Drunk fuel gauge (one more)

[BigBrother-84]

Hi Gentlemen!

My road trip is coming soon, so I will limit Big Brother's enhancements for the moment.

But there's a new issue that I will probably have to address.  The fuel gauge hesitates with the fuel level.
Let me explain:  Starting the engine, it slowly goes to the good level.  But during the drive, it fluctuates with the rpm.
I noticed that, when the gauge shows a lower level than the reality, it's when the engine is going to idling (at a stop light or something like that).  When the engine goes back to higher rpm, the gauge goes up too, reaching its correct level.


It is not constant nor regular.  Just from time to time, but it happens too often in my opinion.


I did the "ground test" and the gauge goes to the max.  So I suppose that my gauge and wiring are ok.

Is it possible that the fuel sending unit is sensitive to the "electrical power"?

[81-F150-Explorer]

Possibly a bad varying sending unit ground. Possibly internal to the sending unit.

Grounding the gauge eliminates the possibility of a bad gauge, but not a bad ground to the sending unit itself.

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It could possibly be related to the ICVR, instrument Cluster voltage regulator.

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How is the charging system? Could the voltage be dropping at idle?

 

[BigBrother-84]

You’re right Ralph, this is logical and needs to be verified.
But electrical diagrams do not show where the sending unit ground is located.
Any advice?

[81-F150-Explorer]

It's grounded through the black wire in the connector.

It connects to this ground via the sweep to the sending unit. The sweep rides on a resistor like spring, thus varies the ground as fuel level decreases.

These sending units are known to have bad sweeps.

I edited my original post with more ideas as to why the gauge can fluctuate.

[Gary Lewis]

If the other two gauges, the coolant temp and oil pressure, don't do this then it probably isn't the ICVR as it serves all three.  But if they do it is the ICVR.

Having said that, I can't imagine why it would be tied to engine RPM save for an issue with the ICVR.  That's because the battery voltage will vary with engine RPM, assuming there's a reasonable load on the battery such that the alternator can't keep the voltage at 14.4v at idle.

So check to see if the other two gauges are acting wonky also.

[grumpin]

The way these gauges "work", I'd think it's from sloshing fuel, if the other gauges aren't affected as Gary said.

[BigBrother-84]

Gary Lewis wrote

If the other two gauges, the coolant temp and oil pressure, don't do this then it probably isn't the ICVR as it serves all three.
[…]
So check to see if the other two gauges are acting wonky also.

All other cluster instruments are perfectly stable.
Just the fuel gauge that fluctuates this way.

[Gary Lewis]

Then it shouldn't be the ICVR as it powers all three of those gauges.

That being the case, maybe it is the slosh that Dane mentioned?  Variations in these gauges aren't usually obvious since they are actually thermometers and the heat from the current running through the coil doesn't change very quickly.  But perhaps you have a gauge that does react quickly?

[81-F150-Explorer]

Gary Lewis wrote

Then it shouldn't be the ICVR as it powers all three of those gauges.

That being the case, maybe it is the slosh that Dane mentioned?  Variations in these gauges aren't usually obvious since they are actually thermometers and the heat from the current running through the coil doesn't change very quickly.  But perhaps you have a gauge that does react quickly?

Yes I agree, all three gauges would react...

As for the sloshing, my fuel gauge will react to slosh, going around a turn, or going up/down hill  but it's not very noticeable.

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Is this something that happened recently?  He did say that he changed the instrument cluster out for one with a tach... (or am I getting people confused?) Perhaps the replaced fuel gauge is more reactive than the older one he swapped in with the cluster?

Still could be sending unit issues too.

[Gary Lewis]

Yes, Jeff recently swapped out his gauge cluster.  I wonder if the movement of the gauge was with the "old" or "new" gauge?  

[BigBrother-84]

Oh-oh…  Maybe you’re touching something, Dane.

You make me realize that maybe it’s not related to the engine rpm, but possibly to acceleration/deceleration/hilly road/turns/etc.

This cluster is made of the best parts of 3 used ones plus NOS gauges.  This fuel gauge is NOS.

And this summer is the first time Big Bro is "really" on the road for long runs, if I can say, since mid 2019.  And so for its "rebuilt" cluster.

