Help With Fuel Gauge Reading

[wazcov]

Since I have replaced the fuel pump in my Superspec I have had doubts about the reading to the fuel gauge (which previously has worked perfectly).

My doubts arise because, while I rarely get chance to use the car, I have been on a couple of reasonable length journeys and yet the fuel gauge still reads 3/4 full, which I sincerely doubt.

I have read that some of these pumps give backwards readings, so it could be 1/4 full. 

I am hoping that someone can look at this picture of the inside of my tank and give me a rough estimate if it looks pretty empty or pretty full.

The ideal way to diagnose this would be to fill the car full and see what the gauge says however it's so incredibly difficult to fill up (you have to squeeze so gently else fuel spills out) that it's hard to know when full is. I generally have just put a tenner in every so often and hoped.

 

[alanrichey]

It’s not the pump that causes the gauge reversal, it’s the way the potentiometer on the sender is wired.  Although Ford used the same unit for many of their models, some were wired one way and some the other way.  So when you replaced the unit you had a 50/50 chance.

As far as the slow fill is concerned you need to check the breather pipes on the tank.   You should have a small one in the middle of the tank that releases air as the petrol level drops.   Then you should have a larger one (roughly 1/2” diameter) the allows the air to be released while you are filling up.  The destination of that pipe varies.  In my case it goes up and joins the filler pipe about 1 1/2” from the outside.  That means the nozzle of the filler goes past the join.  As long as I don’t push it in too far so the rubber collar on the filler seals the opening I have plenty of room for the air to escape out of the pipe.   So I can fill at full speed.  I know other Superspec owners where the breather just goes underneath the car.   Maybe yours is blocked ?

As far as the gauge is concerned I would bite the bullet and fill the tank.  That will tell you if it is wired correctly.   After that you don’t actually need to use the gauge, just fill up every 200 miles (you could probably squeeze another 50 miles but I wouldn’t recommend it).

As far as the gauge behaviour is concerned, it will depend on the status of the potentiometer.  After a few months I know my gauge sits on full for quite some time, then fairly quickly drops to 1/2, then stays there for a very long time.  Then it quickly drops to a 1/4 and waits for a bit, then falls of a cliff  So I always fill her when the gauge is at a 1/4.   But watch out, when I first filled the car the gauge stuck at full as the float had jammed.   Hitting a big bump in the road fixed it, but the next time it happened I had to take the pump/sender unit out and release it.

The joys of kit cars

 

 

[AndyW]

When you replaced the fuel pump, was it just the pump, or did you replace the whole sender, float & pump? If it was the whole in-tank unit, then It’s likely your gauge is now reading back to front. As originally supplied the Superspec fuel gauge read in reverse - full when empty and vise versa. You had to swap the wires on the potentiometer to get the gauge to read correctly.

Although there was an electronic Spiyda module you could buy to correct the calibration. Don’t know if the previous owner might have fitted one of those?

It’s a bit difficult to tell from your photo, but the tank looks near empty to me. If it was 3/4 full you’d clearly see the petrol level a couple of inches from the top if you removed the sender unit.

As Al says, you need to check your filler and vent pipes on the fuel tank and make sure there are no kinks or restrictions. I can fill my Superspec at pretty much normal speed, just have to slow down when it’s nearly full. I regularly put in 30-35litres and drive it for 200 plus miles until it gets down to a 1/4 again. The original tank takes 42 litres. 

[wazcov]

Thanks guys. I managed to put some fuel in and the gauge moved the opposite way by the amount I expected so at least I can rely on it, even if it's backwards.

In terms of filling it up, it took more than 10 mins to put £11 worth in... even the slightest tap of the filler had it cut out. I checked and there's two pipes, a small breather at the top and a thicker pipe as you mentioned in the middle of the tank at the back. Both go to the floor and both don't appear blocked (I put some wire quite far up them). Surprised they aren't enough

[alanrichey]

A bit naughty having the thick pipe just going to the outside in these high enviromental standard days.   But if it passes the MOT then tyou are OK.   As you might know, in the original Rover build it had a carbon canister to absorb any fumes and then dumping them into the intake to be burnt up.   RHE decided not to implement that 

As for the slow filling.  I actually replaced my Filler/Tank pipe with a proper fuel pipe.  Previously it was 2 suspension gaiters tied together.   Having fitted it I could see what route it was taking and made sure it first curved to the left and started being horizontal..   That way I can put the nozzle in almost horizontal and rest it on the spare wheel to ease any pain in my back.  And becasue the fuel then has a strainght shot from the nozzle into the tank without aiming at the sides of the pipe I don't have any problem.   Not sure if there is any mileage in you attempting a sumilar exercise.

Also, one of the favourite modifications of all Robin Hood owners was to move the filler cap to the top of the car and just put a blanking plate over the hole that is left.   That way the filler pipe just drops down before going into the tank.

And now your have determinded the position of the gauge with a full Fuel tank you can calibrate it's behaviour as I have said before.   Once it gets low carry a gallon of fuel in the car and run it to dry then you know exactually how it behaves.   I must admit I haven't done that yet, I just fill up when she hits a quarter 

[phaeton]

Or buy one of these https://spiyda.com/fuel-gauge-wizard-mk3.html

[alanrichey]

At £47 each !!!!    I would buy one of these for £0.75, cut the 2 sender wires and use it to swap them over 

[AndyW]

1 hour ago, alanrichey said:

cut the 2 sender wires and use it to swap them over 

That wouldn’t work as there is only one wire from the sender to the gauge.

There are 3 possible fixes mentioned in early forum posts: 1) rejig/bend the float arm so it operates on the other side of the sender, 2) invert the potentiometer on the sender so the float arm swipes the other way, or 3) unsolder the signal wire on the potentiometer and solder it onto the end of the coil.

[alanrichey]

Ansolutely right   I must admit I had always assumed that it worked like every other potentiometer I have come across.   That is earth at one end, 12V at the other end and the gauge operators as a voltmeter sensing the voltage at the wiper.     Turns it actually works as an ammeter, pushing 12V to the wiper and measuring the current flow (and hence the resistance) which will vary depending on how much of the potentiometer coils it has to go through.