Fuel gauge reads zero

[LANCECASPER]

Tricky because the outboards seem to have been added after, and still trying to determine at what point the inboards start to spillover into the outboards. Seems like right around 20 gal.

It sounds like you have the Jose Monroy Long Range Tanks STC. It will be in your logs and there will be a supplement in your POH.
The outboard fuel moves into the inboard tanks as you use fuel. This happens by gravity since it's a dihedral wing.

[Fly Boomer]

1 hour ago, jamicozzi said:

We've learned on ours that the fuel gauges are useless. Read full or almost full all the time. Was running on fumes on the right tank and showing more than 3/4. 

Tricky because the outboards seem to have been added after, and still trying to determine at what point the inboards start to spillover into the outboards. Seems like right around 20 gal.

Looks like we'll be doing an avionics upgrade early spring which will include new senders. Sure will be nice to have a reference beyond constantly having to do the math to keep track of fuel in the tanks...UGH

Given your model and your reference to "outboard" tanks, I assume you have Monroy long range tanks.  Do you know that there are no senders in the auxiliary tanks?  So you have 4 tanks and 2 fuel gauges.  If you fill all 4 tanks (probably a little over 100 gallons), the fuel gauges won't come off "full" until you have burned a large amount of fuel because, due to the wing dihedral, as fuel is burned, the aux tanks replenish the fuel in the mains until you have burned 20 or 30 gallons -- I don't know what the number is, but with the Monroy long range tanks, the only time the gauge has a chance of being accurate is when you have zero fuel remaining.  The senders and gauges were designed with the as-built tanks in mind, and don't know anything about all the extra fuel in the aux tanks.  You may have other issues with the senders, wiring, or gauges but, even if they are in perfect working order, the gauges will not come off "full" until you have flown several hours.

EDIT:  On review, I see that @LANCECASPER beat me by 12 minutes.

[jamicozzi]

1 hour ago, Fly Boomer said:

Given your model and your reference to "outboard" tanks, I assume you have Monroy long range tanks.  Do you know that there are no senders in the auxiliary tanks?  So you have 4 tanks and 2 fuel gauges.  If you fill all 4 tanks (probably a little over 100 gallons), the fuel gauges won't come off "full" until you have burned a large amount of fuel because, due to the wing dihedral, as fuel is burned, the aux tanks replenish the fuel in the mains until you have burned 20 or 30 gallons -- I don't know what the number is, but with the Monroy long range tanks, the only time the gauge has a chance of being accurate is when you have zero fuel remaining.  The senders and gauges were designed with the as-built tanks in mind, and don't know anything about all the extra fuel in the aux tanks.  You may have other issues with the senders, wiring, or gauges but, even if they are in perfect working order, the gauges will not come off "full" until you have flown several hours.

EDIT:  On review, I see that @LANCECASPER beat me by 12 minutes.

Yes, that is what I have reasoned on this. We seldom fill beyond the Inboards, and shortly later they spillover. Which we can live with. It is pretty clear there is no sender in the outboards. However even when the outboards are dry, and down to about 10 in the Inboards, it still is not reading correctly. Will be working with the A&P to at least check the senders. Sounds like there may be a ground problem. I'd be happy with a semi-accurate read of some kind...At least be correct once the outboards run dry....

[Z W]

Sounds like normal behavior for the long range tanks.

If you fill the inboards, you will have 75 gallons on board (37.5 per side), plus or minus.  The gauges should read full then, but the fuel will slowly push out into the outboards.  After that, the gauges should read 25 gallons or so, even though you have 37.5 gallons in the tank.  They will stay between 20-25 gallons until the outboard tanks burn to empty, as the fuel will drain down into the mains as you fly and burn gas.  Under about 20 gallons, with empty outboards, the gauges become accurate again, assuming they're working.

[Pinecone]

On my 252, you can see a film of fuel at the outboard filler at 25.0 gallons. 

If you try to "fill just the inboards" it will depend on how fast you fill over that 25 gallons, as at the point, fuel is moving into the outboard.  Fairly slowly, but the slower you fill, the more fuel will move to the outboard.

When I calibrated my dip sticks, I found that even after you are nearly full, there is still air bubbling out of the inboard when you add fuel to the last little bit.  I managed to put 55 gallons in mine.  Rated capacity is 52.  And even that last gallon, you could hear the air.

So the gauges don't miss a huge amount of fuel.

As for senders, think about putting in Cies senders.  VERY accurate.

[Fix]

I also have AUX tanks and when I have ~37 gallons in my main tank, around 10 gallons is flowing over to the Aux tank.
So my Main Tank fuel meter says roughly 27 gal.

In my POH for AUX tanks is says when you measure fuel level.

1/2" = 5 gal
2" = 10 gal
4" = 17 gal

[hoot777]

While we are on fuel gages do all m20s have two senders per tank??  I have a M20G.  1968 model. Thanks