Maybe this NOS fuel gauge is much more sensitive than the old one I used to have?

Before more digging the electrical question, I’ll pay attention to its behavior.

If it varies only with truck’s movements, we’ll have found something.  But one thing I can say is that the fluctuations proportion can be important, not just a little needle movement.

Can the fuel sloshing influence so much a sending float, as Ralph described it?

[grumpin]

BigBrother-84 wrote

Oh-oh…  Maybe you’re touching something, Dane.

You make me realize that maybe it’s now related to the engine rpm, but possibly to acceleration/deceleration/hilly road/turns/etc.



If it varies only with truck’s movements, we’ll have found something.  But one thing I can say is that the fluctuations proportion can be important, not just a little needle movement.

Can the fuel sloshing influence so much a sending float, as Ralph described it?

I agree with Ralph that it happens but not radical, noticeable. I don't trust these fuel quantity systems.

Never trusted my 1974 F250, 1986 F250 or 1988 F250 (7.3). My 1992 Bronco and 1994 F250 were and are the only ones I trusted.

I never had enough faith or desire to try to fix them, went by mileage or oops better flip the switch.

The best way I see to fix them is with the Meter Match that Gary is using.

[BigBrother-84]

Gentlemen, you were right, and THANK YOU for your advice:
The fuel gauge behavior is strictly fuel sloshing related.

I am dumbfounded that the fuel level can vary so much just because of the truck movements!


I made a serious test on a hilly winding road near home (had fun driving Big Bro as if Montecarlo ), and the conclusion is clear.
It is not linked to the engine rpm.

The fluctuation can be as much as a quarter of tank(!), for a hairpin turn. The indicated level drops much more faster than it comes back to the “good” fuel level.

But driving quietly on a flat highway is very stable, no matter how fast or idle the engine turns.

I don’t understand why I’d never noticed this kind of fuel gauge behavior.

[Gary Lewis]

Glad you got it figured out and that there's no problem.

You said the fuel gauge is NOS, so you haven't seen it work before.  Apparently it is sensitive and small changes register more on it than the old gauge.  

[Machspeed]

Glad you got that figured out, Jeff! That would be a bit bothersome taking a long trip with a bad fuel gauge. Of course, if you know the MPG's on it, you could work the trip off the math. Do you have duel tanks or single?

The only thing on my truck that does not work is the flipp'in fuel gauge. Strange thing is that once in a blue moon, I'll look down and see it working. That is rare moment though....LOL! I have got to look into this.

Looking forward to meeting you at our show!  

[81-F150-Explorer]

Glad to find out it was only fuel slosh.


John-  From your description, it sounds like the sending unit, or the electrical connector at the sending unit.

[BigBrother-84]

John, Big Bro is big, but has a little stomach… Single tank.

So we’re going to often stop’n fuel, LOL!

Excited about the trip, can’t wait to see y’all at the show!

I will create a thread about this road trip, stay tuned!

[BigBrother-84]

I finally fixed the issue.
It was the fuel tank sending unit.

We never had an accurate fuel gauge since we own Big Bro.
So I didn’t really trusted this float, and I tested the gauge accuracy.

Verdict:
There no “reserve” or warning.
When it shows “empty”… it is.

I ran out of gas.

[Gary Lewis]

You fixed it by dropping the tank and replacing the sending unit?  That's a lot of work if it was the rear, and still a fair amount if the front.

Bummer about running out of gas.  Been there, done that.  My father loaned me his '72 Mustang, and forgot to tell me one important thing - when the needle touches E you just ran out of gas.  I pulled into our driveway and turned it off, and the next morning it wouldn't start.  He said "Oh, I forgot to tell you about that."  

[BigBrother-84]

Gary Lewis wrote

You fixed it by dropping the tank and replacing the sending unit?  That's a lot of work if it was the rear, and still a fair amount if the front.

Nan, Big Bro has only the side tank.
And I didn’t do it myself, I asked my garage if he had time do to it while the truck was up for the summer tires swap.  He charged me half an hour for the job, everything went easy.

When I left Big Brother at the garage, it was so empty that it ran out of gas just going out of the garage.

Is it becoming a habit, Big Bro…?
🧐