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MG MGB Technical - Fuel Tank Sender - any tips?

I think I'm going to have to do one of the jobs I've been dreading!

I'm almost sure that I have about 3/4 tank of fuel, but driving home today I noticed the gauge reading only about 1/4. My first thought was that the tank had sprung a leak - but it all seemed OK.

I reversed onto my ramps, cleaned the connectors at the tank sender and was both pleased and surprised to get a good 3/4 reading again. But, as I came off the ramps the reading fell away to nothing. It's repeatable - raise the rear and it's good, but on the level it isn't.

I think my sender must have failed - does everyone agree?

Now, I don't want to risk picking up sludge by letting the level fall too low before I replace the sender and so what I'm wondering is what's a safe level to have the fuel at when I do the job?

Also, I'm a bit concerned that I might find the seal leaks the first time I fill up afterwards.

So - any words of advice from people who've been there would be welcomed.

Miles Banister

Hey Miles
I did the replacement about seven years ago so I'll try.
I filled the tank and drove until I thought I shoud be low. I topped off again and took note of how much I put in and how many miles I went. Next week I went a little farther and jacked up the right side of the car, blocked it, and make the change. I had no problems with leaks.
Don't forget to check your fuel lines to the carbs just in case you do have a leak over your exhaust manifold. In California we have fuel additives that eat fuel lines so I keep a close eye on mine. Mark
MLS Somers

Mark,

From what you say, and what's in the archive, it seems that I'm a bit over anxious about leaks. I recall changing the sender on a Mini many years ago and realising that it leaked only after filling up! This time I'll be sure to not to put in more than I can syphon out into cans until I know it's a good seal.

Thanks, Miles.
Miles Banister

Hi Miles
A fuel tank may leak if it isn't well vented out through a vented cap or a charcoal canister like my all smoged up MGB. If so on a warm day with a full tank it will escape any where it can.
I didn't think about the type of gasket on my previous post. As I have read in the past, the cork gasket is the way to go.
Again check those hoses!!
HTH. Mark
MLS Somers

Miles

I do not have an engineering answer to the problem, only experience.

The fuel guage on my 1971 B never registers above 3/4, even when the tank is brimming with fuel. 3 years ago the engine died from fuel starvation still reading c 1/4 tank full - twice. So I don't trust the fuel guage! As I get c28 to 30 mpg I reset the odometer ever time I refuel. 28 X 12 gallons (c 53 ltr) should do c 300 miles between refills, leaving a gallons or so for the sludge.


Happy motoring in this lovely weather.

Iain
Iain Hallifax

Miles,

Could be the sender if the wire is broken around the 3/4 mark.

Try filling it up to the brim and see if you get a full reading. If so it points to the sender.

Try earthing the sender wire at the tank and see if you get a full reading, If so it points to the sender.

If not it points to the voltage stabiliser - screwed to the top of the bulkhead above the steering col behind the dash. A voltmeter should show a switching voltage varying between battery and about 8 V - gives an average of 10 V for the fuel guage.

Fuel tank is OK at 1/4 full - jack up on RHS to send fuel the other side from the sender. Use a small screwdriver to lever out the locking ring if it won't turn with gentle tapping. Clean up the tank surface inside the ring - don't worry about ring itself. Use a sealant on the rubber gasket if the tank is rusted -Hylomar is good. Tap new locking ring round gently and smear a little grease on it to help next time!
Chris Betson

Miles, might it be worth 'calibrating' the new tank sensor (i.e. bend the metal rod) before it is sealed in? So many Bs have fuel guages that give innacurate readings, I was thinking it would at least be worth while to see how yours does while you can move the float by hand.

I did this job the other way round, putting an old sensor in a new tank, however I don't think it would be much different unless yours is very rusty. All went quite simply, I used a new gasket and covered it in hermetite, and it's not leaked so far as I've noticed.

Good luck!
--
Olly
Oliver Stephenson

If you take the sender out also check the plastic float hasn't got a leak in it and filled with fuel. I've had two do that on me. In the end I have tried painting one with POR15 to seal it from leaks. I haven't removed it to see if it hasn't leaked but my fuel gauge readings haven't changed since I did so fingers crossed it is fuel free.

Simon
Simon Jansen

Thanks Guys - lots of good advice as usual!

I'm sure now that it is the sender because as the level in the tank has dropped lower I can see that the gauge is completely 'dead' until I go round a left hand bend when the fuel surges towards the sender and kicks it into life for a moment. At that point the needle moves a bit. I suspect that as the level falls lower even that will cease.

It's a pity because my gauge was quite accurate before, but I've ordered the new sender and once I'm sure the fuel level is down to about 1/4 I'll fit it and recalibrate the gauge following the method Paul Hunt describes on his excellent site. I'll also bear in mind Olly's advice and I'll check that the arm has a bend as much like the original's as I can get.

It seems that my worries about leaks round the new sender are groundless - everyone I've heard from or spoken to says they had no problem with that.
Miles Banister

I've had four fail on two cars over the past 13 years and all dropped to below E (same as when the ignition is off) once the tank got down to about 1/3rd full. In all cases the sliding contact had worn through the spiral-wound resistance wire. Once I changed the first one on each car and recalibrated the gauges I found I didn't need to recalibrate again for the subsequent replacement. It can be quite difficult to get the plastic case apart the first time, and springs and things fly in various directions. Make sure only the pip of the slider rubs on the wire and that the flat parts either side of the pip and nearer the pivot don't. I've never had any problems getting the new seal to seal, but remember to fit it between the tank and the sender! You will not pick up sludge or anything else as the level drops - unless it is floating on top of the fuel. Some say there is more turbulence to stir it up when the fuel level is low, which may well be true, but if it were going to pick up sludge it would do so progressively from about 3/4 full, not suddenly start when you get near E.
Paul Hunt

I've now done the job, and it turned out to be quite easy.

First I removed the gauge from the dash and connected the new sender. I intended to adjust the gauge so that extremes of sender arm movement corresponded with Empty and Full on the gauge - but they already did so no adjustment was necessary.

Then I put the back right wheel on a ramp and undid the lockring. No fuss, no rust it just slid open and out popped the old sender. There was a rubber ring around the old float. I didn't have one on the new float so I transferred it. Slipped in the new seal and sender and the lock ring snugged up a treat.

I put back in five litres of fuel and on the level the gauge read bang on Empty. Then I went to the petrol station and put in another 25 litres. The gauge is now reading between half and three quarters (which seems about right), and there's no sign sign of leaking round the sender so I've given it a good coat of Waxoyl.

So, my thanks to those who gave advice and encouragement, and for anyone reading this prior to doing the same job themself then rest assured that's it no big deal.
Miles Banister

This thread was discussed between 11/08/2003 and 16/08/2003

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