Fuel gauge. Why?

Submitted: Monday, Jan 30, 2006 at 16:57
ThreadID: 30252 Views:1969 Replies:7 FollowUps:8
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I have a theory on this but want to know what you all think.

While driving home last night I noticed an anomoly with my fuel gauge. I was watching it pretty closely because it was lower than I would have liked and there was nowhere to fuel up.

While travelling on the dirt, Hillston to Mossgiel the gauge dropped quickly and then seemed to stabalise. When I got onto the bitumen for the run to Ivanhoe it actually came back up. Yes it came up! I watched the gauge come back over the half full mark, I must say it seemed odd. I turned onto the dirt heading for Menindee and once again it dropped quickly before stabalising once again. When I got to Menindee I dropped the sub-tank so did not notice if it climbed again.

I am driving a GU 4.2 Turbo Diesel.

As I say I have a theory, but what's yours?

Had a grreat holiday, will try to post some pics soon. Look forward to catching up on all the EO gossip over the next few days.

Duncs
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Reply By: Notso - Monday, Jan 30, 2006 at 17:12

Monday, Jan 30, 2006 at 17:12
My GU Goes up and down depending on whether the Driving lights are off or on. They are wired active and earth both back to battery so it can't be the dreaded "Nissan Poor Earthing" problem??

Lord knows what you problem is but you weren't hunting roos by any chance with a night sun attached to the roof rack

AnswerID: 151843

Follow Up By: Member - Duncs - Tuesday, Jan 31, 2006 at 09:25

Tuesday, Jan 31, 2006 at 09:25
Notso,

My driving lights were on the whole time. I know it's naughty but I did not even dip them going through Ivanhoe.

The only roo injured was the one that attacked the side of the trailer, so I don't think it was the driving lights.

Duncs
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Reply By: robak (QLD) - Monday, Jan 30, 2006 at 17:36

Monday, Jan 30, 2006 at 17:36
On the dirt the fuel in your tank was being sloshed around while on the bitumen it was level so it showed a higher volume?
AnswerID: 151849

Follow Up By: Member - Duncs - Tuesday, Jan 31, 2006 at 09:27

Tuesday, Jan 31, 2006 at 09:27
Yeah robak, I think you are thinking along the same lines as me. Not exactly the same but in that area.

Duncs
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Follow Up By: Member - Oldplodder (QLD) - Tuesday, Jan 31, 2006 at 18:46

Tuesday, Jan 31, 2006 at 18:46
I agree with the sloshing, right idea.
Up and down hills makes a difference too.

Is the tank a standard tank or aftermarket tank with baffles?
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Reply By: Mike DiD - Monday, Jan 30, 2006 at 17:39

Monday, Jan 30, 2006 at 17:39
Half fill a soft drink bottle with and put it horizontally on the dashboard and go for a drive.

This will give you some idea of the challenge in measuring fuel level in a vehicle - especially an offroad vehicle.

The float will be bouncing up and down wildly, yet your fuel gauge is steady - only because there is an averaging effect built in to the circuit.

If the averaging circuit has a tendency to detect the highest (or lowest) extremes of travel, then as there is more up-and-down movement (with the same amount of fuel) the display will change.

People tend to believe that the needle gives an accurate indication of what's in the tank but that's rarely the case. In Pajero NM/NPs, the needle shows F after driving for 100km.

A relative of mine interpreted it this way in his Camira "what great fuel economy - I can drive 100km on just the fuel that was in the Filler Pipe" :-)

Mike
AnswerID: 151852

Follow Up By: Bonz (Vic) - Monday, Jan 30, 2006 at 19:06

Monday, Jan 30, 2006 at 19:06
Yes what Mike said, Except for the Camira bit
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Follow Up By: Ron173 - Monday, Jan 30, 2006 at 20:47

Monday, Jan 30, 2006 at 20:47
What Mike says is correct.

The fuel gauge consists of a sender unit at the tank, and the gauge on dash, which is actually a volt meter.

The sender unit has a float mechanism which will bounce up and down with bumps etc., the dash doesnt register this as it is heavily damped electrically/electronically to prevent this, and indeed as Mike says basically averages which is what you see.

