fuel from the other side, so there could possibly be issues with the pressure regulator or the venturi device.

[KWESHAZ]

Thanks @ sonicii for your clear explanation. Atleast I now have an idea of how the system operates. I will do my checks and give feedback. Thanks a lot.

 

[V35_Paul]

Also just make sure the Fuel sensor is clean.

VERY common that they give a false reading to your fuel gage. E.g if you Fill "to the top" car up and it shows just under the "Full" line then its wrong.

Just use a cotton tip to rub the metal contact points on the sensor attached to the pump

[sonicii]

^^ yep, that is true too.  Except mine of course, it likes to randomly read high, cleaned sensors and connectors, checked for damage/debris on the level sensor, still happens through.  Almost as if one sensor occasionally decides to read 'full' when it isn't.

[Blakeo]

I had the same issue with mine. I found the line was blocked somehow on mine. I used compressed air and blew out the sludge. 

[KWESHAZ]

Guys does anyone have any idea as to why the Nissan skyline 250 GT has 2 fuel sender units (fuel sensors).Whats  the purpose of the fuel sensor in dead side of the saddle tank.What's the part played by this sensor since there is another sensor in the live side of the tank where the pump is situated.

[KWESHAZ]

Guys let me share with you my observations on the operation of the 2 fuel sender units on the Nissan skyline 250GT.I pulled out the 2 sender units out of the tank and switched the ignition key to ON position.I was moving the floats on both sensors simulating different tank levels while observing the behaviour of the fuel gauge.For my experiment the dead side tank was 2 and the live tank was 1.For the sensor positions Low meant empty, Mid was half tank and High was full tank.

I made the following observations

1. If both tanks 1 & 2 sender units are set on Low, the fuel gauge will indicate empty
2. If sensor in tank 2 is set to Low and  as you move your sensor in tank 1 from Low to High the fuel gauge moves from empty to maximum half tank.
3. If sensor in tank 2 is in Mid position to High and as you move your sensor in tank 1 from Low to High the fuel gauge moves from empty to full tank.

So in conclusion there is some form of synchronization between the 2 sensors.The fuel gauge reading is a result of the synchronized positions of the dead side and live side fuel sender units.

[sonicii]

the sender are in series, there is only 1 fuel gauge circuit that passes through both senders, for the gauge to read full both senders need to have minimal resistance, which is what they have at/near the full position.

If you unplug either sender, you break the circuit and the gauge will read empty no matter how full the tank is.

[Lizxo]

Does anyone know why my fuel gauge reads full but it's only about half a tank. Have cleaned the sensor behind drivers side. What is the sensor in the other tank for. Could that be causing the incorrect fuel reading?

[sonicii]

Possibly, there are 2 level senders as there is 2 halves to the tank (saddle tank), even though they are connected internally, one side can be higher than the other, so to give a genuine reading, there is a sender on both sides.

As for why it is reading high, I wish I could work out that on mine (or my son's car now), it intermittently reads high, I have removed, cleaned and tested both level senders and they seem to be fine, checked the wiring as much as I can, but still can't work out why it sometimes reads high.

Usually when you have a dirty sender it reads low.

[V35_Paul]

Just a guess, maybe the lever that is connected to the float was bent when cleaning. So now it lifts too high??