Dodgy Fuel Gauge?

[ChrisIS200SportTTE]

Hi guys. Apologies for the long post but I have a very strange one for you. Now I've read a lot about faulty fuel gauges and sticking fuel gauges on the forums but nobody seems to have had this problem that I can find anyway.

First of all you should know that I'm one of those people who always knows exactly what the fuel gauge is reading at all times and I always re set the trip when filling up so to monitor MPG.

When I arrived at work this morning the fuel gauge was on or just below the 3rd bar from the bottom and the trip was showing 312 miles since a full tank so exactly where it should be just before the light comes on (1 bar from the bottom)

I had to park on a very steep hill today. When I got in the car tonight and started it, the fuel gauge shot up to between a quarter and a half. I didn't think anything of this as in my Honda if you parked on a gradient, then the fuel gauge would move up or down but would go back to normal once on the flat. So I assumed the IS would do the same.

Well no. I drove the 28 miles home and the fuel gauge did go down but only what I would expect it to after doing a 28 mile trip through towns and villages. I've been home for a few hours now, car parked on my flat drive. I've checked it 3 times and the fuel gauge still reads a lot more than it should!

So has anybody else experienced this or similar? Its like the petrol faires came and put some free petrol in! :)

First thing I'll do tomorrow is put some fuel in to see if the gauge moves up correctly. I'm just unsure now of how much fuel is actually in the tank! I'm worried that it could be reading over a quarter but this morning I was waiting for the light to come to go and fill up so it could be empty!

[BenIS]

Id say itll probably just go back to normal once you fill up again. Let us know what happens.

[krisby1]

I would say it will probably settle down after you next drive it. I wouldn't worry too much, just wait until the light comes on, until then enjoy the free fuel. :P

My 528 did a similar thing this week, even though I parked on a level surface, it doesn't seem to have gone down much, or what I would expect, in the last 52 miles, and I am sure what I saw when I got to work yesterday and when I left last night, it was slightly higher, but then I know it will catch up between now and empty, as it normally does. OBC saying 150 miles to empty still, which is unbelievable after 78miles and only £50 of super, unless the cooler weather is giving me a significantly better mpg.

[noby76]

Let me guess Chris!! Are you using Shell or any form of Top Tier fuel in your IS?? Although I haven’t experienced this spooky behaviour in my IS yet I did experience it in my previous Ford Cougar V6 when I filled up with Shell normal unleaded and the Cougar does have OBC fuel readout so filled up with if I remember £30 quids worth which normally sits at half tank on the fuel gauge when fuel is filled from close to E. after filling I switched to fuel readout on my screen and it showed 120miles. That is 120 miles left before another fill up is needed. drove daily to and from work and on the the third day I noticed although my computer fuel readout was counting down, the fuel gauge was moving but not a lot. I thought this cant be right but I ignored it. so come Friday my fuel readout showed 40 miles left to refill but my gauge had only moved a gauge was still getting baffled by this.. Finished work got in my car turned on my ignition to start and noticed my computer fuel readout has increased from 33miles when I got to work that Friday to 75miles . Was so surprised by this I thought someone or maybe my guardian Angel must have filled in petrol into my car. Normally I tend to fill up on Saturdays for the coming week but the fuel lasted me till Monday which means I got more miles from the same litres of fuel. This has happened twice the second allowed me to drive 15 miles when the fuel read out was showing 32miles after my 15 mile drive it had only gone down to 30miles but this was during heavy snow last year so was only limited to like 20mph maybe it was the weather or the fuel .. but havent experieced it in my IS tho :(

[ChrisIS200SportTTE]

Well actually Noby yeah. I normally use total excellium 97 RON but filled up last time on 99 RON. Do you think the difference in fuel is making the ECU confused or something making it give the wrong reading on the gauge? I put £15 in this morning (99 RON) and the gauge has moved up the correct amount. It's also moved down the correct amount since driving the 28 miles to work. So everything seems to be normal again, I just gained nearly a quarter of a tank from somewhere! :)

[noby76]

Well actually Noby yeah. I normally use total excellium 97 RON but filled up last time on 99 RON. Do you think the difference in fuel is making the ECU confused or something making it give the wrong reading on the gauge? I put £15 in this morning (99 RON) and the gauge has moved up the correct amount. It's also moved down the correct amount since driving the 28 miles to work. So everything seems to be normal again, I just gained nearly a quarter of a tank from somewhere! :)

I couldnt explain it myself when i experienced it in my Cougar and in my case it was normal Shell unleaed i used not V power 99RON during that time i was driving 'normal' i.e without putting my foot down unnecesarily or braking hard...I must admint i havent managed to replicate it again either it might be the fuel was really fresh at the fuel court when we filled up or there is a way of driving i.e the right speed, the right throttle pedal angle etc which causes this to happen!! but it sounds like one could extract excees fuel(mpg) from the same litres of fuel but dont know how it came about!!

[ChrisIS200SportTTE]

I read this on another forum but have no idea how true it is. Interesting if this is correct, any experts let us know :)

The float in the tank has a resistor in it. It sends readings to the ECU of how far away it is from the top of the tank. Something like 7 a minute. The ECU then calculates how much fuel is in the tank by working out an average of these readings. It does this because if your on a steep hill or going around a corner then the petrol obviously sloshes about in the tank giving inaccurate readings. Working out an average gives a more accurate reading. The ECU then sets the fuel gauge on the dash. This avoids your fuel gauge going up or down when on a steep hill or going around a corner.

[Stuart Aspey]

Sorry to reply to an old thread, but the ecu thing is an interesting idea. I've been on snow mode since coming back from Nottingham and I wonder whether the ECU has simply reset its averages and believes that I'm able to get further out of my tank than I had been able to?

Filled my tank to the brim on Friday and had driven about 110 miles. On sunday the gauge read just above 3/4s of a tank. Yesterday, I got in and drove to the supermarket and realised, half the way round, that the needle had gone back up to full. I thought it had stuck, but I've driven a further 34 miles now and the needle is moving, but the reading is about 60 miles out, from what it should be. Unless the needle resets and catches itself up, the car seems to think it has 60 miles worth of fuel.

I'll keep an eye out, of course, but it is an odd occurrence.

[Raymond.]

Fuel gauge over reading is never a bad thing in an IS Chris.....