If it wasnt damped it would be bouncing up and down fast.

Take a look at a portable boat fuel tank, if poss, check gauge n shake it, thats what your fuel gauge would be like without the damping.

Best way to get (as accurate as poss) from gauge is vehicle stationary on level ground.

Rgds

Ron
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Follow Up By: Member - Duncs - Tuesday, Jan 31, 2006 at 09:29

Tuesday, Jan 31, 2006 at 09:29
What Mike says is right. But, I wonder if the variation is as great in a petrol vehicle as it is in a diesel?

Duncs
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Reply By: Lone Wolf - Monday, Jan 30, 2006 at 19:31

Monday, Jan 30, 2006 at 19:31
"While travelling on the dirt, Hillston to Mossgiel the gauge dropped quickly and then seemed to stabalise. When I got onto the bitumen for the run to Ivanhoe it actually came back up. Yes it came up! I watched the gauge come back over the half full mark, I must say it seemed odd. I turned onto the dirt heading for Menindee and once again it dropped quickly before stabalising once again. When I got to Menindee I dropped the sub-tank so did not notice if it climbed again. "

You, my friend, got off light. There were two families abducted by aliens last night, in the same area, must have been the magnetic flux....

Wolfie
AnswerID: 151888

Follow Up By: Member - Duncs - Tuesday, Jan 31, 2006 at 09:32

Tuesday, Jan 31, 2006 at 09:32
Wolfie,

You are soooo in tuch with other worlds.

I did wonder what happened to the dual cab Triton I was following. I assumed he had turned into a station but maybe you are onto something. That was just before Ivanhoe too. Maybe they left me his fuel. Hhhmmmm. Thanks aliens.

Duncs
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Reply By: Member - Blue (VIC) - Monday, Jan 30, 2006 at 19:57

Monday, Jan 30, 2006 at 19:57
Mine never fluctuates whether on dirt or tar... It's not a Patrol though and as stated about the Paj, my Triton reads full until between 70-100km have been travelled.
AnswerID: 151890

Reply By: Member - Duncs - Tuesday, Jan 31, 2006 at 09:46

Tuesday, Jan 31, 2006 at 09:46
Ok This is what I think.

The soft drink bottle is a very good example of my theory. Much better than a bottle of water.

When you fill the tank with diesel you have to wait for the froth on top to settle before you can squeeze the last bit in. Petrol you can just pour in till full.

I reckon that on the dirt the diesel gets shaken up and develops a head, like a nice cold beer type head. Now, unless your drinking Guiness that head will not support the float so it drops to the level of the liquid. When you get on the black-top and slow down a bit, going through a town, the foam settles and the liquid level rises lifting the sender float and giving a higher reading. Just a theory.

What this tells me really is that, despite concentrating on the road, dodging kangaroos, potholes, narrow grids and some slippery bits (it rained on me too) too much driving late at night on country roads will lead the brain to do strange things.

Have you been driving late at night recently Wolfie?

Duncs

AnswerID: 152022

Follow Up By: HJ60-2H - Tuesday, Jan 31, 2006 at 20:16

Tuesday, Jan 31, 2006 at 20:16
In tha tcase it would happen with all diesel vehicles?? Doesn't happen with my 60.....
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FollowupID: 405693

Reply By: traveller2 - Wednesday, Feb 01, 2006 at 15:15

Wednesday, Feb 01, 2006 at 15:15
It is a common problem with a few late model diesel vehicles which may apply in this case. While the engine is running the fuel lift pump is pumping fuel from the tank to the injection pump, the injection pump uses some of this fuel to run the engine the rest is recirculated back into the tank lubricating and cooling the injection pump along the way.
Depending on the vehicle it and how the fuel tanks are connected and how the return lines are plumbed you will find that the fuel may be sucked from the auxilary down to the engine and back to the main tank.
Most Toyota's keep the two tanks seperate switching both feed and return lines but weird things happen when say the return changover solenoid sticks in the wrong position or partially changes etc.
If the driving lights being on changes the fuel level then it will be a voltage or grounding problem or a combo of both.
AnswerID: 152283

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