Rover 75 Classic SE 1.8 - Misleading petrol gauge problem

[Gazinho7]

For a number of months now, I have had a problem with my petrol gauge, in so far as I've found it difficult to believe, like after a journey of some 25 miles or so, finding I have more petrol than I had when I started. I've always regarded this as an irritation, and one of thse jobs I'd eventually get round to. However, last weekend, with about a quarter of a tank indicated, I filled the tank up, and to my surprise, the petrol gauge didn't budge. I've become used to this and after a hard drive on the motorway, this usually rights itself and indicates a level I'm prepared to believe. This time, it was different. The indicated level dropped as I used up the quarter tank I'd started with.
I was going to just continue until it decided to work again, until I suddenly remembered something an AA man said to me. Before I go on, I'm not that technical, so please forgive my ignorance. If I'm remembering it right, he told me that on the 75, the petrol tank is split into two, and the pump in one half feeds petrol into the other, which eventually heads off to the engine. The second half of the tank has a filter and a sender. Would I be right in thinking that this sender is the one connected to the fuel gauge, and that the reason I'm not seeing any fuel indicated on the gauge is that this half of the tank is empty, probably because the pump in the other side has been intermittent for ages and has now stopped?
Any ideas gratefully received.

[T-Cut]

The system you describe is correct in principle, but it doesn't work like that. The tank is divided by a central ridge and fuel is drawn from the drivers side by the pump module. The high pressure fuel flow is divided and operates two venturi (static) pumps within the tank, which distribute the contents to ensure the supply to the pump inlet is maintained at all times. There is a float level sender in both sides of the tank - one on the pump module and one on the filter unit. The gauge integrates their signls to display the net contents. Erratic gauge output may be due to erratic floats. If one or both has been disturbed when the pump and/or filter module has been serviced, it is difficult to get the floats repositioned right. This may cause binding in certain positions. The floats operate a rheostat with a linear resistance value against float level. You can check the float resistances using an ohm meter. There's a connector socket under the rear seat, driver's side. This diagram may help.



TC

[berkshirelad]

You can also use the OBD to tell how much is being measured in each side of the tank.

[Gazinho7]

Originally Posted by berkshirelad

You can also use the OBD to tell how much is being measured in each side of the tank.

Thanks, berkshirelad, now what's the OBD and how does a numpty like me use it? By the way, base on what I've said (the gauge reads zero and the nagging light is on, but I know I've got loads of pertol in there), what's the chance of me technically running out of petrol if I drive it?

[Gazinho7]

Originally Posted by T-Cut

The system you describe is correct in principle, but it doesn't work like that. The tank is divided by a central ridge and fuel is drawn from the drivers side by the pump module. The high pressure fuel flow is divided and operates two venturi (static) pumps within the tank, which distribute the contents to ensure the supply to the pump inlet is maintained at all times. There is a float level sender in both sides of the tank - one on the pump module and one on the filter unit. The gauge integrates their signls to display the net contents. Erratic gauge output may be due to erratic floats. If one or both has been disturbed when the pump and/or filter module has been serviced, it is difficult to get the floats repositioned right. This may cause binding in certain positions. The floats operate a rheostat with a linear resistance value against float level. You can check the float resistances using an ohm meter. There's a connector socket under the rear seat, driver's side. This diagram may help.



TC

Thanks T-Cut, I'll get a multimeter on it. Should I expect a high resistance on an empty or full tank? By the way, base on what I've said (the gauge reads zero and the nagging light is on, but I know I've got loads of pertol in there), what's the chance of me technically running out of petrol if I drive it?

[T-Cut]

Originally Posted by berkshirelad

You can also use the OBD to tell how much is being measured in each side of the tank.

You can't, it doesn't work. The original diagnostic notes have been updated to that effect. However, if someone does fathom out how to do this correctly let me know and I'll add it to the routines.

TC

[Gazinho7]

Hi T Cut,

Thanks for your invaluable help. I thought I'd sussed this one out not long after it had happened. I found the multi plug, disconnected it, gave it a quick clean and reconnected it. Suddenly the gauge went from 0 to nearly full instantaneously.

Unfortunately it has reoccurred. As in the first instance, this occurred right after filling up (I rarely fill right up). This time, the needle parked itself at 3/4 full. Never mind, I thought. Yesterday, as we're planning a short trip, I topped it up (from slightly above half a tank), but the petrol pump stopped short of 14 vquid's worth. Strange, but again, never mind. However, this time the needle didn't budge, and the only direction it seems to be going in is downwards. So this afternoon, I repeated the trick with the plug - but this time with no success. The diagnostic routine indicated a problem with the filter sender, and the multimeter suggests it's open circuit.

So,okay, this'll have to be replaced as soon as is practicable, but my real worry is simply this:- Providing I keep putting petrol in, will I be okay? I mean I wouldn't want to run out of petrol with a full tank.

Any ideas?

Thanks,

G7

[david75]

My car exhibits the same problem as you have described, intermittently.
Being a non-mechanic I asked my garage for a quote to fix it and they gave the same diagnosis as indicated in this thread and came up with something like £300, about half of which was for the parts!
Needless to say I decided to leave it alone, and I just reset the trip meter each time I fill up, and then fill up again after 350 miles or so. When I establish an accurrate mpg for the car (I've only had it a couple of months, but appears to do 35-36 mpg) I may feel brave enough to go to 400+miles

[eurorover]

it could be a ground problem try running another wire to ground