fuel gauge reading high with new senders

[mat in tn]

I have been following along as neither of my new senders were reading correctly. i went through verifying grounds first. then proving the gauge. with 5 gal in the rear tank, it read empty. with five in the front, it reads below empty. so, I jumped off the dock and filled up the rear tank. now it reads full as you would expect. but it " ranges" last I looked it red 1/4 tank. but while driving around it seemed to work somewhat randomly. I only drove 30 miles or so and I would expect it to still be near full. when switching between tanks it seems to do as it is told. I may try a different tank switch just to test

[Megiddo]

I tracked my gauge readings (full = 100%) vs miles traveled on my odometer for 5 fill ups on each tank



Miles traveled is on the vertical axis, fuel level in % is along the bottom.
A perfectly linear 10 mpg is shown as the red line. This has been my consistent average for the 3 1/2 years I've had the truck
My front tank is the blue line.  It show a lot of movement early then "hangs out below" 1/2 tank
My back tank is the green line that shows full for quite a while then quickly moves to empty

In the middle of December my mileage fell way off and I had a few backfires and black residue in the exhaust.  I took it in had had my catalytic converters taken off and new mufflers put on.   The cats had come apart and clogged the mufflers.  I thought the back pressure had to be the cause for the sudden change in "efficiency", but my mileage still hasn't recovered all the way and now I'm only getting about 8 mpg.  Any thoughts on that?

[Gary Lewis]

Interesting graph.  But that's basically my experience as well.

On the MPG, if you had backfires then your power valve is probably blown, which will kill your mileage.

[Megiddo]

Thanks, I'll check that

[archeman]

Perhaps the experts on this site (and they are indeed experts!) can suggest the Parts Vendors who are able to point us to the 'correct' bullnose version of the tank fuel pump/sending units?

When I purchased mine, I ended up with the 'newer' pump/sending unit. I saw no such distinction for old/new when shopping but clearly there is.

[Gary Lewis]

I can't get to a specific part number as you've not given us enough info.  However, you can do it yourself as you know the details.

First, I'd go to the page at Documentation/Fuel & Air Systems/Fuel System Part #'s/Fuel Senders & Pumps.  There you can find the part numbers.

And the next thing I'd do is go to Rock Auto and see what parts they have.  Compare those part numbers with those from our documentation, which is from Ford's master parts catalog.

Then, with that info you can go to other sites looking for those parts.

[mat in tn]

I just thought I would post some test results in hopes of helping someone along the way. I used a little science to check my tank level. we had a few cool nights so the other day as it was warming up quickly, I drove the truck out onto the drive in the sun. then crawled underneath and watched the condensation accumulate on the tank at the liquid level. voila! rear tank about 5/8 full and the front tank 1/4. now I know. so, I pumped out the front tank, dropped it, and removed the sending unit. tested it for ohms across its range. all was good except the float arm needed a little "tuning". I used a bucket of gas to set the level per the sock referenced at the bottom of the tank. re installed then transferred all gas from rear tank to the front and did the same. this was a little more interesting. after a little study I noticed the sending unit was made where the pump, sock and level sensor were dead level in the tank. horizontal! and right about seam level. meaning that it will read dead empty at half tank and even run out of gas there too. that certainly explains why it went down so fast and read empty. I used a different pump to transfer and moved ten gallons.  I needed to "tune" the entire assembly to put the pickup are in the well of the tank and tune the level sender also. all good now and I wasted a LOT of time and effort triple checking all of my wiring and still having to disassemble finish painted systems because I trusted new parts. check everything! i know better and I still learn something new every day. sometimes again. 

[Gary Lewis]

Good info, Matt. Glad you got it sorted.