Fuel Gauge Drops from 1/2 to Empty

[irishjag]

Since getting the car the fuel gauge has been misbehaving but with a consistent pattern

Fill it up it gauge shows full.
Drops very slowly until just over half full, then gauge drops to zero, light comes on and get low fuel warning on the touchscreen
Fill it up immediately and it takes about 50 litres which means there's about 30 litres in it when it drops to zero.
Have read codes and I get 'B2879 Fuel tank jet pump fault' coming up on the instrument cluster module, no codes relating to the fuel pump senders.
I understand from reading up that the fuel tank is a saddle arrangement, with a combined fuel tank sender/pump unit on both sides of the tank. The drivers side pump delivers the fuel to the engine compartment while the pump on the passenger side, (referred to as both the 'Jet pump' and the 'transfer pump') moves fuel from the passenger side of the saddle to the drivers side.

So any ideas welcome on what the next step is with solving this or what exactly is going on.

[J44EAG]

We`ve not had this fault here before, Chris, as far as I know. Info on the net is few and far between.

Removal of the back seat base gives access to the tank top panel(s). Obviously the fault code points towards an issue with the jet pump but not necessarily the pump itself. I`d suggest the first things to check are the condition of any grounding connections...that has to be the case with so many of the faults we experience.

The frustrating thing about diagnostics on modern cars is the complexity of circuitry and how one component interfaces with so many others. For the likes of so many of us not working professionally on these cars or having attended manufacturers training courses, we often find ourselves scratching. When that happens we have to resort to local assistance or our Club gurus. You might now drop an email to Matt Quail our new technical advisor asking for his advice. Details on the 350 pages in the magazine.

Sorry I`ve not been more helpful.

Mike

[irishjag]

Mike

Thanks for that
I can only see contact details for Gavin Jones/Ian Kelsall on the page in the Mag?
Is it they or Matt Quail I should contact?

[J44EAG]

Definitely, Matt. See one of the latest mags for details. Gavin and Ian no longer operate for us which is a shame.

Mike

[irishjag]

Mike

Ok yes, was looking at an older copy, got that now

[irishjag]

So a bit more info on the fuel gauge thing.

As I said in an earlier post and probably as many of you know the X350 xjr has two fuel pumps and two fuel senders one each side of the saddle type tank which sits across the axle. In the XJR both pumps deliver fuel to the engine and transfer fuel to the other side of the saddle tank. The pumps/senders are accessed through two access bungs which you can get to by removing the rear seat.

I've also tracked down a TSB dealing with erratic fuel gauge readings on the x350 which gives a procedure for putting the instrument cluster into 'test mode': Open the drivers door, press the trip computer button and turn ignition on. Release the trip computer button after between 5 and 8 secs after the ignition was turned on.

So now if you keep pressing the trip button the message display scrolls through various tests on the IC lamps, gauges and displays lots of other stuff which I really don't understand.
However, eventually it gives you the resistance reading/signal from each of the fuel senders either side of the tank which should each be between 27 and 240.
My gauge shows its 3/4 full (but its not, from the amount of fuel I've used since the last fill should show nearer half full)
The readings I got were 160 and 130 for the two senders, so not the same for each side of the tank.

As recommended by the TSB I rocked the car while looking at each reading and each one did go up and down a bit, indicating that the gauge sender arm is sweeping across the tracks as the level in the tank is affected by rocking the car.

So it seems that the gauge senders are working. The levels are not the same so it looks like the fuel is not being distributed between the two sides of the tank equally or one of the pumps is not functioning.
This points towards one of the two pumps being duff as one job they do is transfer the fuel across the axle to the other side. The code I've got is a 'jet pump failure'. Or maybe the pipes between the two sides of the saddle are blocked.

So next step is drive it till its down to half and as soon as its does its drop to zero thing, I'll put the IC into test mode and read the levels on each side again.

Regards

Chris

[J44EAG]

Really sorry you have that issue, Chris, but many thanks for highlighting the test procedure. I`m sure all here with a 350 will be most interested in learning how you achieve the fix.

Have you tried dumping some Liqui Moly Injector Cleaner in the tank? This is a good strong cleaning brew and my car has improved since using it. If it can clean injectors, it might also help remove any blockages in tank components. Worth a try. Loads on eBay.

Mike

[irishjag]

The latest on my fuel gauge thing

Used the test function on the IC to read the level signals again after a bit of driving and
130 has reduced to 90 (this is the right hand side of the tank)
160 is still at 160 ( this is the left hand side of the tank)

A schematic of the arrangement in the XJR tank is below showing the dual fuel pumps which also transfer between the tanks via the cross over pipes.

So either my left hand fuel sender float is stuck, or the left hand pump is knackered?
Anybody see any other possibilities.
Question is if the left hand pump was knackered would the engine still run? The two pumps are there to provide the increased flowrate required for the SC engine according to the blurb in the technical guide. When I floor the pedal there's no hesitation, its getting enough fuel to move pretty quickly!!

So I'am going to drive it till I get the drop to zero situation, read the sender values again, fill it up to the top, read the values again and see if any insight can be gained by that.

The fuel pump modules (which includes housing, gauge sender, non return valve, pipe connections and fuel pump) are 780 sterling EACH on the SNG Barratt website. The actual pump is not listed as a seperate and replaceable part. There is however info on other forums about just replacing the pump unit which appears to be one shared by the BMW E39 for less than 100 Euro, although this is a DIY job dismantling the fuel pump module

[J44EAG]

Gulp! as regards prices, Chris.

I wonder if it is worth buying a complete used tank and parts from Eurojag. Worth asking what they would require for a known working and warranted unit. Carriage might be an issue in view of the somewhat explosive nature of petrol. Perhaps a compromise might be to get them to send you parts alone in a sealed bag. Eurojag are great guys to do business with. Suggest you give them a call and discuss possibilities. Their prices are always fair and delivery very rapid...next day usually in the UK.

Mike