Advice sought.
1984 XJ6 Vanden Plas.
Two days ago, after workign fine for 2 years, the left fuel (petrol) gauge started bouncing around wildly. Still , in motion, didn't matter. It did seem to bounce more around actual fuel level when stopping and bounce down to empty-ish when slowing down, but this wasn't 100%

I have made no modifications. I did nothing to the fuel system. It just started doing this.

So thinking to always look at the ground first, I went to G10 in the trunk and the grounding spot had some rust. I cleaned all that up and liberally smeared Di-Electic grease. Now it still bounces, but much less and it mostly pegs at empty ( I know there is about 5/8 tank in there). It when driving it stays pretty much like well below empty. When I brake from motion, the gauge swings up to near where it should be momentarily before sinking back to empty. All I did was clean the ground and also the two contacts on the sender.

Any ideas on what this might be. I thinking the sender is stuck or the float is holed. Any suggestions for order of troubleshooting. And also, how do you check fuel level without a gauge?

 

You have two tanks. Select the good (non-bouncing) tank. If the care runs, switch back to the faulty one and drive until it stumbles. Switch to the good tank to get home. Now you have an empty - or almost empty tank. Or, shove a plastic hose in there and siphon it off.

 

To confirm, the fuel gauge reads normally when the right side fuel tank is selected? This would suggest the the gauge itself is OK, and the wiring from the gauge to the switch is OK.

Since you've already cleaned the ground, this leaves us with the sending unit, switch, and wiring to the switch.

The fact that braking changes the reading virtually confirms that you have a sending unit problem. Time has come to remove it for inspection, which means draining the tank or driving until you're 100% sure is is well below half-full

Not hard to replace, replacements are $25-$50.

If the problem is only the float you can replace it separately with a generic. Someone will come along with some good info on that, I'm sure.

Cheers
DD

 

it sounds like the float is absorbing fuel and is being weighed down and bouncing up and down by the weight of fuel.

must remove the entire sender with float to inspect. Sender is at Rear of tank, first thing is to disconnect Battery and open the fuel filler cap.
then remove tail lights lens,
then tail lights housing,
then disconnect plugs off tail lights housing.

you will see two wires connected to sender.

mark where black wire goes with magic marker, then disconnect wires.

Use a brass or wood punch or tool only, no steel tools, to back off the sender slowly and counterclockwise to remove assembly.

NOTE: Fuel vapors are explosive, if you cause a spark it could cause an explosion.

 

Thanks for terrifying me Jose.

Of course, you are right. Thanks for those tips.

 

Muttony I don't like messing with gasoline. burned myself once. Any spark turns it into a ball of fire. Think matches. You are welcome.

 

Sometimes being terrified is what keeps us safe

I had a car catch on fire (gasoline) once while I was working on it. Not fun. Almost got out of control. Could've been much worse.

Some things will always be a bit scary. I've removed umpteen suspension springs, for example, and I still get a little edgy doing that job!

Cheers
DD

 

LOL. I thought I was going to have a coronary while I was doing the springs my heart was beating so hard.

 

I removed one of those tank senders one time, VERY CAREFULLY, it almost looks like the old toilet tank valves with a plastic float? That is what you will be removing, a toilet valve.

fortunately J.D. of J.D.'s British Cars taught me how before I screwed up.

There was a parade of hot rods today. maybe 100 of them. Something like $2 billion dollars worth of chromed parts. All candy colors imaginable, those guys are something else. The noise was incredible. Opened headers? Girls in bikinis out the sunroofs smiling at police escorting them. No masks. What a sight. No idea where they were going, maybe to Daytona Beach. And behind them the Harleys. Talk about a roar. Same low frequency vibration as when a helicopter passes over.