Sudden improvement in instrument readings

Greetings all,

I have a mystery i cant quite explain.

My fuel gauge has read lower than expected for some years. I would fill the tank…and the fuel gauge never read more than 3/4 of a tank. I just dealt with it…not a super big deal for me.

However, i just finished servicing the distributor and TPS as well as replacing the ignition module to distributor shielded wiring harness.

The car performs SOoo much better…

I took it for a run yesterday including topping off the gas tank with fresh fuel.

To my surprise…the fuel gauge is now reading full…a condition it has never reached in 10 years.

The battery volt meter also seems to be reading a bit higher as well.

Now, i never touched anything but the distributor etc…so how could this have changed???

Was maybe the ignition harness shorting to the shielding ground causing a low voltage condition?

Im just mistified…while im excited with this improvement…im quietly fearing that its a sign the car will burn down next week :grimacing:


I’m assuming you have replaced your fuel sender previously without any change to the fuel gauge.

I’m going to assume that your tank had collapsed a little bit, most likely following a failure at the Rochester valve. The tank may have simply, finally, popped back into place? Temperature change maybe caused it?

The likely cause was a blocked fuel vent, your car was sucking in fuel and air could not replace that fuel fast enough, so the tank collapsed, or cavitated. It doesn’t take a lot of air pressure to force it back into place, but be careful because it only took a little more to pop a leak! That is a tank out problem you do not want to deal with!


Years ago i replaced the fuel sending unit and float…the fuel gauge had read fine until it didnt…the new sender never read full straight out of the box.

I just figured the resistance was off a bit…i was disappointed…but reading a bit low when full never seemed a problem.

It was just surprising that all of a sudden…it now read full. But i must admit one additional item, this was the first time I fueled up with 93 octane gas instead of the usual 87.



That is what bad connections add: resistance.

Moving (bumping) wires around can change ‘how bad’ a bad connection is. Maybe you moved some other wires around doing the work you did.

Some causes of ‘bad’ connections could be: corrosion; damage wires (strands breaking and other issues); push on/in connectors losing tension; and other causes.

A collapsed tank doesn’t make sense. If let’s say it now holds 23 gallons instead of 24, when filled up the float would still be at the top and register full.
Only if the collapse was exactly at the upper travel of the float, although I’m not sure where that is.

I’ve replaced my sender. I have to put in an extra half gallon after gas pump clicks to get it to read Full. I also notice it goes down too fast from full to 3/4, but then goes slow around 1/2 and eventually reads. correct. The needle moves a lot too while driving. Car needs to sit a bit to get a steady read. So the gas gauge on these cars seems a bit flakey by design.

The sender is sending a 12v signal. When it is empty, it is grounded. I replaced mine with an aftermarket, no change. Then I replaced it with an OEM, no change. I did everything you could do at the cluster, no change.

When the tank collapses the top contracts down. That now limits how high the sender arm can go, hence the lower maximum reading.

If you want to cross off the resistance question, take a reading at the sender, then pull the cluster and see if you’re seeing the same voltage at the gauge. If it’s the same, you can cross that off?

Although since the problem has been resolved, there’s nothing to test.

I think i understand the instrument issues with the instruments…now trying to find the cause.

During todays drive…i began to notice an ever so slight irregularity or what felt like a sure while cruising along a a steady pace. At slower speeds…i began to notice a blip in the tachometer…

Ive also had an issue with the stereo doing some strange things…like changing into demo mode or color change of the ilumination. Usually a touch of the mode button…brings it back to normal.

But today i noticed the stereo problem seems to coincide with the tach blip and engine hiccup.

Im suspecting i have a poor power connection or ground in the engine compartment.

Over the next couple days…i think i will start with the firewall terminals and give them a good clean up.

Any other locations to check???



Based on other than Jag autos I would be looking at the ignition switch. No reason the Jag would be any different.